diploma at UNSW or any other educational institution, except where due ... ABSTRACT. Open pit mines may experience water ingress into blast holes. ...... zone surrounding the blast hole and a reduction of far field ground vibration. ..... Numerical models can be used to calculate and predict deformation, strains and stresses.
i
AN INVESTIGATION INTO WATER DECKED BLAST HOLES USING NUMERICAL MODELLING
XU DONG FENG
2018
A thesis submitted in partial fulfilment of the requirements for the award of Bachelor of Engineering (Mining) The University of New South Wales
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Originality Statement I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged. Copyright Statement I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in all forms of media, now or hereafter known. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. Third Party Copyright Statement I have either used no substantial portions of third party copyright material, including charts, diagrams, graphs, photographs, maps, in my thesis, or I have obtained permission for such material to be made accessible worldwide via the Web. If permission has not been obtained, I have asked/will ask the Library to remove the third party copyright material from the digital copy. •
Signed on this 7th day of October 2018
•
Xu Dong Feng
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ABSTRACT Open pit mines may experience water ingress into blast holes. Laboratory studies, numerical models and field applications have demonstrated a reduction in explosive consumption, oversize boulders, fines, dust, noise, and fly rock by utilising this water as decking in production blasts or coupling medium in presplit blasts. This thesis is an investigation into the effects of water deck location, water deck length, rock mass strength and explosive density applied to the water decking technique. Multiple blast column models were established and compared using ANSYS Autodyn®. Numerical model geometries were based on DynoNobel guidelines. Material models used the JWL equation of state (EOS) for explosives, the P-alpha EOS for brittle rock material, the Compaction EOS for stemming material and the Shock EOS for water. Principal stress behaviour at the spacing distance was examined in the top, middle and lower regions. Near field damage was examined using the Rankine-Hugoniot Shock damage output. No significant results were obtained by modifying explosive density and velocity of detonation. 50% greater stress magnitudes were seen at the spacing distance when rock strength was increased from 35 MPa to 140 MPa. Top water decks slightly reduced near field damage. Bottom water decks increased the duration of stress application in the toe region at the spacing distance and led to greatly more uniform explosive energy distribution in the near field. The greatest bottom water deck effects were found when deck length was 10% of entire blast hole length. The findings of this thesis suggest that the application of bottom water decking, even in high strength rock mass is beneficial. The use of the bottom water decking over top water decking is suggested for production blasts and air remains the ideal presplit blasting decking medium.
ii
ACKNOWLEDGEMENTS
This thesis would not have been successfully completed without the support and guidance I received from the following people. To them, I extend my sincere gratitude. •
Associate Professor Paul Hagan;
•
Dr Chengguo Zhang;
•
Dr Faham Tahmasebinia;
•
My family;
•
Michelle; and
•
Jack Smith.
iii
CONTENTS 1
INTRODUCTION .......................................................................................................... 1 1.1
2
3
4
Project Objectives ......................................................................................................2
LITERATURE REVIEW ............................................................................................... 3 2.1
Introduction .................................................................................................................3
2.2
Mechanism of Water in blasting ..........................................................................4
2.3
Physical Modelling ................................................................................................. 12
2.4
Field Studies ............................................................................................................. 14
2.5
Numerical Modelling of Water Decked and Water Coupled Blasts ..... 19
2.6
Numerical Modelling of Other Blast Phenomena ....................................... 22
2.7
Comparison with Air Decking ............................................................................ 23
2.8
Conclusions ............................................................................................................... 24
RISK ASSESSMENT AND MANAGEMENT PLAN ...............................................26 3.1
Hazard Identification and Risk assessment ................................................. 26
3.2
Contingency Plan .................................................................................................... 31
3.3
Analysis of the Risk assessment and Management Plan ......................... 31
METHODOLOGY ........................................................................................................33 4.1
Numerical Modelling ............................................................................................. 33
4.2
Material models....................................................................................................... 34 4.2.1 Jones-Wilkins-Lee (JWL) Equation of State .................................... 34 4.2.2 Riedel- Hiermaier-Thoma (RHT) Concrete Model ....................... 37 4.2.3 P-alpha Equation of State ....................................................................... 38 4.2.4 Compaction Equation of State .............................................................. 39 4.2.5 Ideal Gas Equation of State .................................................................... 40 4.2.6 Rankine-Hugoniot Shock Equation of State .................................... 41
4.3
ANSYS 3D Explicit Dynamics Model ................................................................ 42 4.3.1 ANSYS Explicit Dynamics Background ............................................. 42 4.3.2 3D Base Model Geometry ....................................................................... 43 4.3.3 Solver, Meshing and Initial conditions .............................................. 44 4.3.4 Model Investigations................................................................................ 45 4.3.5 Initial Results .............................................................................................. 46 4.3.6 Discussion of Initial Results .................................................................. 49
iv 4.3.7 Secondary Results ..................................................................................... 52 4.3.8 Discussion of Secondary Results ......................................................... 56 4.4
ANSYS 2D Autodyn Model ................................................................................... 58 4.4.1 ANSYS Autodyn Background ................................................................ 58 4.4.2 Base Model Geometry .............................................................................. 58 4.4.3 Solver Zoning and Solver Controls ..................................................... 59 4.4.4 Initial Conditions ....................................................................................... 60 4.4.5 Model Investigations................................................................................ 60 4.4.6 Gauge Locations ......................................................................................... 61 4.4.7 Model Validation ....................................................................................... 62
5
ANSYS AUTODYN MODEL RESULTS ...................................................................63 5.1
Model Near Field Damage ................................................................................... 63
5.2
Maximum Principal Stress History .................................................................. 68
5.3
Minimum Principal Stress History ................................................................... 73
5.4
Maximum Principal Stress Comparison at the Spacing Distance......... 78 5.4.1 Maximum Principal Stress History Comparisons at Gauge 9 ... 79 5.4.2 Maximum Principal Stress History Comparisons at Gauge 11 84 5.4.3 Maximum Principal Stress History Comparisons at Gauge 13 89
6
ANSYS AUTODYN MODEL DISCUSSION .............................................................95 6.1
Effect of Parameter on Near Field Damage .................................................. 95 6.1.1 Deck Location ............................................................................................. 95 6.1.2 Deck Length ................................................................................................. 95 6.1.3 Comparison with Air Deck ..................................................................... 96 6.1.4 Comparison with No Deck ..................................................................... 96 6.1.5 Rockmass Strength ................................................................................... 96 6.1.6 Explosive Density ...................................................................................... 96
6.2
Effect of Parameter on Stress Action at the Spacing Distance .............. 97 6.2.1 Deck Location ............................................................................................. 97 6.2.2 Deck Length ................................................................................................. 97 6.2.3 Comparison with Air Deck ..................................................................... 97 6.2.4 Comparison with No Deck ..................................................................... 97 6.2.5 Rockmass Strength ................................................................................... 98 6.2.6 Explosive Density ...................................................................................... 98
6.3
Conclusions ............................................................................................................... 98
v 7
CONCLUSIONS ......................................................................................................... 100
8
RECOMMENDATIONS ........................................................................................... 102
9
REFERENCES ........................................................................................................... 103
vi
LIST OF FIGURES Figure 1. The application of water in blast holes. ............................................................ 4 Figure 2 - The pressure on the blast hole wall in water coupled blast holes ................. 11 Figure 3. Experiment set up by Lin and Ma (1992) ......................................................... 12 Figure 4. Experiment set up by Chen (2000)................................................................... 13 Figure 5. Gas bag operation procedure .......................................................................... 17 Figure 6. Bai and Zhang (2017) bottom water deck set up............................................. 18 Figure 7. Numerical model geometry definition by Zong, Yan and Wang (2011) .......... 20 Figure 8. Liu and Zhang field study blast fragmentation photographs. .......................... 22 Figure 9. Pareto analysis of research project failure modes. ......................................... 30 Figure 10. DynoNobel Blasting Guidelines ...................................................................... 43 Figure 11. Base case model geometry. ........................................................................... 44 Figure 12. Base case model meshing. ............................................................................. 45 Figure 13. Maximum Principal Stress at the (0.075,0.075,1) coordinate. ...................... 47 Figure 14. Maximum Principal Stress at the (0.075,0.075,6) coordinate. ...................... 47 Figure 15. Maximum Principal Stress at the (0.075,0.075,8) coordinate. ...................... 48 Figure 16. Maximum Principal Stress at the (3,3,3.5) coordinate. ................................. 48 Figure 17. Maximum Principal Stress at the (3,3,6) coordinate. .................................... 49 Figure 18. Air Deck Sections (Lu and Hustrulid, 2003) .................................................... 50 Figure 19. Maximum Principal Stress at Section A-A. ..................................................... 50 Figure 20. Maximum Principal Stress at Section B-B. ..................................................... 50 Figure 21. Maximum Principal Stress at Section C-C. ..................................................... 51 Figure 22. Maximum Principal Stress at Section D-D...................................................... 51 Figure 23 - Maximum Principal Stress at Section E-E. .................................................... 51 Figure 24. Principle Stress History at the (0.075,0.075,0.5) coordinate. ........................ 53 Figure 25. Principle Stress History at the (0.075,0.075,3.5) coordinate. ........................ 53
vii Figure 26. Principle Stress History at the (0.075,0.075,6.5) coordinate. ........................ 54 Figure 27. Principle Stress History at the (0.075,0.075,7.5) coordinate. ........................ 54 Figure 28. Principle Stress History at the (0.075,0.075,8.5) coordinate. ........................ 55 Figure 29. Principle Stress History at the (0.075,0.075,9.5) coordinate. ........................ 55 Figure 30. Principle Stress History at the (6,6,0.5) coordinate. ...................................... 56 Figure 31. ANSYS Autodyn base model geometry. ......................................................... 59 Figure 32. ANSYS Autodyn model gauge locations. ........................................................ 61 Figure 33. Model 1 Damage. ........................................................................................... 64 Figure 34. Model 2 Damage. ........................................................................................... 64 Figure 35. Model 3 Damage. ........................................................................................... 65 Figure 36. Model 4 Damage. ........................................................................................... 65 Figure 37. Model 5 Damage. ........................................................................................... 66 Figure 38. Model 6 Damage. ........................................................................................... 66 Figure 39. Model 7 Damage. ........................................................................................... 67 Figure 40. Model 8 Damage. ........................................................................................... 67 Figure 41. Model 9 Damage. ........................................................................................... 68 Figure 42. Scenario 1 Maximum Principal Stress History. ............................................. 69 Figure 43. Scenario 2 Maximum Principal Stress History. ............................................. 69 Figure 44. Scenario 3 Maximum Principal Stress History. .............................................. 70 Figure 45. Scenario 4 Maximum Principal Stress History. .............................................. 70 Figure 46. Scenario 5 Maximum Principal Stress History. ............................................. 71 Figure 47. Scenario 6 Maximum Principal Stress History. .............................................. 71 Figure 48. Scenario 7 Maximum Principal Stress History. .............................................. 72 Figure 49. Scenario 8 Maximum Principal Stress History. .............................................. 72 Figure 50. Scenario 9 Maximum Principal Stress History. .............................................. 73 Figure 51. Scenario 1 Minimum Principal Stress History. ............................................... 74
viii Figure 52. Scenario 2 Minimum Principal Stress History. ............................................... 74 Figure 53. Scenario 3 Minimum Principal Stress History. ............................................... 75 Figure 54. Scenario 4 Minimum Principal Stress History. ............................................... 75 Figure 55. Scenario 5 Minimum Principal Stress History. ............................................... 76 Figure 56. Scenario 6 Minimum Principal Stress History. ............................................... 76 Figure 57. Scenario 7 Minimum Principal Stress History. ............................................... 77 Figure 58. Scenario 8 Minimum Principal Stress History. ............................................... 77 Figure 59. Scenario 9 Minimum Principal Stress History. ............................................... 78 Figure 60. Gauge 9 Maximum Principal Stress Comparison 1. ....................................... 80 Figure 61. Gauge 9 Maximum Principal Stress Comparison 2. ....................................... 80 Figure 62. Gauge 9 Maximum Principal Stress Comparison 3. ....................................... 81 Figure 63. Gauge 9 Maximum Principal Stress Comparison 4. ....................................... 81 Figure 64. Gauge 9 Maximum Principal Stress Comparison 5. ....................................... 82 Figure 65. Gauge 9 Maximum Principal Stress Comparison 6. ....................................... 82 Figure 66. Gauge 9 Maximum Principal Stress Comparison 7. ....................................... 83 Figure 67. Gauge 9 Maximum Principal Stress Comparison 8. ....................................... 83 Figure 68. Gauge 9 Maximum Principal Stress Comparison 9/....................................... 84 Figure 69. Gauge 11 Maximum Principal Stress Comparison 1. ..................................... 85 Figure 70. Gauge 11 Maximum Principal Stress Comparison 2. ..................................... 85 Figure 71. Gauge 11 Maximum Principal Stress Comparison 3. ..................................... 86 Figure 72. Gauge 11 Maximum Principal Stress Comparison 4. ..................................... 86 Figure 73. Gauge 11 Maximum Principal Stress Comparison 5. ..................................... 87 Figure 74. Gauge 11 Maximum Principal Stress Comparison 6. ..................................... 87 Figure 75. Gauge 11 Maximum Principal Stress Comparison 7. ..................................... 88 Figure 76. Gauge 11 Maximum Principal Stress Comparison 8. ..................................... 88 Figure 77. Gauge 11 Maximum Principal Stress Comparison 9. ..................................... 89
ix Figure 78. Gauge 13 Maximum Principal Stress Comparison 1. ..................................... 90 Figure 79. Gauge 13 Maximum Principal Stress Comparison 2. ..................................... 90 Figure 80. Gauge 13 Maximum Principal Stress Comparison 3. ..................................... 91 Figure 81. Gauge 13 Maximum Principal Stress Comparison 4. ..................................... 91 Figure 82. Gauge 13 Maximum Principal Stress Comparison 5. ..................................... 92 Figure 83. Gauge 13 Maximum Principal Stress Comparison 6. ..................................... 92 Figure 84. Gauge 13 Maximum Principal Stress Comparison 7. ..................................... 93 Figure 85. Gauge 13 Maximum Principal Stress Comparison 8. ..................................... 93 Figure 86. Gauge 13 Maximum Principal Stress Comparison 9. ..................................... 94
x
LIST OF TABLES Table 1. HAZOP failure mode identification. .................................................................. 27 Table 2. Risk rating matrix. ............................................................................................. 28 Table 3. FMECA. .............................................................................................................. 28 Table 4. Default ANSYS ANFO material parameters. ...................................................... 35 Table 5. Orica Fortis JWL Parameters. ............................................................................ 36 Table 6. Rockmass RHT Model Parameters. ................................................................... 37 Table 7. Rockmass P-alpha EOS Parameters. ................................................................. 39 Table 8. Stemming Compaction EOS Parameters. .......................................................... 40 Table 9. Air Idea Gas EOS Parameters. ........................................................................... 41 Table 10. Water Linear Shock EOS Parameters. ............................................................. 42 Table 11. ANSYS Explicit Dynamics Numerical Model Scenarios. ................................... 46 Table 12. ANSYS Autodyn Numerical Model Scenarios................................................... 60 Table 13. ANSYS Autodyn Numerical Model Gauge Point Locations. ............................. 61 Table 14. Maximum Principal Stress Comparisons. ........................................................ 79
1
1 INTRODUCTION South 32 Ltd’s GEMCO is a large surface operation mining manganese ore on Groote Eylandt, Australia. GEMCO is located in a tropical climate and experiences extreme rainfall events during its wet season. Groote Eyelandt also overlies a water table in close proximity to the surface. At the GEMCO mine it is not uncommon to encounter high levels of water in blast holes and outright flooded pits. The source of the water in the blast holes can be from one or a combination of infiltration of groundwater; ingress of rainwater; and, ingress of surface water from nearby groundwater and rainwater pools. The flooded blast holes and pits are problematic to drill and blast operations where extra resources are required to dewater the flooded pits. Knowledge on this mine site about the effects of water was limited and in general, water was seen to have a negative effect on drill and blast operations (Pope, 2018). An investigation into the literature surrounding the use of water in blast holes indeed found difficulty in sourcing English language literature. It was found that the majority of research on this topic was undertaken in China and published in Chinese literature. Chinese laboratory studies, numerical models and field applications have demonstrated positive effects when water is used either as decks in the blasting column during production blasts or as a surrounding coupling medium in presplit blasts. The literature unanimously finds the use of water decking and water coupling associated with a reduction in explosive consumption, oversize boulders, fines, dust, noise, and fly rock. A review of the literature has established that while numerical models have been established investigating and comparing water coupling rations, attempts to numerically model the application of water decks in blast holes have been by large specific models tailored to specific site conditions. This thesis has identified a gap in the knowledge in comparing different water decking parameters such as water deck length, water deck location, comparisons between air decking, and applicability to varying rock mass strengths and explosive types. This thesis was undertaken to investigate and compare the effects of water deck location, length, comparison with air decking, rock mass strength and explosive type on the near field damage at the blast hole wall and far field stress behaviour at the spacing distance.
2
1.1
PROJECT OBJECTIVES
The following project objectives were established to satisfy the aim of the research project: 1. To source literature surrounding the use of water in blast holes; 2. To translate any Chinese language literature; 3. To design, implement and review a numerical model that quantitatively measures far field stress behaviour at the spacing distance and qualitatively displays near field damage at the blast hole wall; 4. To interpret and evaluate results to determine a generalised guideline regarding water deck location and length; and 5. To interpret and evaluate results to determine the applicability of water decking in different strength rock masses and the applicability of different explosive types.
3
2 LITERATURE REVIEW 2.1
INTRODUCTION
In investigating the thesis topic “To investigate and compare the effects of water deck location, length, comparison with air decking, rock mass strength and explosive type on the near field damage at the blast hole wall and far field stress behaviour at the spacing distance” it is important to apply a progressive approach to building knowledge. In this literature review, the mechanism of water in blast holes during blasting is first investigated. This is followed by an examination of recent attempts to physically model the water decking technique, field applications and finally numerical modelling. In recent years, the application of water in blasting has seen much investigation and application in China where conventionally fully coupled drill and blast practices have been met with inefficiencies in water filled blast holes. In reviewing the literature surrounding the use of water in blast holes it was found that there was very little research available in English. Much, if not all relevant studies and examinations of the topic have been done in China and the literature is in Chinese language publications. Most of the literature has been compiled from the China National Knowledge Infrastructure (CNKI) database. While the literature compiled from the CNKI is available with English abstracts that detail key findings and conclusions, the rest of the literature has not been translated. While best efforts have been made to translate and understand the Chinese literature, a significant part of the Chinese literature is beyond mechanical translator capabilities. This presents a potential problem in that some key parameters relating to water decking theory and best practice may have been missed in this literature review. Additionally, objective and critical assessments of the literature were made with difficultly as access to the completely translated, grammatically correct literature was limited. In reviewing Chinese language literature, it would appear that the current application of water in blast holes has led to two classes of its application. The first is known as the water decked blast hole technique as initially described by Baranov, Gopanyuk and Shvets (1986). In this method, a water spacer is placed axially along the length of the blast hole and the explosive column may sit above or below the water spacer. Two subclasses of its applications have been identified. Where the water deck is located below the explosive column it is known as the bottom water decking technique and, in this
4 report, will be known as such, and where the water deck is located above the explosive column it is known as the top water decking technique and, in this report, will be known as such. The second class of water application in blast holes is known as the water coupled technique as initial field trialled by Zhang, Shang and Gan (2002). In the water coupled technique the water is placed radially along the blast hole and forms a coupling medium between the blast hole wall and the explosive column. A subclass of this technique is known as the pressurised water coupled blast hole technique as implemented by Yang, Liu and Yu (2017). In this subclass, the water is still placed radially in the blast hole as a coupling medium between the explosive column and blast hole wall but with the addition of additional confining pressure on the water. Figure 1 shows a schematically representation of definitions used in this literature review.
Figure 1. The application of water in blast holes.
2.2
MECHANISM OF WATER IN BLASTING
When shock waves are transmitted from one material to another, wave reflection and refraction at the material interface will result from shock wave impedance mismatching. The energy transmitted from one material to another at the material interface will not be equal. The theory of wave transmission and wave refraction can be used to calculate the resulting shock wave energy reflection and refraction to establish best principles for impedance matching in blasting design. Du and Luo (2003) investigated the initial formation and propagation of explosive shock waves when a detonated explosive charge was coupled with water in a blast hole. Du and Luo (2003) applied elastic wave theory
5 and used an impedance mismatch method to find the shockwave impact pressure on the borehole wall in a water coupled blast hole. The shockwave impact pressure on the borehole wall as derived by Du and Luo (2003) is given by Equation 1, Equation 2 and Equation 3: 𝑃𝑟 =
2𝜌𝑚 𝐶𝑝 2𝛽 𝑄𝑉𝑆 𝛼 ( π 𝜌 )2 𝑒 𝐾𝑑𝛼 𝜌𝑚 𝐶𝑝 + 𝜌0 𝐷1 𝑄𝑉𝑇
(1)
where 𝛽 =2+
2𝜇 1−𝜇
(2)
and 𝜋=
4𝑄𝑆 𝑑𝑐2 𝜌𝑒
where 𝑃𝑟 is the initial impact pressure of the rock (MPa); 𝜌𝑚 𝐶𝑝 is the impedance of the rock; 𝐶𝑝 is the shock wave velocity in the rock (m/s); 𝜌𝑚 is the density of the rock (kg/m3); 𝜌0 𝐷1 is the impedance of the coupling medium; 𝐷1 is the shock wave velocity in water (m/s); 𝜌0 is the density of the coupling medium (kg/m3); 𝜌𝑒 is the density of the explosive (kg/m3); 𝑄𝑉𝑆 is the specific energy of the explosive (MJ/kg); 𝑄𝑉𝑇 is the specific energy of TNT (MJ/kg); 𝐾𝑑𝛼 is the coupling ratio of the explosive and blast hole; 𝛼 is a site specific constant; 𝛽 is a site specific decay constant; π is a site specific constant;
𝜇 is Poisson’s ratio; 𝑄𝑆 is the linear charge density (kg/m); 𝑑𝑐 is the diameter of the explosive charge (m); and 𝜌𝑒 is the density of the explosive (kg/m3).
(3)
6 Duo and Luo (2003) found that the weak compressibility of water acted as an efficient coupling medium for the transmission of explosive energy. They concluded that by using water as a coupling medium in blast holes, explosive energy could be more efficiently transferred to the rock to result in more uniform fragmentation, reduced ground vibration and noise, and reduce flyrock. Zong, Li and Xu (2004) expanded on Duo and Luo’s (2003) Equation 1 by examining elastic and wave reflection theory and found that the coupling factor 𝐾𝑑 for water coupled blasting could be calculated using Equation 4: 𝐾𝑑 = (
1 2𝜌𝑚 𝐶𝑝 2𝛽 𝑄𝑉𝑆 1 )𝛼 (π𝜌𝑒 )2 𝑁𝑆𝑐 𝜌𝑚 𝐶𝑝 + 𝜌0 𝐷1 𝑄𝑉𝑇
(4)
where: 𝑆𝑐 is the uniaxial compressive strength of the rock (MPa); and N is a coefficient estimating rock dynamic strength a value between 5 and 10 is suggested. In these theoretical investigations by Duo and Luo (2003) and Zong, Li and Xu (2004) into water coupling, a case is made encouraging the use of the technique however based on the limited assessment of these sources, the methodology used to reach this conclusion is unclear. Zong and Meng (2003) also investigated the influence of different coupling and decking mediums within a blast hole and the resulting transmission of explosive energy. Using an impedance mismatch method Equation 5 was found to describe the impedance factor of rock for a blast hole charged with a bottom water deck: 𝑍𝑚 = (𝑘𝑤 𝜌𝑤
𝐷 𝐿 3 ( ) 𝑍𝑒 1 8𝑘 𝐿+ℎ 𝑤 𝑒 )2
where: 𝑍𝑚 is the impedance of the rock; 𝑘𝑤 is the coupling coefficient; 𝜌𝑤 is the density of water (kg/m3); L is the length of the bottom water deck (m); h is the total height of the blast hole (m); and 𝑍𝑒 is the impedance of water.
(5)
7 This equation can be rearranged in a number of ways to provide engineering guidance in the design of bottom water decked and coupled blast holes, however, additional physical, numerical modelling and field studies are needed to verify the practical implementation of the equation. Wang (2003) noticed that conventional drill and blast practices in Chinese local and township level coal mines often generated significant fines resulting from a mismatch between initial shock wave pressure and the low strength of coal. It was identified that while the composition of the explosives could be changed to rectify excessive fines generation, the application of water coupling in the blast holes could also reduce initial blast hole pressure and reduce explosives consumption as well. Wang (2003) opined that the low compressibility of water aided transfer of pressure, increased explosive energy utilisation, and the driving force of the detonation products increased the width and length of fractures. Wang (2003) noted that the greater density of water over air slowed the expansion of detonation products to lengthen the duration of the explosive shock wave. Wang (2003) also reported that some explosive energy is consumed by the high heat capacity of water which weakened the initial explosion shock wave pressure, however, this is inconsistent with the greater initial shock wave pressures in water decked holes found by Wu et al (2002). While Wang (2003) also gives some guidance to the design of the water coupled blast holes, the methodology in which his conclusions were made are unclear and clear translation of his work is required for further assessment. Wang, Li, Shi and Fang (2008) discussed the mechanism of shockwave propagation and attenuation in air and water coupled blast holes. Wang, Li, Shi and Fang (2008) solved continuity conditions at the blast hole wall based on pressure and velocity equilibrium and extended the attenuation equation of the stress wave from spherical charges to cylindrical charges to reflect blast hole charging methods in practice. Using a cylindrical charge based stress wave attenuation equation, Wang, Li, Shi and Fang (2008) found that the initial shock wave pressure in an air coupled blast hole is ten to hundreds of times larger than the ultimate compressive strength of rock and resulted in a crushed zone immediately surrounding the blast hole wall. It was observed that when compared to a fully coupled blast, air decoupled blasting decreased the initial shock wave pressure at the blast hole wall and increased the shock wave pressure in the blast holes back rock. In line with these observations, it was concluded that the adoption of a reasonable decoupling factor could eliminate the crushed zone, improve explosive energy utilisation.
8 It was suggested that the use of water as the decoupling medium could increase the propagations of energy, extend the duration of the shockwave and extend the region of influence of the shock wave. A review of Wang, Li, Shi and Fang’s (2008) work also reveals a need for a clear translation to properly explain the theory and mathematics behind this theoretical discussion. Du, Zhou and Zong (2007) conducted a theoretical study comparing water coupled charges to air coupled charges using Equation 1 to model shock wave pressure on the blast hole wall. They found that regardless of coupling medium, blast hole pressure decreased alongside increased decoupling of the explosive. Du, Zhou and Zong (2007) found that the shockwave from the water coupled blast acted over a much longer period of time and generated a larger shockwave pressure on the blast hole wall compared to the air decked blast. Du, Zhou and Zong’s (2007) findings support that water can be used as an alternate medium to air in decoupled control blasts such as smooth, perimeter and contour blasting. Luo, Cui, and Lu (2009) expanded on Wang, Li, Shi and Fang (2008)’s work in comparing air coupled and water coupled blasts and expanded on the mechanism of action of the water decked charge. Luo, Cui and Lu (2009) reported that when air is used as the coupling medium the detonation products expand inside the blast hole and compress the air to generate shock waves that impact the blast hole wall. Due to the high compressibility of air, the expansion of the detonation products is not diminished and the detonation products expand to fill the entire blast hole. As the total volume of air and detonation products inside the blast hole increases, the density of this gas mixture increases to reduce the speed of sound and thus alter the impedance of the gas mixture. Additionally, due to the rapid expansion of detonation products, it is assumed that no heat exchange occurs between the gas mixture and the blast hole wall. However, when water is used as the coupling medium, Luo, Cui and Lu (2009) reported when the detonation products reach the interface with water the explosive shock wave undergoes transmission and additional reflection. This reflection is a centripetal rarefaction wave. A similar centripetal rarefaction wave is generated when the centrifugal shock wave through the water reaches the interface between the blast hole wall and the process repeats. As a result, the magnitude of pressure in the water is increased for a longer duration of time.
9 This mechanism of action described by Luo, Cui and Lu (2009) was replicated by Zhang and Huang’s (2013) studies conducted into improving the penetrability of coal using pressurised water coupled blasting. Zhang and Huang (2013) additionally describe that when the shock wave through the water reaches the blast hole wall and generates a centripetal rarefaction wave, the water body reaches a quasi-static stress state which extends the duration of the shock wave. Zhang and Huang (2013) report that the fluctuations caused by the repeating generation of centripetal and centrifugal rarefaction shock waves and the application of the quasi-static stress on the rock subsequently causes the blast hole wall to fluctuate, deform and crack. Radial fracturing is generated as the dynamic strength of the rock is exceeded by the tangential tensional stress caused the shock wave passing through the rock mass. Zhang and Huang (2013) report that the radial fracturing initiates from the microfractures generated at the blast hole wall. According to Liu, Yang and Yu (2017), the commonly accepted theory of blast hole damage involves the action of both the initial and rapid shock wave generated by the explosion and the slower expansion of detonation products. It is not clear whether shock wave action described in Luo, Cui, and Lu (2009) and Zhang and Huang (2013) is based upon the initial explosion or the action of the later and slower expansion of detonation products. Liu, Yang and Yu (2017) clearly define that when the explosive in the borehole is detonated, it is the detonation wave that first acts on the surrounding confined medium. The detonation products are described to then enter radial cracks created by the blast wave and resultant stress wave in the rock mass. A further wedging effect is created by the detonation products which propagate and widen cracks further and increase damage in the rock. Liu, Yang and Yu (2017) then derive that the extent of the fracturing zone surrounding a blast hole can be found combining Equation 6, Equation 7, Equation 8, Equation 9 and Equation 10. −1 𝛽
1 √2𝜎𝑐 𝜉 3
−1 𝛼
𝜎𝑡 𝐷𝑡 = ( 1 ) ( ) 𝐶𝑝𝑐 𝜎𝑐 𝜉 3 [ ]
𝑟𝑏
(6)
where 𝐶 = [(1 + 𝜆)2 − 2𝜇𝑑 (1 − 𝜇𝑑 )(1 − 𝜆)2 + 𝜆2 + 1]2 and
(7)
10 𝛼 =2+
𝜇𝑑 1 − 𝜇𝑑
(8)
and 𝜇𝑑 1 − 𝜇𝑑
(9)
𝜇𝑑 = 0.8𝜇
(10)
𝜆= and
where: 𝑟𝑏 is the radius of the blast hole (m); 𝜎𝑐 is the uniaxial compressive strength of rock (MPa); 𝜉 is the loading strain rate of surrounding rock; 𝑝𝑐 is the initial pressure of shock wave acting on the blast hole wall (MPa); and 𝜇 is the static Poisson’s ratio. An alternate view of the mechanism of water in water coupled blasting is presented by Huang et al (2011). Huang et al (2011) investigated the effect of pressurised water coupled control blasting in aiding subsequent hydraulic fracturing and opined that when a water coupled blast hole is detonated, the shock wave in the water medium will cause high strains in the blast hole wall. Furthermore, a rapidly expanding gas filled cavity known as a bubble is created by the detonation products. An account of the behaviour of the bubble is given by Reyes (2007). The pressure within the bubble eventually falls below the ambient pressure at which point the bubble is compressed until it reaches a minimum diameter and the process of bubble formation is created again. The bubble follows this oscillating nature and each re-expansion generates shockwaves of diminishing strength and of equal duration of action of the initial detonation bubble. When the stress imposed by the initial and subsequent bubble driven shockwaves on the blast hole wall exceeds the rock dynamic critical fracture strength, the rock fragments and tangential and radial fractures surrounding the blast hole are formed. It should be noted that the shockwaves generated from bubble oscillation have not been detected by experiments investigating the acoustic emissions generated during pressurised water coupled blasting in cement blocks conducted by Huang et al (2011) and Huang et al
11 (2013). The bubble oscillation behaviour is believed to occur in during the detonation of suspended, free hanging underwater explosive charges where the magnitude of the initial shockwave, in this case, is much greater (Reyes, 2007). The attenuation of the shock wave through the rock mass generated by pressurised water coupled blasting was investigated by Shao et al (2017). Shao et al (2017) investigated the formation and propagation of the shock wave in water and found that the stress induced by the shockwave initially decreased rapidly then gradually decreased thereafter. It is noted however that no comparison is made between the shock wave attenuation characteristic observed in unpressurised air or water coupled blasting. It was identified that this investigation into pressurised water blasting by Shao et al (2017) was undertaken to assess the viability of applying the water coupled blasting technique in fracture rock, as similarly undertaken by Yang, Liu and Yu (2017) and Yang and Liu (2017). Figure 2 shows the pressure on the blast hole wall in water coupled blast holes as theoretically described by Shao et al (2017).
Figure 2 - The pressure on the blast hole wall in water coupled blast holes. (after Shao et al, 2017)
Shao, Yang, Mi and Zhao (2017) also studied the reflection and transmission of shock waves at the water and rock interface. They found that with the increase of the noncoupling coefficient, the initial incident shockwave pressure and the initial transmission pressure change acting on the wall of the water in the underwater shock wave are the same, and all show an exponential decay. The transmitted wave pressure generated by the shock wave is greater than the incident wave pressure, and the difference between the two becomes smaller and smaller as the non-coupling coefficient increases.
12 Yang, Liu and Yu (2017) also found that the magnitude of the pressure of the shock wave through the rock mass increased in an approximately linear manner with the increase in the initial density of the coupling medium. By introducing additional pressure on the coupling medium, full coupling is achieved between the explosives and the coupling medium. An increased shock wave pressure in the coupling medium is experienced and the greater fragmentation is seen as a result.
2.3
PHYSICAL MODELLING
Lin and Ma (1992) conducted preliminary experiments in cement mortar blocks using high speed photography to capture fragmentation of bottom water decked blast holes. Lin and Ma found that bottom water decked blast holes as shown in Figure 3 lead to an improvement in explosive energy distribution which resulted in increased uniformity of fragmentation.
Figure 3. Experiment set up by Lin and Ma. 1. Stemming material 2. Explosive column 3. Water (after Lin and Ma, 1992).
Field trials of Lin and Ma’s water decking technique were conducted at the Baoguo iron ore mine in China. Bench heights of 10 m were subdrilled 2 m to create 12 m deep, 200 mm diameter blast holes. 1 m of water was left at the bottom of the blast holes to form the bottom water deck. Over eight bottom water decked production blasts it was found that the technique resulted in uniform energy distribution throughout the length of the
13 blast hole, smaller and more uniform fragmentation, and reduction in explosive consumption compared to dewatered conventionally loaded holes. The most optimum bottom water decking height was determined to be 1.5 m for the 12 m blast holes. Chen (2000) conducted control blasting experiments in cement mortar blocks comparing fragmentation distribution between different water coupling scenarios. In his experiment modelling control blasts, a main explosive column was omitted and only the use of primers was modelled. The primers were placed into water filled blast holes drilled into cement blocks and stemmed with a watertight mud plug. Chen determined that the optimum control blast designs for water coupled charges were when two primers were placed on either end of the water spacer and when a single primer was placed at the top of the water spacer. Diagram (b) and Diagram (c) in Figure 4 the top primer and boundary primer placements for optimum water decked control blast design as determined by Chen.
Figure 4. Experiment set up by Chen. 1. Detonating cord 2. Mud plug 3. Water 4. Primer (after Chen, 2000).
Wu et al (2002) conducted experiments in concrete blocks comparing air and water coupled charges using copper-manganese piezoresistive sensors and an oscilloscope to record shock wave action. It was found that when air is used as the decoupling medium the initial shock wave pressure at the blast hole wall was large and the duration of shockwave was also long, however the peak initial shockwave pressure decreased rapidly as radial distance away from the blast hole increased and the duration of the initial shockwave pulse also decreased as this radial distance increased. In the case of the water coupled charge it was found that the initial shock wave pressure at the blast hole wall was
14 higher, and a greatly increased total shockwave duration was observed. A similar decrease in initial shockwave pressure and duration of shockwave pulse was observed in the water coupled blast hole as radial distance away from the blast hole increased. The higher pressure and longer duration of the shockwave indicated that the water coupled blast holes have greater explosive energy utilisation and more uniform explosive energy distribution. Zong and Luo (2006) continued Wu et al’s (2002) blasting experiments in concrete blocks comparing water coupled and air decoupled charges. By taking strain measurements throughout the cement blocks similar conclusions were reached that water coupled charges demonstrate higher peak stress and a longer duration of effect compared to an air decoupled charge. Zong and Luo’s work support Wu et al’s (2002) conclusion that water coupled charges demonstrate greater explosive energy utilisation and a greater blasting strength compared to air decoupled charges. Zong and Luo (2006) and Wu et al’s (2002) work identified an opportunity to replace air with water coupling in presplit and control blasting. Yang and Liu (2010) identified that while Lin and Ma (1992) studied the fragmentation from water decked blasts, the fragmentation resulting from water coupled blasts had not been studied. Continuing the use of the cement mortar block as test models for rock, air coupled and water coupled blast holes were physically modelled and blasted and the resulting blasted rock sieved and classified by size. It was found in comparing air and water coupled blasts the 80% cumulative passing size P80 and the 50% cumulative passing size P50 were greatly reduced in the water coupled blast case. The P50 decreased by 16% and the P80 decreased by 18% indicating that greater explosive energy utilisation was achieved when the blast hole was coupled with water.
2.4
FIELD STUDIES
The use of water in blast holes in surface mining was first described by Baranov, Gopanyuk and Shvets (1986). An account was given of the middle water decked blasting process used in the open pits in the Ukrainian Soviet Socialist Republic. Baranov, Gopanyuk and Shvets described the application of the water decked blast holes in the lower benches of the open pits where 50-60% of blast holes were flooded with water of column heights ranging from 0.5 to 6 m. It was determined that the optimum water deck height for the site fell between 1.2 to 1.8 m. It was found that when the water decked blasts were compared to conventional blasts without the use of water a greater volume of
15 rock was broken. The water decked blast exhibited greater seismic vibration in the near zone surrounding the blast hole and a reduction of far field ground vibration. Increased intensity and duration of stress waves in the rock mass were seen as a result of the dynamic loading. A 15% reduction in explosives consumption and 21% reduction of oversize material was reported as a result of the application of water decked blasting in the Ukrainian Soviet Socialist Republic. Xie (1999) conducted some bottom water decked blast hole field trials in the Longban quarry in China. Groundwater infiltration into blast holes was commonly seen and the quarry was bounded by large electricity substations 150 m to the east, high voltage lines to the north and south and private property and mining infrastructure to the west. Blasting operations needed to ensure minimisation of flyrock, ground vibration and dust. The bottom water decking technique was applied and ground vibration, flyrock were all deemed acceptable. A passing comment was made that the bottom water decking technique also improved explosive energy utilisation. Song, Liu and Jiang (2000) also conducted field trials of the bottom water decked blast hole technique to the Cangshang gold mine in China. The Cangshang open cut gold mine displayed similar ground water conditions to the Longban quarry investigated by Xie (1999) and experienced groundwater infiltration in 95% of blast holes. The presence of water in the blast holes deteriorated the blasting quality of conventionally loaded blast holes. The bottom water decking technique was applied to 400 blast holes to reduce ground vibrations received by nearby construction facilities and maintain slope stability in the weak rock mass. Analysis of the application of bottom water decking blast holes found a decrease in dust generation, explosive consumption was reduced by 16%, oversize boulders were reduced by 56%, ground vibration was reduced by 10% and shovel dig rate was increased by 20%. Similar dust mitigation effects involving the use of water were also reported by Jin, Liu and Yu (2007) in their investigation into use of a mud stemming mixture of water and stemming found that the optimum water content to combine with stemming was 10% which results in a 91.3% reduction in coal dust generation in the Yang Zhuang Group coal mines in Huaibei, China. Zhang (2000) noted that the density of water resistant emulsion is 1200 t/m3 and denser than water. Zhang investigated the properties of top water decked blasts and performed
16 field studies at the Changchun River mine in China. Zhang gives guidance that when top water decking is used, the parameters for blast design should be: 𝑎 = (1.2~1.3)𝑎0 𝑤 = (1.1~1.2)𝑤0 ℎ = (1.2~1.3)ℎ0 𝑞 = (0.9 ~ 0.95) 𝑞0 where: a is the top water decked blast hole spacing (m); 𝑎0 is the fully loaded blast hole spacing (m); w is the top water decked blast hole burden (m); 𝑤0 is the fully loaded blast hole burden (m); h is the top water decked blast hole depth (m); ℎ0 is the fully loaded blast hole depth (m); q is the top water decked blast hole explosive amount (m); and 𝑞0 is the fully loaded blast hole explosive amount (m). Zhang (2000) also noted that the loading of explosives into water decked holes is a critical task. According to empirical data, Zhang proposed that when the water column of a top decked water blast hole is great than 30 times the height of the explosive column then water can be used in place of stemming material. It was proposed that when the water column is less than this amount additional rock stemming of height not exceeding twenty times the blast hole diameter is to be used. Zhang noted that when the depth of the water column in the blast hole is deep the blast hole cannot be directly filled with stemming as the explosive may float on the water. Additional thin layers of cotton yarn or explosive packaging paper must be loaded between the emulsion and rock stemming to prevent stemming from settling into the emulsion. The top water decked blast hole technique was used over a period of a year at the Changchong River mine and reduced oversize fragmentation by 11%, increased overall fragmentation and eliminated misfires. Cao (2009), and Zhang et al (2010) reported that in developing underground drives it is common to encounter groundwater ingress which is problematic to conventional drill and blast techniques. According to Cao, flooded holes are flushed with high pressure air to dewater the blast holes. Cao (2009) undertook a field study and applied water coupling
17 to the development of dipping roadways in the Chengzhong coal mine in China. Cao (2009) confirmed that the conventional blast hole loading technique could be replaced by bottom water decking the dipping development blast holes to reduce blast hole loading time, reduce dust and fly rock, concentrate resulting blasted muck pile, reduce blasting noise and ground vibration. Bottom water decked blast holes were used over a period of one year and improved overall roadway development rate between 30% and 50%. Bai and Zhang (2017) applied the application of bottom water decked blast holes to the flooded benches of Kyisintaung mine in Myanmar. Bai and Zhang (2017) designed the bottom water decked blast holes with a water column height of 17% of the overall blast hole length as suggested by Lin and Zhang (2017). Bai and Zhang (2017) gave guidance to the loading procedure of a bottom water decked blast hole using a gasbag. The loading procedure for the gasbag is shown in Figure 5. Bai and Zhang (2017) determined that the optimum blast hole design parameters based upon their empirical field tests suggested an inflated gas bag height of 10% of the explosive column height, and a sub drill of 20% of the bench height to accommodate the bottom water deck. An example of the overall blast hole design based on Bai and Zhang’s (2017) guidelines is shown in Figure 6. The application of bottom water decked blast holes in production blasts at the Kyisintaung mine in Myanmar reduced consumption of explosives in blasting between 5.71% and 19.23% and improved production blasting efficiency.
Figure 5. Gas bag operation procedure. 1. Gasbag initial positioning 2. Raising of gasbag 3. Gasbag inflation (after Bai and Zhang, 2017).
18
Figure 6. Bai and Zhang bottom water deck set up. 1. Stemming column 2. Explosive column 3. Gasbag 4. Bottom water deck (after Bai and Zhang, 2017).
The application of water as the coupling medium in blast holes was first investigated in field studies conducted by Zhang, Shang and Gang (2002) at the Bangmoshan mine in China and it was verified that water coupling was effective in presplit blasting. A reduction in the magnitude of peak stress wave pressure, an extension of the duration of the shock wave and greater explosive energy utilisation were found. It was also noted that the water coupling technique required a more complex loading procedure and additionally, the increase in magnitude and uniformity of explosive energy distribution resulted in a diminished effect of hole spacing in presplit blasting. It was suggested that strict control of detonation timing is required for effective water coupled presplit blast design. Zhang et al (2010) undertook a similar field study in an underground mine driving roadways of 4.6 m width and 3.5 m height where water inflow was quantified between 10 to 30 m3/h. Analysis of the blast vibration generated from saturated blast holes found that shock wave intensity velocity increased as water content in the hole increased. It was also found that saturated blast holes demonstrated increased ground vibration magnitude
19 in the near zone and a greater distance of propagation, however, the magnitude of the ground vibration in the far field was much less than the unsaturated blast holes. Experimental studies on concrete blocks were conducted by Yang and Liu (2017) involving the application of a confining pressure to the water coupling medium. Yang and Liu (2017) found that the pressurised water coupling technique successfully solved the problem of water drainage in highly fissured rock and coal to allow the application of water coupled blasting. The pressurised water coupling technique was subsequently applied to control blasts in the Datong coal mine in China by Yang, Liu and Yu (2017) to promote the creation and propagation of fractures to allow gas drainage and relieve strong stresses in the surrounding rock strata. The effectiveness of the confined water blasting technique was overestimated and the resulting zone of fragmentation around the blast hole was 2000% that of a conventionally loaded blast hole. Damage was overserved to a nearby coal rib and it was concluded that the confined water blasting technique required more investigation into optimum loading parameters. Yang, Liu and Yu (2017) concluded that the tensile fracture zone created by a pressurised water coupled blast hole extended to 10 to 15 times the blast hole diameter, and additional hydrofracking could extend the water propagated cracks up to 83 times the blast hole diameter. Mu, Wang and Yu (2014) conducted a vibration comparison between air, water, fine sand and coarse soil used as a coupling medium. Ground vibration was measured at distances ranging from 5 to 65 m away from the blast holes and Fast Fourier transform was used to classify vibrations into four categories. It was concluded from the field tests that the seismic wave energy generated when water was used as a coupling medium was the highest followed by air. The water coupled blast holes demonstrated additionally demonstrated greatest peak vibrations in the 25 Hz to 100 Hz frequency range, the greatest frequency and lowest frequency attenuation.
2.5
NUMERICAL MODELLING OF WATER DECKED AND WATER COUPLED BLASTS
Zong, Yan and Wang (2011) conducted numerical simulation analysis of the stress field generated by different blast hole decking scenarios. Zong, Yan and Wang (2011) used ANSYS LS-DYNA software to compare top air and water decked and water and air coupled blast holes with fully loaded blast holes. The Jones-Wilkins-Lee (JWL) equation
20 of state (EOS) was used to model explosives, however, it is unclear what equations of state were used to model the air, water, rock and stemming material. The numerical analysis found that the magnitude of the stress wave peak was greatest in the fully loaded blast hole followed by the water coupled charge and the minimum peak stress wave was generated by the air coupled charge. However, it is unclear what specific stress Zong, Yan, and Wang (2011) measured. This is in line with Wu, Yang, Huang and Zhong’s (2002) observations of concrete block models and Wang, li, Shi and Fang’s (2008) theoretical findings. It was also found that the magnitude of the stress wave peak decreased as the ratio of coupling to explosive increased and this effect was greater when air was the coupling medium. The simulation also quantified compressive and tensile stress magnitude in the near and far fields surrounding the blast hole. It was determined that compressive stress was greater than tensile stress in the near field and the opposite in the far field. The investigation into axial decoupling or decking found that the magnitude of stresses in the decked zone were much smaller than the charged zone in the near field, and this effect diminished until the magnitude of stresses in the decked and charged zone were approximately equal in the intermediate zone surrounding the blast hole. Figure 7 shows the geometry definition in ANSYS LS-DYNA as defined by Zong, Yan and Wang (2011).
Figure 7. Numerical model geometry definition by Zong, Yan and Wang. 1. Stemming 2. Top water deck 3. Explosive column. (after Zong, Yan and Wang, 2011).
21 Chen et al (2013) used ANSYS LS-DYNA software and compared bottom water decked blast holes with fully loaded blast holes. It was found that the bottom water decked blast holes resulted in greater uniformity in explosive energy distribution and increased the duration of the shock wave. It should be noted that Chen et al (2013) measured the von Mises equivalent stresses in their numerical model. It should be noted that the von Mises yield criterion is most suited in describing the yielding of ductile materials (Mises, 1913) and is not the most suitable yield criterion for brittle rock masses. A field study was conducted on 12.4 m long blast holes comparing fully loaded blast holes with blast holes with a 2 m high bottom water deck. The result of the field study was a reduction in explosive consumption of 12.5%, increase in shovel dig rate by 12.8% and a reduction in oversize fragmentation by 21.1%. Lin and Zhang (2017) also used ANSYS LS-DYNA software to model and compare bottom water decked blast holes with standard fully loaded blast holes. Lin and Zhang similarly also measured the von Mises equivalent stress in their numerical model. Lin and Zhang (2017) found that the bottom water deck acted to raise the centroid of the explosive column and resulted in a more uniform distribution of explosive energy. The duration of the shock wave was also increased and was suggested to reduce oversize fragmentation. It was found that a bottom water deck height around 20% of the overall blast hole length yielded the best results. A field study was conducted in an open pit mine located in Heshun County, Shanxi Province in China where 95% of blast holes were found to experience infiltration of groundwater. The bottom water deck technique was applied and a great reduction in oversize fragmentation, increase in explosive energy utilisation and fragmentation uniformity and a great decrease in dust. The results of the field study are shown in Figure 8.
22
Figure 8. Liu and Zhang field study blast fragmentation photographs. Bottom water decked blast on left. Conventionally loaded blast on right. (after Liu and Zhang, 2017).
Weng, Li, Tao and Wang (2014) investigated the fracture plane formation of water coupled blast holes applied to quarry cutting blasts in ANSYS LS-DYNA software. The effect of different coupling ratios and blast hole spacings were investigated. It was found that a large coupling ratio and small blast hole spacing results in a rough fracture plane and additional undesired fracturing. However, a small coupling ratio of 0.15 and blast hole spacing of around 0.55 m for the granite simulated in the study resulted in the creation of an ideal smooth fracture plane with minimal radial fracturing. Weng, Li, Tao and Wang’s (2014) work investigated the formation of fracture planes and were thus more concerned with material erosion than stress magnitudes and thus did not give guidance on which stresses to measure.
2.6
NUMERICAL MODELLING OF OTHER BLAST PHENOMENA
Abdalla, Hagan, and Chalmers (2013) used ANSYS Autodyn software to model air decked blasts. A Euler mesh was used to model explosives and air while the rock mass and stemming where modelled using a combination of Lagrange and Arbitrary LagrangeEuler (ALE) elements. Park and Jeon (2010) conducted some modelling using ANSYS Autodyn software to investigate the effect of bottom air decked blasts for ground vibration reduction in tunnelling. Park and Jeon (2010) modelled singular air decked blast holes under 3D axial symmetry and 2D plane strain conditions. The 3D axial symmetry model was conducted
23 using only Lagrangian elements. The 2D plane strain model was conducted using 2D the smooth particle hydrodynamics (SPH) method. Park and Jeon’s (2010) modelling was further verified with a field study of air deck induced ground vibration. Upon analysis, the 3D results appeared to be an order of a magnitude off and this was attributed to the use of only Lagrangian elements in the 3D numerical model. Johnson (2014), conducted some laboratory experiments and used ANSYS Explicit Dynamics to model the interaction of stress waves and resulting fragmentation in conventionally loaded blast holes. Johnson used a Euler mesh to model the explosive and a Lagrangian mesh to model the rock mass and stemming material. Unfortunately, Johnson (2014) found that ANSYS Explicit Dynamics was unable to model the interaction between extremely short duration stress wave interaction when only using the Lagrangian mesh to model rock mass and stemming material. It should be noted that in Abdalla, Hagan and Chalmers (2013); Park and Jeon (2010); Johnson (2014); and, Chen et al’s (2013) numerical modelling, the JWL EOS was used to model explosive material and detonation product expansion, the RHT material strength model and P-alpha EOS was used to model the rock mass, the Linear Compaction EOS was used to model the stemming material, the Ideal Gas Law EOS was used to model air and the Rankine-Hugoniot Shock EOS was used to model water.
2.7
COMPARISON WITH AIR DECKING
The mechanism by which explosive energy is transferred to the rock mass in water decked blast holes is similar to that of air decked blasts. In his state of the art review of air decked blasting, Jhanwar (2011) noted that an air deck also allows explosive energy to act in pulses rather than instantly as in the case of a conventionally, undecked blast. The expansion of gaseous detonation productions and the multiple interacting shock wave pulses of an air decked blast increases the duration of shock loading and explosive energy utilisation. Effective application of air decked blasts in presplit blasts to control over break and ground vibration have been found by Mel’Nikov and Marchenko (1971), Mel’Nikov et al (1979) and fragmentation, oversize and explosive consumption were also found to be reduced by 50-60%, 50-90% and 10-30% respectively. Bussey and Borg (1988) similarly used air deck blasting to control high wall stability and reduced explosive costs by 50%. Chiappetta and Memmele’s (1987) field trials of air decked blast holes in
24 production blasts in a coal mine also resulted in cost reductions up to 46%. Generally, air deck design is carried out by rule of thumb (Jhanwar, 2011) and air deck lengths range between 10-30% of the original charge length. Mid column air decks are overall preferred over bottom and top air decks. Top air decks are used to increase fragmentation in the stemming region and bottom air decks are generally not suggested for use (Jhanwar, 2011).
2.8
CONCLUSIONS
A review of the current literature surrounding the use of water as a medium in coupling and decking blast holes has been examined. It was found that English literature surrounding the use of water in this way is very limited. Instead, it was found that much research and application of the technique was limited to China and Chinese literature. Chinese laboratory studies, numerical models and field applications have demonstrated positive effects when water is used either as decks in the blasting column during production blasts or as a surrounding coupling medium in presplit blasts. Chinese laboratory studies, numerical models and field applications have found that use of water decked blast holes in production blasts and use of water coupled blast holes in presplit blasts effectively reduce fly rock, dust, ground vibration, increase explosive energy utilisation to reduce oversize boulders, and improve the uniformity of fragmentation. Comparisons have been made with air coupled blast holes and it was found that the use of explosive energy utilisation was greater when water was used instead. The low compressibility, higher density relative to air and high specific heat of water has been attributed to increased utilisation of explosive energy in water coupled and water decked blast holes. The mechanism of action in water coupled blast holes that are given by Zhang and Huang (2013) describes when the explosion shock wave through the water reaches the blast hole wall and generates a centripetal rarefaction wave, the water body reaches a quasi-static stress state which extends the duration of the shock wave. The application of the quasi-static stress and fluctuating centripetal and centrifugal rarefaction waves on the blast hole wall causes the blast hole wall to fluctuate, deform and crack. Radial fracturing is then generated as the dynamic strength of the rock is exceeded by the tangential tensional stress caused the shock wave passing through the rock mass.
25 It can, therefore, be concluded that where water in blast holes is faced as an issue by drill and blast, water decking of production blasts or water coupling of presplit blasts is a viable method to reduce explosive consumption and meet environmental and engineering requirements regarding dust and noise emission, ground vibration and slope stability. A review of Chinese literature has established that while numerical models have been established investigating and comparing water coupling rations, attempts to numerically model the application of water decks in blast holes have been by large specific models tailored to specific site conditions. This thesis has identified that there is a gap in knowledge in comparing different water decking parameters such as water deck length, water deck location, comparisons between air decking, and applicability to varying rock mass strengths and explosive types. This thesis was undertaken to investigate and compare the effects of water deck location, length, comparison with air decking, rock mass strength and explosive type on the near field damage and far field stress behaviour using numerical modelling.
26
3 RISK ASSESSMENT AND MANAGEMENT PLAN 3.1
HAZARD IDENTIFICATION AND RISK ASSESSMENT
Risk of hazards negatively impacting on the research project were mitigated through hazard identification and the establishment of appropriate courses of action. To ensure that the thesis was safely completed on time and relevant to industry, the hazards associated with the disruption of this objective were identified. The relevant controls that managed these hazards were also identified. A hazard and operability study (HAZOP) was conducted to identify the function failures and failure modes associated with safely completing the thesis that will contribute to the mining industry on time. A failure mode, effects and criticality analysis (FMECA) was combined with a risk priority number (RPN) analysis quantifying the frequency, severity and risk of the failure modes. Appropriate controls were then identified to manage the failure modes. A Pareto analysis of the failure modes was then conducted which determined the critical failure modes. The potential functional failures of this project were identified as: •
the thesis is not completed;
•
the project is party completed;
•
the thesis is delayed past deadline;
•
the thesis is complete but irrelevant to industry;
•
the thesis is complete but of sub-par standard; and
•
people are injured due to actions related to the completion of the thesis.
The results of the HAZOP study are shown in Table 1.
27 Table 1. HAZOP failure mode identification. Functional failure
Failure Modes Complete loss of critical data
The thesis is not completed (Project Hazard)
File corruption No submission Lost time owing to injury
The project is partially completed (Project Hazard)
Lost time owing to illness Insufficient data File corruption Scope of thesis too large Poor time management
The thesis is delayed past deadline (Project Hazard)
Lost time owing to injury Lost time owing to illness Poor time management Scope of thesis too large
The thesis is complete but irrelevant to industry
No new contribution to the thesis topic
(Project Hazard)
Irrelevant scope of works Inappropriate and irrelevant sources
The thesis is complete but of sub-par
Lost time owing to injury
standard
Lost time owing to illness
(Project Hazard)
Incorrect scope of works Irrelevant scope of works Inappropriate and irrelevant sources Inadequate proof reading Poor time management Marking criteria not adhered to
Injury to people as a result of actions related
An electrical shock from faulty electrical
to the completion of the thesis
equipment
(Physical Hazard)
Eye strain from using a computer monitor Repetitive strain injury from using a computer
28 The risk rating matrix is shown in Table 2. Table 2. Risk rating matrix. Rating
Likelihood
Severity
1
Rare (less than once per year)
2
Unlikely (once per year)
3
Moderate (a few times per year)
4
Likely (ten times per week)
Insignificant
(no
consequence
on
delivery and quality of thesis) Minor (minor delay and a minor decrease in quality of thesis) Moderate
(moderate
delay
and
a
moderate decrease in quality of thesis) Major (major delay and a major decrease in quality of thesis) Severe (unable to safely complete the
5
Highly likely (once per week)
project on time and to satisfactory standard)
The results of FMECA analysis applied with risk rankings and risk priority numbers is shown in Table 3. Table 3. FMECA.
Failure mode
Complete
Control
loss
of
critical data
weekly Back up data and files
File corruption
weekly Constant
No submission
Lost injury
time
Back up data and files
owing
supervisor
consultation to
Be safety conscious at all times
Likelihood
Severity
of failure
of failure
mode
mode
3
Risk
RPN
Rating
(%)
4
12
11
3
4
12
11
1
5
5
4
1
3
3
3
29
Lost
time
owing
to
illness
Insufficient data Scope of thesis too large
Observe good hygiene practise throughout the
1
3
3
3
2
3
6
4
3
3
9
8
2
4
8
7
2
5
10
9
1
5
5
4
2
4
8
7
2
2
4
4
2
4
8
7
1
4
4
4
1
5
5
4
Take regular breaks
3
2
6
5
Take regular breaks
3
2
6
5
semester Adhere
to
project
schedule Constant
supervisor
consultation Insufficient scope of
Constant
works
consultation
Poor time management No new contribution to the thesis topic
supervisor
Adhere to the project schedule Constant
supervisor
consultation Irrelevant scope of works
Constant
supervisor
consultation Inappropriate and irrelevant sources
Constant
supervisor
consultation Inadequate proofreading
Constant
supervisor
consultation Marking criteria not adhered to
Constant
review
of
marking criteria An electrical shock from faulty electrical equipment
Inspect
equipment
before use Eye strain from using a computer monitor
Repetitive strain injury from using a computer
30 RPN was graphed alongside cumulative RPN and the most critical hazards that have the capacity to disrupt the project have been defined by the 80% cumulative RPN value. The RPN and cumulative RPN graph is shown in Figure 9.
RPN vs Cumulative RPN values 12.0%
100.0% 90.0% 80.0% 70.0% 60.0% 50.0% 40.0% 30.0% 20.0% 10.0% 0.0%
10.0% 8.0% 6.0% 4.0% 2.0% 0.0%
RPN
Cumulative RPN
Figure 9. Pareto analysis of research project failure modes.
The most critical thesis hazards that disrupt the object of ensuring the safe completion of the thesis that is completed on time and relevant to industry were found to be: •
complete loss of critical data;
•
file corruption;
•
poor time management;
•
scope of thesis is too large;
•
insufficient scope of works;
•
irrelevant scope of works;
•
inadequate proofreading;
•
insufficient data;
•
eye strain from using computer monitor;
31 •
repetitive strain injury from using a computer; and
•
no submission.
The most critical controls that prevented these failure modes as determined by the FMECA were: •
back up data and files weekly and as required;
•
constant supervisor consultation;
•
adhere to the project schedule; and
•
take breaks when working.
3.2
CONTINGENCY PLAN
In the event that it was unfeasible to proceed with the project outlined above in 1.1 Project Objectives, it was determined that the project may be amended by implementing the contingency plan. The contingency plan objective was outlined as follows: “To conduct an in-depth review and summarise the Chinese investigations, discussions, experiments and field trials in relation to the safe and economic application of water decking and water coupling in blast holes.” The review that was to be conducted in accordance with the contingency plan was to be written in English. While a review in English would have not contributed anything new to the state of knowledge of the application of water in blast holes to Chinese scholars, it was expected that an English review of the untranslated Chinese literature would positively contribute to the English-speaking mining industry and increase understanding on the application of this technique.
3.3
ANALYSIS OF THE RISK ASSESSMENT AND MANAGEMENT PLAN
In completing the thesis, an analysis of the effectiveness of the risk assessment and management plan was undertaken. It was found that following critical controls indeed prevented the critical thesis hazards such as complete loss of critical data, file corruption, poor time management and scope of the thesis. However, additional hazards were found
32 to negatively affect the thesis. These additional hazards that were not originally identified have been listed alongside their estimated respective resultant loss of time: •
Running out of storage space on university computers (3 days);
•
Delays in increasing storage space on university computers (3 days);
•
Corruption of numerical model files whilst using the numerical modelling program (4 days); and
•
Inability of original numerical modelling program ANSYS Explicit Dynamics to model stress wave oscillation interaction behaviour inside rock mass (21 days).
The main thesis disruption was the inability of the original numerical modelling program to model stress wave oscillation behaviour inside the rock mass. This was only encountered after the construction and running of the original model and was confirmed to be a software limitation by Johnson (2014) and her work on fragmentation analysis in the dynamic stress wave collision regions during blasting. Considering that Johnson’s (2014) work was initially unrelated to the thesis topic, it was impossible to identify this hazard when the risk assessment and management plan was undertaken. Similarly, the corruption of the numerical model files during use was also unforeseen as the ANSYS software used was a stable release and this hazard was unable to be identified during the initial risk assessment. The delays associated with university computer storage should have been anticipated due to the large file sizes of the numerical models and this was a lack of foresight that resulted in lost time.
33
4 METHODOLOGY 4.1
NUMERICAL MODELLING
Numerical models can be used to calculate and predict deformation, strains and stresses that are difficult to measure physically. Numerical modelling software suites such as ANSYS, Abaqus FEA and LS-DYNA are able to model the high pressure short duration shock loading of explosives and are commonly used to model blast related phenomena. A number of solvers and methods are available in these software packages including: •
Lagrange and Euler finite element solvers for computational structural dynamics (FE);
•
Finite volume solvers for computational fluid dynamics (CFD);
•
Smoothed-particle hydrodynamics (SPH) for simulating the mechanics of continuum media in high velocity, large deformation and fragmentation problems;
•
Coupled multiphysics solvers that combine the above FE, CFD and SPH methods.
Finite element analysis is a continuum based approach and is the preferred over discontinuum based software environments in modelling stress distribution (Oh, 2018). ANSYS, Abaqus FEA and LS-DYNA software packages are multi-physics simulation environments where finite element analysis may be conducted through explicit time integration. ANSYS Autodyn software has been used by Abdalla, Hagan and Chalmers (2013) to model the stress distribution in rock mass surround air decked blast holes and the combined ANSYS/LS-DYNA software suite was used by Chen, Wu, Li and Chang (2013) in modelling the stress distributions in rock mass surrounding water decked blast holes. It should be noted that whilst finite element methods dominate historical modelling of blast phenomena, SPH methods are rapidly progressing and present an alternative to FE modelling as shown by Zhang, Yang and Yao’s (2012) SPH modelling of underwater blasts and Park and Jeon’s (2010) SPH modelling of conventionally loaded blasts. Due to student software licensing limitations, the ANSYS Student software suites were found to be least restrictive in terms of element and node limitations and provided access
34 to FE, CFD, SPH and coupled multiphysics solvers and thus modelling was undertaken using ANSYS software.
4.2
MATERIAL MODELS
The ANSYS Student software suite provides access to a materials database which includes material models for common materials. These material models characterise the material’s volumetric and deviatoric response to phenomena such as pressure, strain, thermal softening, strain hardening, damage, failure and directional material properties. The material volumetric response is characterised by an equation of state (EOS) and the material deviatoric response is characterised by a strength model. The JWL EOS was used for detonation product expansion and was previously also used by Abdalla, Hagan, and Chalmers (2013) to model air decked blasts, Chen, Wu, Li and Chang (2013) to model water decked blasts and Park and Jeon (2010) to model conventional blasts. The rock mass in the thesis numerical model is modelled using the RHT material strength model and P-alpha EOS. This decision was based on the multiple successful models published by Abdalla, Hagan, and Chalmers (2013), Park and Jeon (2010), and Johnson (2014), among others. Similarly, stemming material is modelled by the Linear Compaction EOS. Comparisons between water and air decked blasts were done using the Ideal Gas Law EOS to model air and the Rankine-Hugoniot Shock EOS to model water. The Ideal Gas Law EOS and Linear Shock EOS material models for air and water were the default material models for ANSYS explicit analyses and deemed suitable for use in modelling the project. These material models were used in all the numerical models constructed in ANSYS Explicit Dynamics and ANSYS Autodyn. 4.2.1
Jones-Wilkins-Lee (JWL) Equation of State
The Jones-Wilkins-Lee Equation of State describes the detonation product expansion of high energy explosive materials. The JWL EOS is shown in Equation 11 and Equation 12: 𝑃 = 𝐴 (1 −
𝜔 𝜔 𝜔𝑒0 ) 𝑒 −𝑅2𝑉 + 𝐵 (1 − ) 𝑒 −𝑅2𝑉 + 𝑅1 𝑉 𝑅2 𝑉 𝑉
(11)
35 and, 𝑉=
𝜌𝑒 𝜌𝑑
(12)
where: A, B, 𝑅1 , 𝑅2 , 𝜔 are reference explosive constants; 𝜌𝑒 is the density of the explosive (kg/m3); 𝜌𝑑 is the density of the detonation products (kg/m3); and 𝑒0 is the chemical energy of the explosive (kJ/m3). The default material parameters for JWL EOS ANFO in the ANSYS material database are shown in Table 4. Table 4. Default ANSYS ANFO material parameters. Parameter
Value
Density
931 kg/m3
JWL Parameter A
49.46 GPa
JWL Parameter B
1.891 GPa
JWL Parameter R1
3.907
JWL Parameter R2
1.118
JWL Parameter W
0.33333
C-J Detonation Velocity
4160 m/s
CJ-Energy / unit mass
2.668 MJ/kg
CJ - Pressure
5.15 GPa
Orica Fortis is a bulk emulsion blend that is used in wet hole blasting conditions (Orica, 2009). The Orica Fortis product is available in Australasia and was modelled in the thesis as the water-resistant explosive. As such, the default ANSYS ANFO material was modified to reflect the Orica Fortis product. The Orica Fortis bulk emulsion blend is available from densities ranging from 1100 kg/m3 to 1250 kg/m3. The relative weight
36 strength of the 1100 kg/m3 and 1250 kg/m3 density products were obtained from the Orica Fortis technical data sheet (Orica, 2009) and the corresponding C-J Detonation Velocity or velocity of detonation (VOD) was obtained by empirical Equation 13.
𝑅𝑊𝑆 = (
𝜌𝑒 𝑉𝑂𝐷𝑒2 1 )3 𝜌𝑜 𝑉𝑂𝐷𝑜2
(13)
where: 𝜌𝑒 is the density of the explosive (kg/m3); 𝜌𝑜 is the density of ANFO (kg/m3); 𝑉𝑂𝐷𝑒 is the velocity of detonation of the explosive; and 𝑉𝑂𝐷𝑜 is the velocity of detonation of the ANFO. Thus, the parameters of the 1100 kg/m3 and 1250 kg/m3 density Orica Fortis products that were modelled are shown in Table 5. Table 5. Orica Fortis JWL Parameters. Parameter
Orica Fortis 1100 kg/m3
Orica Fortis 1250 kg/m3
Density (kg/m3)
1100
1250
JWL Parameter A (GPa)
49.46
49.46
JWL Parameter B (GPa)
1.891
1.891
JWL Parameter R1
3.907
3.907
JWL Parameter R2
1.118
1.118
JWL Parameter W
0.33333
0.33333
C-J Detonation Velocity (m/s)
3656
3974
CJ-Energy / unit mass (MJ/kg)
2.668
2.668
CJ – Pressure (GPa)
5.15
5.15
37 4.2.2
Riedel- Hiermaier-Thoma (RHT) Concrete Model
The Ridel-Hiermaier-Thoma concrete model is an advanced plasticity model for brittle materials. The RHT model was initially developed to model the dynamic loading of concrete, however, can be also used to model other brittle materials such as rock and ceramic. The RHT model is a combined shear damage and plasticity model and phenomena such as pressure hardening, strain hardening, strain rate hardening, strain softening and porous collapse damage are modelled. Two default RHT concrete materials are provided in the ANSYS materials database for explicit analyses. These are the CONC-35MPA and CONC-140MPA materials representing two concrete materials with compressive strengths of 35 MPa and 140 MPa respectively. There is precedence and prevalence of prior numerical modelling of blast phenomena such as Park and Jeon (2010) and Abdalla, Hagan and Chalmers (2013) using the RHT CONC-35MPA concrete material. It was then decided that the RHT CONC35MPA and CONC-140MPA concrete materials would be suitable for modelling a lower strength and higher strength rock mass for the thesis numerical model. The RHT parameters for the CONC-35MPA and CONC-140MPA materials are shown in Table 6. Table 6. Rockmass RHT Model Parameters. Parameter
CONC-35MPA
CONC-140MPA
Shear modulus, G (GPa)
16.7
22.06
Compressive strength, fc (MPa)
35
140
Tensile strength, ft / fc
0.1
0.1
Shear strength, fs / fc
0.18
0.18
Intact failure surface constant, A
1.6
1.6
Intact failure surface exponent, N
0.61
0.61
Tens/Comp. meridian ratio, Q
0.6805
0.6805
Brittle to ductile transition,
0.0105
0.0105
38
4.2.3
G (elastic)/G (elastic-plastic)
2
2
Elastic strength / ft
0.7
0.7
Elastic strength / fc
0.53
0.53
Residual strength constant, B
1.6
1.6
Residual strength exponent, M
0.61
0.61
Compressive strain rate exponent, α
0.032
0.032
Compressive strain rate exponent, δ
0.036
0.036
Max. fracture strength ratio
1 x1020
1 x1020
P-alpha Equation of State
The P-alpha EOS is a commonly used material model to model shock compaction. The P-alpha EOS assumes that the internal energy of a porous material is equal to the internal energy of a solid material at the same pressure and temperature. The P-alpha EOS describes the material’s elastic behaviour up to a point and plastic behaviour beyond that point. The P-alpha EOS is given by Equation 14: 𝛼 = 1 + (𝛼𝑝 − 1)(
𝑝1 − 𝑝 2 ) 𝑝𝑠 − 𝑝𝑒
(14)
where: 𝛼𝑝 is the initial compaction pressure, 𝑝𝑠 is the solid compaction pressure; and 𝑝𝑒 is the initial compaction pressure. The P-alpha parameters for the CONC-35MPA and CONC-140MPA materials are shown in Table 7.
39 Table 7. Rockmass P-alpha EOS Parameters.
4.2.4
Parameter
CONC-35MPA
CONC140MPA
Reference density (g/cm3)
2750
2.75
Porous density (g/cm3)
2.314
2.520
Porous sound speed (m/s)
2.92 x103
3.242 x103
Initial compaction pressure (kPa)
2.33 x104
9.33 x104
Solid compaction pressure (kPa)
6.00 x106
6.00 x106
Compaction exponent
3.00
3.00
Bulk modulus, A1 (kPa)
3.527 x107
3.527 x107
Parameter, A2 (kPa)
3.958 x107
3.958 x107
Parameter, A3(kPa)
9.04 x106
9.04 x106
Parameter, B0 (kPa)
1.22
1.22
Parameter, B1
1.22
1.22
Parameter, T1 (kPa)
3.527 x107
3.527 x107
Parameter, T2 (kPa)
0.00
0.0
Reference temperature (K)
300
300
Specific heat (J/kgK)
654
654
Thermal conductivity (J/mKs)
0.00
0.00
Compaction Equation of State
The Compaction EOS using linear unloading describes the response of porous materials to pressure. A plastic compaction path is defined as a linear function of pressure versus density and the elastic unloading and reloading path is defined as a linear function of sound speed and density. The Compaction EOS bulk stiffness equation is given by Equation 15. 𝐾 = 𝜌0 𝑐 2
(15)
40 where: K is the bulk stiffness; 𝜌0 is the density at sound speed; and c is the sound speed. The Sand material using Compaction EOS in the ANSYS material database was used by Abdalla, Hagan and Chalmers (2013) to model stemming material in air decked blasts. It was deemed appropriate that the Sand material could also be used to model the stemming material in water decked blasts. The parameters of the Sand material using Compaction EOS in ANSYS are shown in Table 8. Table 8. Stemming Compaction EOS Parameters.
4.2.5
Parameter
Value
Reference density (kg/m3)
2641
Density 1 (kg/m3)
1674
Soundspeed 1 (m/s)
265.12
Density 2 (kg/m3)
1740
Soundspeed 2 (m/s)
852.1
Ideal Gas Equation of State
The Ideal Gas EOS relates the density, pressure, and temperature of gases. It allows good approximation of the behaviour of many gases under many conditions. The Ideal Gas EOS is given by the Equation 16: 𝑃𝑉 = 𝑛𝑅𝑇 where: P is the pressure of the gas; V is the volume of the gas; n is the number of moles of the gas; R is the ideal gas constant; and T is the absolute temperature of the gas.
(16)
41 The default ANSYS material properties for air using the Ideal Gas EOS are given in Table 9. Table 9. Air Idea Gas EOS Parameters. Parameter
Value
Gamma
1.4
Adiabatic Constant
0
Pressure shift (kPa)
0
Reference Temperature (K)
288.200012
Specific Heat (J/kgK)
717.599976
Thermal Conductivity (J/mKs)
0.000000
It should be noted that for modelling purposes this air material must be given additional internal energy to simulate atmospheric pressure. The additionally internal energy added is 0.2068 MJ and is set as an initial condition in the ANSYS models. 4.2.6
Rankine-Hugoniot Shock Equation of State
The Rankine-Hugoniot Shock EOS describes the relationship between density, pressure, energy, particle velocity and shock velocity of a material. The Linear Shock EOS describes, in particular, a linear relationship during dynamic loading of solids and many liquids between shock velocity and particle velocity. This relationship is given by Equation 17. 𝑈 = 𝑐0 + 𝑠𝑢𝑝 where: U is shock velocity; 𝑐0 is a constant; 𝑠 is the specific gradient of the relationship; and 𝑢𝑝 is particle velocity.
(17)
42 The Shock EOS may be additionally established into a Mie-Gruneisen EOS form which relates pressure and volume of the material at a given temperature. This Mie-Gruneisen form is given by the Equations 18, 19 and 20. 𝑝 = 𝑝𝐻 + Γ𝜌(𝑒 − 𝑒𝐻 )
(18)
𝑝0 𝑐02 𝜇(1 + 𝜇) [1 − (𝑠 − 1)𝜇]2
(19)
1 𝑝𝐻 𝜇 ( ) 2 𝑝0 1 + 𝜇
(20)
𝑝𝐻 =
𝑒𝐻 =
The Linear Shock EOS parameters for the water material are shown in Table 10. Table 10. Water Linear Shock EOS Parameters. Parameter
Value
Reference Density (kg/m3)
998
Gruneisen coefficient
0
Parameter C1 (m/s)
1647
Parameter S1
1.921
4.3
ANSYS 3D EXPLICIT DYNAMICS MODEL
4.3.1
ANSYS Explicit Dynamics Background
ANSYS Explicit Dynamics (Explicit Dynamics) is a numerical analysis system available in the ANSYS Workbench. The Explicit Dynamics numerical analysis system is an explicit finite element solver and can simulate the response of structures to loadings. Explicit Dynamics is able to process numerical models that involve short duration, severe loading, large deformation and material failure. Explicit Dynamics is able to capture the physics of highly nonlinear, transient and dynamic forces. It was initially decided to proceed with ANSYS Explicit Dynamics to model the thesis.
43 4.3.2
3D Base Model Geometry
A 3D base model scenario was created in ANSYS Explicit Dynamics which modelled a top water decked blast hole. Variations of this model were created in order to investigate the effects of deck location, deck length, deck medium, rockmass strength and explosive density on the principal stress behaviour at the blast hole wall and spacing distance. The blast hole was 10 m in total length. 3 m of stemming was at the top of the blasting column, followed by 1 m of the decking material and subsequently 5 m of explosive at the bottom. Based on the 10 m blast hole length, the hole diameter was modelled at 150 mm as per the DynoNobel Explosive Engineers’ Guide (2017) shown in Figure 10.
Figure 10. DynoNobel Blasting Guidelines (after DynoNobel, 2017).
As per the 10 m long, 150 mm diameter blast hole, the spacing distance was determined to be 6.9 m. The model width and length were set to 20 m by 20 m respectively. This was to examine the stress behaviour slightly beyond the spacing distance. For similar reasons, the rock mass model height was set at 15 m to investigate stress behaviour in the toe region. Figure 11 shows the base case model geometry.
44
Figure 11. Base case model geometry.
4.3.3
Solver, Meshing and Initial conditions
ANSYS Explicit Dynamics is limited to a Lagrangian solver only. A virtual Euler solver utilising the Lagrangian solver to display results is also available. The rock mass and stemming materials were assigned to the Lagrangian solver. The explosive and decking materials were assigned to the virtual Euler solver. The interactions between the Lagrangian and Euler elements were automatically calculated by ANSYS Explicit Dynamics. Meshing was automatically done by ANSYS Explicit Dynamics. Mesh type was assigned as hex-dominant for the explicit solver and grading was applied to increase mesh density closer to the blast hole. This is shown in Figure 12. Unfortunately, a denser mesh at the rockmass surfaces was unable to be achieved due to ANSYS Explicit Dynamics academic licence meshing node and element limitations.
45 Impedance boundaries were assigned to all free faces of the rockmass excluding the top surface to transmit stress waves. This was done to model a continuous, infinitely long and wide rock mass and reduce potential stress wave reflection interactions. The detonation point was placed 500 mm above the bottom of the explosive material and set to detonate when the model was initialised. This was done to simulate realistic primer and booster placement.
Figure 12. Base case model meshing.
4.3.4
Model Investigations
Multiple scenarios were established in ANSYS Explicit Dynamics in order to investigate and compare the near field damage at the blast hole wall and stress action at the spacing distance between decking length, decking medium, decking location, rock mass strength and explosive type. Table 11 summarises the dimensions of the constructed scenarios. The model was analysed to 6 ms where it was believed a balance between stress history and run time would be achieved.
46 Table 11. ANSYS Explicit Dynamics Numerical Model Scenarios. Rockmass
Explosive
strength
density
(MPa)
(g/cm3)
1
35
1.10
6
2
35
1.10
1
6
3
35
1.10
Water
1
6
3
35
1.10
Water
Water
1
6
3
35
1.10
6
Top
Water
1
6
3
140
1.10
7
Top
Water
1
6
3
35
1.25
8
None
None
0
7
4
35
1.10
Deck
Decking
location
medium
1
Top
Water
2
Top
3
Deck
Explosive
Stemming
length (m)
length (m)
3
6
Water
2
Top
Air
4
Bottom
5
ID
4.3.5
length (m)
Initial Results
The initial ANSYS Explicit Dynamic model scenarios were run one by one. Each scenario took approximately 6 hours to solve. Figure 13 shows the maximum principal stress time history comparison between a conventional blast without any air or water decks, a 1 m top water decked blast hole and a 1 m top air decked blast hole at the (0.075,0.075,1) gauge point on the blast hole wall. Note that the gauge coordinate is in the form (x,y,z) and units are meters.
47
Maximum Principal Stress (0.075,0.075,1) 100000000 0 0.00E+00 -1E+08
1.00E-03
2.00E-03
3.00E-03
4.00E-03
5.00E-03
6.00E-03
7.00E-03
-2E+08 -3E+08 -4E+08 -5E+08 -6E+08 Maximum Principle (Pa) No Deck
Maximum Principle (Pa) 1m Top Water Deck
Maximum Principle (Pa) 1m Top Air Deck
Figure 13. Maximum Principal Stress at the (0.075,0.075,1) coordinate.
Figure 14 shows the maximum principal stress time history comparison between a conventional blast without any air or water decks, a 1 m top water decked blast hole and a 1 m top air decked blast hole at the (0.075,0.075,6) gauge point on the blast hole wall.
Maximum Principal Stress (0.075,0.075,6) 50000000 0 0.00E+00 -50000000
1.00E-03
2.00E-03
3.00E-03
4.00E-03
5.00E-03
6.00E-03
7.00E-03
-1E+08 -1.5E+08 -2E+08 -2.5E+08 -3E+08 Maximum Principle (Pa) No Deck
Maximum Principle (Pa) 1m Top Water Deck
Maximum Principle (Pa) 1m Top Air Deck
Figure 14. Maximum Principal Stress at the (0.075,0.075,6) coordinate.
Figure 15 shows the maximum principal stress time history comparison between a conventional blast without any air or water decks, a 1 m top water decked blast hole and a 1 m top air decked blast hole at the (0.075,0.075,8) gauge point on the blast hole wall.
48
Maximum Principal Stress (0.075,0.075,8) 7000000 6000000 5000000 4000000 3000000 2000000 1000000 0 0.00E+00 -1000000
1.00E-03
2.00E-03
3.00E-03
4.00E-03
5.00E-03
6.00E-03
7.00E-03
-2000000 Maximum Principle (Pa) No Deck
Maximum Principle (Pa) 1m Top Water Deck
Maximum Principle (Pa) 1m Top Air Deck
Figure 15. Maximum Principal Stress at the (0.075,0.075,8) coordinate.
Figure 16 shows the maximum principal stress time history comparison between a conventional blast without any air or water decks, a 1 m top water decked blast hole and a 1 m top air decked blast hole at the (3,3,3.5) gauge point 3 m radially away from the centre of the blast hole.
Maximum Principal Stress (3,3,3.5) 4000000 3500000 3000000 2500000 2000000 1500000 1000000 500000 0 0.00E+00 -500000
1.00E-03
2.00E-03
3.00E-03
4.00E-03
5.00E-03
6.00E-03
7.00E-03
-1000000 Maximum Principle (Pa) No Deck
Maximum Principle (Pa) 1m Top Water Deck
Maximum Principle (Pa) 1m Top Air Deck
Figure 16. Maximum Principal Stress at the (3,3,3.5) coordinate.
49 Figure 17 shows the maximum principal stress time history comparison between a conventional blast without any air or water decks, a 1 m top water decked blast hole and a 1 m top air decked blast hole at the (3,3,6) gauge point in the rock mass
Maximum Principal Stress (3,3,6) 3000000 2500000 2000000
1500000 1000000 500000 0 0.00E+00 -500000
1.00E-03
2.00E-03
3.00E-03
Maximum Principle (Pa) No Deck
4.00E-03
5.00E-03
6.00E-03
7.00E-03
Maximum Principle (Pa) 1m Top Water Deck
Maximum Principle (Pa) 1m Top Air Deck
Figure 17. Maximum Principal Stress at the (3,3,6) coordinate.
4.3.6
Discussion of Initial Results
In order to validate the model, the maximum principal stress histories at various points along the blast hole for the no deck, 1 m top water and 1 m top air deck scenarios were compared. This was done as previous numerical modelling of water decked blasts in literature were unclear in methodology and presented a poor basis for model validation. Instead, the 1 m top air decked maximum principal stress histories were compared to other top air deck maximum principal stress histories. Air decking shares similarity with water decking as air decking, according to Jhanwar (2011) and Lu and Hustrulid (2003), also generates stress wave oscillation within the blast hole. Thus, there was a good basis to assume if the numerical model could demonstrate the stress wave oscillation behaviour for the air deck then it could for the water deck as well. Figures 18 to 23 show the maximum principal stress histories obtained by Lu and Hustrulid’s (2003) 2D numerical modelling of top air decks.
50
Figure 18. Air Deck Sections. (after Lu and Hustrulid, 2003)
Figure 19. Maximum Principal Stress at Section A-A. (after Lu and Hustrulid, 2003)
Figure 20. Maximum Principal Stress at Section B-B. (after Lu and Hustrulid, 2003)
51
Figure 21. Maximum Principal Stress at Section C-C. (after Lu and Hustrulid, 2003)
Figure 22. Maximum Principal Stress at Section D-D. (after Lu and Hustrulid, 2003)
Figure 23 - Maximum Principal Stress at Section E-E. (after Lu and Hustrulid, 2003)
It should be noted that while the Lu and Hustrulid (2003) air deck numerical model is a 2D simplified plane strain one, the maximum principal stress histories clearly show a minimum of two loading and unloading cycles throughout the blast hole. Consulting Figures 13 to 15, the maximum principal stress histories for the 1 m top air deck modelled in ANSYS Explicit Dynamics do not show more than one loading and unloading cycle anywhere through the length of the blast hole. It was thought that the lack of multiple loading and unloading cycles were attributed to the duration of the stress histories. While
52 Lu and Hustrulid (2003) recorded maximum stress history up to 20 ms, the ANSYS Explicit Dynamics numerical model was only run to 6 ms due to the excessive run time of 6 hours per model scenario. It was hypothesised that the lack of additional loading and unloading cycles was attributed to the short duration of analysis. Thus, a secondary model set to run to 10 ms was constructed in an attempt to validate the appropriate application of ANSYS Explicit Dynamics. 4.3.7
Secondary Results
The secondary ANSYS Explicit Dynamics model was identical to the initial 3D ANSYS Explicit Dynamics model and only differed in extended model analysis time. The secondary model was set to run up to 10 ms of analysis, 4 ms longer than the initial 6 ms analysis of the initial model. The secondary model was constructed in order to observe multiple air deck stress wave loading and unloading cycles as recorded by Lu and Hustrulid (2003) and validate the subsequent modelling of water decks. Due to the length of model computation time being around 10 hrs per run it was prudent that the model validation through air deck comparisons was conducted before proceeding with water decked models. Figure 24 shows the maximum and minimum principal stress time history comparison between a conventional blast without any air or water decks, 1 m top air decked blast hole at the (0.075,0.075,0.5) gauge point on the blast hole wall. Note that the gauge coordinate is in the form (x,y,z) and the units are meters.
53
Maximum and Minimum Principal Stress (0.075,0.075,0.5) 100000000 0 0.00E+00 -1E+08
2.00E-03
4.00E-03
6.00E-03
8.00E-03
1.00E-02
1.20E-02
-2E+08 -3E+08 -4E+08 -5E+08 -6E+08 -7E+08 -8E+08 (Maximum Principal) [Pa] No Deck
(Maximum Principal) [Pa] 1m Top Air Deck
(Minimum Principal) [Pa] No Deck
(Minimum Principal) [Pa] 1m Top Air Deck
Figure 24. Principle Stress History at the (0.075,0.075,0.5) coordinate.
Figure 25 shows the maximum and minimum principal stress time history comparison between a conventional blast without any air or water decks, 1 m top air decked blast hole at the (0.075,0.075,3.5) gauge point on the blast hole wall.
Maximum and Minimum Principal Stress (0.075,0.075,3.5) 100000000 0 0.00E+00 -1E+08 -2E+08 -3E+08 -4E+08 -5E+08 -6E+08 -7E+08 -8E+08 -9E+08
2.00E-03
4.00E-03
6.00E-03
8.00E-03
1.00E-02
1.20E-02
(Maximum Principal) [Pa] No Deck
(Maximum Principal) [Pa] 1m Top Air Deck
(Minimum Principal) [Pa] No Deck
(Minimum Principal) [Pa] 1m Top Air Deck
Figure 25. Principle Stress History at the (0.075,0.075,3.5) coordinate.
Figure 26 shows the maximum and minimum principal stress time history comparison between a conventional blast without any air or water decks, 1 m top air decked blast hole at the (0.075,0.075,6.5) gauge point on the blast hole wall.
54
Maximum and Minimum Principal Stress (0.075,0.075,6.5) 20000000 0 0.00E+00 -20000000
2.00E-03
4.00E-03
6.00E-03
8.00E-03
1.00E-02
1.20E-02
-40000000 -60000000
-80000000 -1E+08 -1.2E+08 (Maximum Principal) [Pa] No Deck
(Maximum Principal) [Pa] 1m Top Air Deck
(Minimum Principal) [Pa] No Deck
(Minimum Principal) [Pa] 1m Top Air Deck
Figure 26. Principle Stress History at the (0.075,0.075,6.5) coordinate.
Figure 27 shows the maximum and minimum principal stress time history comparison between a conventional blast without any air or water decks, 1 m top air decked blast hole at the (0.075,0.075,7.5) gauge point on the blast hole wall.
Maximum and Minimum Principal Stress (0.075,0.075,7.5) 20000000 0 0.00E+00 -20000000
2.00E-03
4.00E-03
6.00E-03
8.00E-03
1.00E-02
1.20E-02
-40000000
-60000000 -80000000 -1E+08 -1.2E+08
-1.4E+08 (Maximum Principal) [Pa] No Deck
(Maximum Principal) [Pa] 1m Top Air Deck
(Minimum Principal) [Pa] No Deck
(Minimum Principal) [Pa] 1m Top Air Deck
Figure 27. Principle Stress History at the (0.075,0.075,7.5) coordinate.
Figure 28 shows the maximum and minimum principal stress time history comparison between a conventional blast without any air or water decks, 1 m top air decked blast hole at the (0.075,0.075,8.5) gauge point on the blast hole wall.
55
Maximum and Minimum Principal Stress (0.075,0.075,8.5) 10000000 5000000 0 0.00E+00 -5000000
2.00E-03
4.00E-03
6.00E-03
8.00E-03
1.00E-02
1.20E-02
-10000000 -15000000 -20000000 (Maximum Principal) [Pa] No Deck
(Maximum Principal) [Pa] 1m Top Air Deck
(Minimum Principal) [Pa] No Deck
(Minimum Principal) [Pa] 1m Top Air Deck
Figure 28. Principle Stress History at the (0.075,0.075,8.5) coordinate.
Figure 29 shows the maximum and minimum principal stress time history comparison between a conventional blast without any air or water decks, 1 m top air decked blast hole at the (0.075,0.075,9.5) gauge point on the blast hole wall.
Maximum and Minimum Principal Stress (0.075,0.075,9.5) 10000000 5000000 0 0.00E+00
2.00E-03
4.00E-03
6.00E-03
8.00E-03
1.00E-02
1.20E-02
-5000000 -10000000 -15000000 (Maximum Principal) [Pa] No Deck
(Maximum Principal) [Pa] 1m Top Air Deck
(Minimum Principal) [Pa] No Deck
(Minimum Principal) [Pa] 1m Top Air Deck
Figure 29. Principle Stress History at the (0.075,0.075,9.5) coordinate.
Figure 30 shows the maximum and minimum principal stress time history comparison between a conventional blast without any air or water decks, 1 m top air decked blast hole at the (6,6,0.5) gauge point in the rock mass.
56
Maximum and Minimum Principal Stress (6,6,0.5) 4000000 2000000 0 0.00E+00 -2000000
2.00E-03
4.00E-03
6.00E-03
8.00E-03
1.00E-02
1.20E-02
-4000000 -6000000 (Maximum Principal) [Pa] No Deck
(Maximum Principal) [Pa] 1m Top Air Deck
(Minimum Principal) [Pa] No Deck
(Minimum Principal) [Pa] 1m Top Air Deck
Figure 30. Principle Stress History at the (6,6,0.5) coordinate.
4.3.8
Discussion of Secondary Results
The secondary model set to run to 10 ms was constructed in an attempt to validate the appropriate application of ANSYS Explicit Dynamics to water decks. The maximum and minimum principal stress histories of the 1 m top air decked blast hole and conventionally loaded blast hole were recorded along the blast hole column at the blast hole wall. At the 0.5 m and 3.5 m blast hole wall gauge points, there were no appreciable differences between the air decked blast hole and the conventionally loaded blast hole. At these gauge points, the medium inside the blast hole column is ANFO. While there is an initial maximum and minimum principal stress loading and unloading caused by the detonation of ANFO, this is not repeated again throughout the 10 ms of analysis. A significantly different maximum and minimum principal stress behaviour is seen at the 6.5 m and 7.5 m blast hole wall gauge points. At the 6.5 m height, the centre of the air deck is reached for the air decked blast hole. The material inside the blast hole at this location is still ANFO for the conventionally loaded blast hole. At the 7.5 m height, the material inside the blast hole is the sand stemming material for both blast holes. At both 6.5 m and 7.5 m gauge points the air decked blast hole experiences an initial principal stress loading and unloading cycle and then experiences another loading cycle. At this stage, this loading cycle continues and does not unload throughout the duration of the analysis. This conflicts with Lu and Hustrulid’s (2003) observations of Principal stress
57 behaviour in a top air decked blast hole. While Lu and Hustrulid observe multiple loading and unloading cycles, the duration of the stress loading is brief and this behaviour has not been replicated by the ANSYS Explicit Dynamics model. An examination of the 8.5 m gauge point reveals multiple principal stress loading and unloading cycles in the 1 m top air decked blast hole, however, the duration of the third minimum principal stress loading cycle appears to take more than 2 ms. Again, while Lu and Hustrulid’s (2003) investigations emphasise the multiple stress loading and unloading cycles in air decks, they note that the duration of the loading cycle is very brief. This has not been replicated here. At the 9.5 m gauge point on the blast hole wall and the (6,6,0.5) gauge point 6 m radially away from the blast hole centre, principal stress behaviour of the air decked and conventionally decked blast holes are very similar. Only one stress loading and unloading cycle are seen in the upper stemming region where Lu and Hustrulid recorded two. Whilst Lu and Hustrulid did not record stress behaviour away from the blast hole wall, there appears to be two maximum principal stress loading and unloading cycles in at the (6,6,0.5) gauge point. Unfortunately, this behaviour appears in both the air decked and conventionally loaded blast holes and the stress behaviour cannot be linked to the results of the air deck. The secondary ANSYS Explicit Dynamics model was unable to replicate the Lu and Hustrulid (2003) air deck stress behaviour and in fact was unable to show a difference between an undecked and air decked blast hole at certain locations. It was deemed that since the secondary ANSYS Autodyn model could not be validated when air decks were modelled then it would also be unable to provide valid water decked models. It was hypothesised that the inability of the secondary ANSYS Explicit Dynamics model to model air decked blasts was due to the inability of the software to model high speed stress wave interaction. This is supported by Johnson’s (2014) modelling of blast wave interaction in ANSYS Explicit Dynamics. Johnson (2014) found that that as two shock fronts inside a modelled rock mass met, no reflection between the two wavefronts were generated. Thus, a software limitation was discovered and it was determined that ANSYS Explicit Dynamics would be unable to model further water decked blast holes.
58
4.4
ANSYS 2D AUTODYN MODEL
4.4.1
ANSYS Autodyn Background
ANSYS Autodyn is a top level explicit solver available from ANSYS. Autodyn can simulate the response of materials to short duration severe loadings from impacts, high pressure and explosions. ANSYS Autodyn was used by Abdalla, Hagan and Chalmers (2013) to model blast phenomena in air decked blast holes and Neetu and Kiran (2014) to validate pressure prediction of underwater explosions. ANSYS Autodyn has additional solvers such as SPH, FE, Euler and multi solver coupler that ANSYS Explicit Dynamics does not and is able to model the repeated stress wave loading behaviour associated with air and water decks. ANSYS Autodyn was not initially used as the program was greatly more complicated and less user-friendly than ANSYS Explicit Dynamics. However, due to ANSYS Explicit Dynamics’ limitation in modelling high speed stress wave reflection, it was necessary to proceed with the top level ANSYS Autodyn software. 4.4.2
Base Model Geometry
A 2D plane strain model was created in ANSYS Autodyn to simulate rock mass with one blast hole. The 2D model was chosen to be modelled over a 3D model into order to increase the resolution of the model mesh and obtain more accurate results, as well as to significantly decrease computation time compared to a 3D model of similar geometry and mesh resolution. Most importantly, however, the 2D model was created to meet the mesh element limitations whilst using the ANSYS student license. The blast hole diameter was 15 mm, and the overall length of the blast hole was 1000 mm. This was in accordance with general DynoNobel (2017) rules of thumb shown in Figure 10 regarding blast hole length and blast hole diameter. According to the DynoNobel (2017) Explosive Engineer’s Handbook guidelines, the burden for a blast hole should be 25 to 40 times the blast hole diameter. Guidance for spacing is at 1.15 times burden distance to generate an equilateral blasting pattern. At these recommendations, the extent rock mass was modelled to be a 2000 mm by 2000 mm square. Stemming height in accordance with DynoNobel (2017) guidelines for a conventional, non-decked blast hole of 20 times blast hole diameter, was set to 300 mm.
59 Figure 31 shows the geometry for the base model without any decks. A vertical plane symmetry was imposed along the horizontal axis running through the centreline of the blast hole to reduce solver computations and decrease run time.
Figure 31. ANSYS Autodyn base model geometry.
4.4.3
Solver Zoning and Solver Controls
The Arbitrary Lagrangian-Eulerian (ALE) finite element method is available in ANSYS Autodyn. The ALE solver is a finite element solver where the computation is neither Eulerian nor Lagrangian. Instead, the ALE solver provides automatic rezoning of distorted grids. The rock mass and stemming were modelled using the ALE solver. This ALE/Eulerian approach to match blasting column materials and the rock mass was similarly used by Abdalla, Hagan and Chalmers (2013) to model blast phenomena in air decked blast holes and Neetu and Kiran (2014) to validate underwater explosion pressure prediction. The Multi-material Euler solver was utilised to model the explosive and decking medium materials. The Lagrangian solver zones were meshed by 10 mm squares. The Euler solver zone was meshed by 5mm by 5mm squares. This ensured sufficient result resolution. The interaction between the ALE and Euler solvers was automatically calculated in ANSYS Autodyn. The length of the analysis was set to 4 ms. This duration was chosen as it was expected that ANSYS Autodyn would be able to correctly model blast phenomena. A lengthier analysis time was not needed for further validation and would increase computation time.
60 4.4.4
Initial Conditions
The detonation point was placed 50 mm above the bottom of the explosive material and set to detonate when the model was initialised. This was done to simulate a realistic primer and booster placement. 4.4.5
Model Investigations
Multiple scenarios were established in ANSYS Autodyn in order to investigate and compare the near field damage at the blast hole wall and stress action at the spacing distance between decking length, decking medium, decking location, rock mass strength and explosive type. These multiple scenarios are variations of the conventionally loaded base model without any decks. Table 12 summarises the dimensions of these constructed scenarios. Table 12. ANSYS Autodyn Numerical Model Scenarios. Deck
Explosive
Stemming
Rockmass
Explosive
length
length
length
strength
density
(mm)
(mm)
(mm)
(MPa)
(g/cm3)
None
0
700
300
35
1.10
Bottom
Water
100
600
300
35
1.10
3
Bottom
Water
100
600
300
140
1.10
4
Bottom
Air
100
600
300
140
1.10
5
Bottom
Water
200
500
300
35
1.10
6
Top
Water
100
600
300
35
1.10
7
Top
Air
100
600
300
35
1.10
8
Top
Water
200
500
300
35
1.10
9
Top
Water
100
600
300
35
1.25
Deck
Decking
location
medium
1
None
2
ID
61 4.4.6
Gauge Locations
Numerous fixed gauges were set up to monitor the principal stress behaviour throughout the rock mass. The gauge locations were created at a distance of 50 mm away from the symmetry plane to measure stress history close to the blast hole wall and additionally 700 mm away at the suggested DynoNobel (2017) spacing distance based on blast hole diameter to measure stress history at the spacing distance. The gauges extend throughout the length of the rockmass and were spaced 250 mm apart. Figure 32 shows the location of the gauge points in the rockmass.
Figure 32. ANSYS Autodyn model gauge locations.
Table 13 shows the coordinates of the gauge points. The coordinate origin was at the exact centre of the model, adjacent the bottom of the blast hole. Table 13. ANSYS Autodyn Numerical Model Gauge Point Locations. Gauge ID
X value
Y value
Gauge ID
X value
Y Value
1
-500
50
8
-500
700
62 2
-250
50
9
-250
700
3
0
50
10
0
700
4
250
50
11
250
700
5
500
50
12
500
700
6
750
50
13
750
700
7
1000
50
14
1000
700
4.4.7
Model Validation
The use of ANSYS Autodyn, the selected material models and the selected solvers has been used by Abdalla, Hagan and Chalmers (2013) to investigate air decked blasts. Similarly, ANSYS Autodyn, the selected material models and the selected solvers have been used by Neetu and Kiran (2014) to validate pressure prediction of underwater explosions and Huang et al (2011) to model underwater one-dimensional blasts. Abdalla, Hagan and Chalmers (2013) work was presented at the Tenth International Symposium on Rock Fragmentation by Blasting, Neetu and Kiran’s (2014) article was published in the International Journal of Science and Research and Huang et al’s (2011) work was published in the Journal of Energetic Materials. These are respected conferences and journals. The numerical modelling undertaken for the thesis in ANSYS Autodyn was an extension of the valid methodology presented by Abdalla, Hagan and Chalmers (2013), Neetu and Kiran (2014) and Huang et al (2011). Thus, there was a reason assumption that the numerical modelling undertaken for the thesis in ANSYS Autodyn was valid.
63
5 ANSYS AUTODYN MODEL RESULTS The RHT damage output was seen as a greater overall indicator of blast hole performance than gauge point stress history in the near field. This is because damage output represents overall blasting performance rather than performance at a specific point. Unfortunately, relevant damage output is restricted to the near field. Additionally, since stress behaviour is a function of time, it is not appropriate to characterise stress behaviour at any particular point in time. Gauges placed at the spacing distance measuring stress behaviour are thus used to record farther blast behaviour.
5.1
MODEL NEAR FIELD DAMAGE
The RHT material strength model encompasses a damage model. The damage output is a value from 0 to 1 where 0 is completely undamaged material, and 1 is completely pulverised material. Damage output values greater than 0.7 denote broken material and is also known as rubblized material (Johnson, 2014). The damage output is a good indicator of the crushed zone around the blast hole and qualitatively serves to show regions of fines generation and near field explosive energy distribution. Figures 33 to 41 below show the damage output of the ANSYS Autodyn modelled scenarios as described in Section 4.4.5 Model Investigations.
64
Figure 33. Model 1 Damage.
Figure 34. Model 2 Damage.
65
Figure 35. Model 3 Damage.
Figure 36. Model 4 Damage.
66
Figure 37. Model 5 Damage.
Figure 38. Model 6 Damage.
67
Figure 39. Model 7 Damage.
Figure 40. Model 8 Damage.
68
Figure 41. Model 9 Damage.
5.2
MAXIMUM PRINCIPAL STRESS HISTORY
Maximum principal stress histories have been recorded up to 4 ms after detonation at the spacing distance at gauges 9, gauge 11 and gauge 13 corresponding with the upper, middle and lower regions in the rock mass. These stress histories are shown in Figures 42 to 50.
69
Figure 42. Scenario 1 Maximum Principal Stress History.
Figure 43. Scenario 2 Maximum Principal Stress History.
70
Figure 44. Scenario 3 Maximum Principal Stress History.
Figure 45. Scenario 4 Maximum Principal Stress History.
71
Figure 46. Scenario 5 Maximum Principal Stress History.
Figure 47. Scenario 6 Maximum Principal Stress History.
72
Figure 48. Scenario 7 Maximum Principal Stress History.
Figure 49. Scenario 8 Maximum Principal Stress History.
73
Figure 50. Scenario 9 Maximum Principal Stress History.
5.3
MINIMUM PRINCIPAL STRESS HISTORY
Minimum principal stress histories have been recorded up to 4 ms after detonation at the spacing distance at gauges 9, gauge 11 and gauge 13 corresponding with the upper, middle and lower regions in the rock mass. These stress histories are shown in Figures 51 to 59.
74
Figure 51. Scenario 1 Minimum Principal Stress History.
Figure 52. Scenario 2 Minimum Principal Stress History.
75
Figure 53. Scenario 3 Minimum Principal Stress History.
Figure 54. Scenario 4 Minimum Principal Stress History.
76
Figure 55. Scenario 5 Minimum Principal Stress History.
Figure 56. Scenario 6 Minimum Principal Stress History.
77
Figure 57. Scenario 7 Minimum Principal Stress History.
Figure 58. Scenario 8 Minimum Principal Stress History.
78
Figure 59. Scenario 9 Minimum Principal Stress History.
5.4
MAXIMUM PRINCIPAL STRESS COMPARISON AT THE SPACING DISTANCE
Maximum principal stress histories at gauge 9, gauge 11 and gauge 13 have been compared with each other in order to investigate the effect of deck location, deck medium, deck length, rockmass strength and explosive type. They have been examined at the spacing distance where any distance further away the stress behaviour of the blast hole would realistically be dominated by the neighbouring blast hole. In order not to overcrowd the principal stress comparison graphs, these comparisons have been conducted on a pairwise basis. The list of maximum principal stress comparisons is shown in Table 14. Detailed information on each scenario can be found in 4.4.5 Model Investigations.
79 Table 14. Maximum Principal Stress Comparisons. Comparison
Scenario
ID
1 ID
1
2
1 m bottom water deck
1
2
2
1 m bottom water deck
3
3
2
1 m bottom water deck
4
4
2
1 m bottom water deck
5
2 m bottom water deck
5
2
1 m bottom water deck
6
1 m top water deck
6
6
1 m top water deck
1
No deck
7
6
1 m top water deck
7
1 m top air deck
8
6
1 m top water deck
8
2 m top water deck
9
6
1 m top water deck
9
5.4.1
Scenario 1
Scenario 2 ID
Scenario 2
No deck 1
m
bottom
water
deck,
CONC140MPA 1
m
bottom
air
deck,
CONC140MPA
1 m top water deck, 1.25 g/cm 3 explosive
Maximum Principal Stress History Comparisons at Gauge 9
Figures 60 to 68 show the maximum principal stress comparisons described in Table 14 at gauge 9. Gauge 9 is located in the outer toe region, at the spacing distance.
80
Figure 60. Gauge 9 Maximum Principal Stress Comparison 1.
Figure 61. Gauge 9 Maximum Principal Stress Comparison 2.
81
Figure 62. Gauge 9 Maximum Principal Stress Comparison 3.
Figure 63. Gauge 9 Maximum Principal Stress Comparison 4.
82
Figure 64. Gauge 9 Maximum Principal Stress Comparison 5.
Figure 65. Gauge 9 Maximum Principal Stress Comparison 6.
83
Figure 66. Gauge 9 Maximum Principal Stress Comparison 7.
Figure 67. Gauge 9 Maximum Principal Stress Comparison 8.
84
Figure 68. Gauge 9 Maximum Principal Stress Comparison 9.
5.4.2
Maximum Principal Stress History Comparisons at Gauge 11
Figures 69 to 77 show the maximum principal stress comparisons described in Table 14 at gauge 11. Gauge 11 is located in the outer middle region, at the spacing distance.
85
Figure 69. Gauge 11 Maximum Principal Stress Comparison 1.
Figure 70. Gauge 11 Maximum Principal Stress Comparison 2.
86
Figure 71. Gauge 11 Maximum Principal Stress Comparison 3.
Figure 72. Gauge 11 Maximum Principal Stress Comparison 4.
87
Figure 73. Gauge 11 Maximum Principal Stress Comparison 5.
Figure 74. Gauge 11 Maximum Principal Stress Comparison 6.
88
Figure 75. Gauge 11 Maximum Principal Stress Comparison 7.
Figure 76. Gauge 11 Maximum Principal Stress Comparison 8.
89
Figure 77. Gauge 11 Maximum Principal Stress Comparison 9.
5.4.3
Maximum Principal Stress History Comparisons at Gauge 13
Figures 78 to 86 show the maximum principal stress comparisons described in Table 14 at gauge 13. Gauge 13 is located in the outer upper region, at the spacing distance.
90
Figure 78. Gauge 13 Maximum Principal Stress Comparison 1.
Figure 79. Gauge 13 Maximum Principal Stress Comparison 2.
91
Figure 80. Gauge 13 Maximum Principal Stress Comparison 3.
Figure 81. Gauge 13 Maximum Principal Stress Comparison 4.
92
Figure 82. Gauge 13 Maximum Principal Stress Comparison 5.
Figure 83. Gauge 13 Maximum Principal Stress Comparison 6.
93
Figure 84. Gauge 13 Maximum Principal Stress Comparison 7.
Figure 85. Gauge 13 Maximum Principal Stress Comparison 8.
94
Figure 86. Gauge 13 Maximum Principal Stress Comparison 9.
95
6 ANSYS AUTODYN MODEL DISCUSSION Damage output represents overall blasting performance rather than performance at a specific point. Thus, the damage output was seen as a greater overall indicator of blast hole performance than gauge point stress history in the near field. Damage is not discussed at the spacing distance as damage output is restricted to the near field. Instead, stress history has been recorded at the spacing distance. Gauges placed at the spacing distance measuring stress behaviour were used to record farther blast behaviour. At the spacing distance, gauge 9 was selected to represent the bottom region. Gauge 11 represents the middle region. Gauge 13 corresponds with the upper region.
6.1
EFFECT OF PARAMETER ON NEAR FIELD DAMAGE
6.1.1
Deck Location
An examination of the top 1 m water decked blast hole and bottom 1 m water decked blast hole damage output showed a similar amount of damaged material over 0.7 damage. This suggests that both top and bottom decks generate a similar quantity of fines. Additionally, the bottom water deck was found to have the largest damaged region in the stemming area suggesting its use in improving fragmentation in the stemming region. The top water deck displayed a sausage like damaged region and suggests its application for the greatest homogeneity in explosive energy distribution. 6.1.2
Deck Length
A reduction in overall damage extent and damage over 0.7 was seen when the bottom water deck length was increased from 1 m to 2 m. This is in line with less explosive use leading to less explosive energy. The increase in bottom water deck length also raised the centroid of the explosive in the blasting column and thus also acted to slightly increase explosive energy utilisation in the stemming region. Thus, a bottom water deck length 10% of overall blast hole length should be used when maximum near field fragmentation is required. An increase in the top water deck length from 1 m to 2 m found that the stemming region was less damaged and received less explosive energy. There was little appreciable difference in damage between the 1 m and 2 m deck lengths elsewhere throughout the
96 rock mass. Thus, a top water deck length 20% of overall blast hole length should be used to minimise explosive use and near field fragmentation. 6.1.3
Comparison with Air Deck
In his literature review of air decking, Jhanwar (2011) reported that the top deck was the most optimum location of air deck placement for improved breakage in the air deck and stemming regions. When the top air deck was compared to the top water deck it was found that the top water deck showed much greater damage and breakage in the stemming region. 6.1.4
Comparison with No Deck
When the top 1 m and bottom 1 m water decked blasts were compared with the undecked, conventionally loaded blast it was found that both decking scenarios displayed an equal reduction in the damaged material over 0.7. This suggests the application of both top and water decks in reducing fines at the blast hole wall. Additionally, the bottom water deck was found to have the largest damaged region in the stemming area suggesting its use in improving fragmentation in the stemming region. The top water deck displayed a sausage like damaged region and suggests its application for the greatest homogeneity in explosive energy distribution. 6.1.5
Rockmass Strength
When the rockmass strength was increased from 35 MPa to 140 MPa a reduction in the damaged area over 0.7 and an increase in overall damaged area was seen. This suggests that the higher rock strength is more resistant to pulverisation and thus leads to fewer fines generation and greater rock breakage in the near field around the blast hole wall. 6.1.6
Explosive Density
No appreciable difference in damage output was seen when the bulk explosive product density was increased from 1.10 g/cm3 to 1.25g/cm3. This suggests that modifying explosive density has no effect on near field damage.
97
6.2
EFFECT OF PARAMETER ON STRESS ACTION AT THE SPACING DISTANCE
The examination of maximum principal stress history at gauges 9, 11, and 13 focuses on the magnitude of the peak stress in each of the stress loading and unloading cycles and the duration of each loading cycle. 6.2.1
Deck Location
No appreciable difference was seen in maximum principal stress behaviour in the upper region at the spacing distance when the 1 m top water deck blast hole was compared to the 1 m bottom water decked blast hole. The two scenarios displayed very similar stress patterns in the middle region where the bottom water decked blast hole was able to achieve 5% greater peak stress magnitudes in the stress loading and unloading cycles. The bottom region saw greatly different stress behaviour by the two decking scenarios. The bottom water decked blast hole was consistently able to achieve 30% greater peak stress magnitudes in the stress loading and unloading cycles. 6.2.2
Deck Length
No appreciable difference in any region at the spacing distance was observed when the top water deck was increased in length from 1 m to 2 m. No appreciable difference in the top and middle regions at the spacing distance was observed when the bottom water deck was increased in length from 1 m to 2 m. At the bottom region at the spacing distance, the 1 m water deck showed a 20% reduction in peak stress magnitude, however, the stress loading and unloading cycles were significantly more homogenous and consistent. 6.2.3
Comparison with Air Deck
No appreciable difference in any region at the spacing distance was observed when the top 1 m water decked blast hole was compared with the top 1 m air decked blast hole. 6.2.4
Comparison with No Deck
No appreciable difference in any region at the spacing distance was observed when the top 1 m water decked blast hole was compared with the conventionally loaded, undecked blast hole. No appreciable difference was also seen in the top region when the bottom 1m
98 water decked blast hole was compared with the conventionally loaded, undecked blast hole. In the middle region, the water decked blast appeared to follow the same stress behaviour as the undecked blast but experienced slightly greater peak stress magnitudes during loading cycles. At the lower region, the water decked blast appeared to follow the same stress behaviour as the undecked blast but experienced 10% smaller peak stress magnitudes during loading cycles. 6.2.5
Rockmass Strength
The peak stresses in the loading and unloading cycles were found to be 50% greater when the rockmass strength was increased from 35 MPa to 140 MPa. In conjunction with the reduction in near field damage, it is theorised that less explosive energy is utilised in fines generation and thus the propagating stress wave experiences reduced attenuation. 6.2.6
Explosive Density
No appreciable difference in maximum principal stress behaviour at the upper, middle and lower regions at the spacing distance were seen when the bulk explosive product density was increased from 1.10 g/cm3 to 1.25g/cm3. This suggests that changing explosive density has no effect on far field rock breakage.
6.3
CONCLUSIONS
Absolute water deck conclusions have not been made here as the performance of water decked blasts are subject to site-specific conditions such as rock porosity, density, strength, weathering to name a few. However, in saying so qualitative conclusions have been made below from the damage output and stress behaviour histories: •
Variance in explosive density has no effect on rock damage;
•
Increase in rock strength reduces near field crushing and fines generation and lead to greater peak stress magnitudes at the spacing distance;
•
Top air decking results in less near field damage and breakage in the stemming region and remains air the preferred decking medium for sensitive presplit blasting.
99 •
The top water deck only appears useful to distribute near field explosive energy distribution evenly. The top water deck should be applied to presplit blasting applications rather than production blasts.
•
Lengthier top water decks slightly reduce explosive energy in the stemming region in the near field with no appreciable differences at the spacing distance.
•
The bottom water deck raises the centroid of the blasting column and increases explosive energy in the toe and stemming regions in the near field to improve rock breakage. At the spacing distance, a slight reduction in stress magnitude is seen in the bottom region.
•
Lengthier bottom water decks reduce fines generation and improve rock damage in the stemming region in the near field. Lengthier bottom water decks also increase peak stress magnitude in the stress loading and unloading cycles at the spacing distance.
100
7 CONCLUSIONS A discussion with mining engineers at the South32 Ltd’s GEMCO mine found little knowledge on the effect of water in blast holes. An examination into the literature surrounding the use of water in blast holes found that there is a lack of literature in the western developed world regarding the use of water as a decking and coupling medium in blast holes. Current literature in this subject has been investigated in the literature review and it was determined that while numerous investigations surrounding the subject have taken place in China, the resulting literature has not been translated to English and there is a lack of knowledge on the practice of using water as a decking and coupling medium in blast holes outside China. Chinese laboratory studies, numerical models and field applications have demonstrated positive effects when water is used either as decks in the blasting column during production blasts or as a surrounding coupling medium in presplit blasts. Chinese laboratory studies, numerical models and field applications have found that use of water decked blast holes in production blasts and use of water coupled blast holes in presplit blasts effectively reduce fly rock, dust, ground vibration, increase explosive energy utilisation to reduce oversize boulders, and improve the uniformity of fragmentation. Comparisons have been made with air coupled blast holes and it was found that the use of explosive energy utilisation was greater when water was used instead. The low compressibility, higher density relative to air and high specific heat of water has been attributed to increased utilisation of explosive energy in water coupled and water decked blast holes. A review of Chinese literature has established that while numerical models have been established investigating and comparing water coupling rations, attempts to numerically model the application of water decks in blast holes have been by large specific models tailored to specific site conditions. Initial numerical models were set up using ANSYS Explicit Dynamics software however solver and element node licencing limitations generated invalid results. A later numerical model was set up using ANSYS Autodyn software and multiple blast hole loading scenarios were constructed to investigate the effects of water deck location, length, comparison with air decking, rock mass strength and explosive type on the near field damage and far field stress behaviour.
101 The following conclusions were made from the damage output and stress behaviour histories: •
Variance in explosive density has no effect on rock damage;
•
Increase in rock strength reduces near field crushing and fines generation and lead to greater peak stress magnitudes at the spacing distance;
•
Top air decking results in less near field damage and breakage in the stemming region and air remains the preferred decking medium for sensitive presplit blasting.
•
The top water deck only appears useful to distribute near field explosive energy distribution evenly. The top water deck should be applied to presplit blasting applications rather than production blasts.
•
Lengthier top water decks slightly reduce explosive energy in the stemming region in the near field with no appreciable differences at the spacing distance.
•
The bottom water deck raises the centroid of the blasting column and increases explosive energy in the toe and stemming regions in the near field to improve rock breakage. At the spacing distance, a slight reduction in stress magnitude is seen in the bottom region.
•
Lengthier bottom water decks reduce fines generation and improve rock damage in the stemming region in the near field and increase peak stress magnitude in the stress loading and unloading cycles at the spacing distance.
A risk assessment and management plan were implemented in order to manage the risks of file loss, thesis delays, and overall scope of the project. Multiple file backups were created to prevent file loss and corruption and constant consultation with thesis supervisors ensure management of the scope of the thesis. Unforeseen delays were experienced due to university storage limitations, poor choice of initial numerical modelling software and undiagnosed corruption of numerical model files. These unforeseen delays contributed to an increase in workload, however, the thesis was able to successfully meet thesis deadlines. A contingency plan was developed to ensure backup thesis objectives however the successful completion of the main thesis objectives ensured that the contingency plan was not implemented.
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8 RECOMMENDATIONS The numerical modelling of the water decked blasts undertaken by this thesis has not considered the hydraulic fracturing effect of the water on crack propagation and potential improvement in fragmentation. This was because it was deemed that the investigation into crack propagation was beyond the scope of this thesis. Further research into water decked blast holes which appropriately model the hydraulic fracturing effect of water may result in different conclusions to those drawn here. Additionally, it is recommended that further research into the simulation of stress behaviour of non-isolated blast holes in realistic blast layout be conducted. Such simulations would better characterise overall rock mass behaviour during blasting. The modelling of water decked blasts under such conditions would involve the investigation into multiple parameters such as blast timing, spacing distance, burden distance, bench height and was not conducted in this thesis due to time constraints. The numerical model in the thesis was not undertaken in 3D due to licencing limitations limiting the application of appropriate element sizes. Most critically, it is recommended that further research is conducted using full licenses of the chosen numerical modelling software. This is so that solver element and node limitations are removed and the numerical model is able to provide appropriate model resolution and model geometry.
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