A dissertation submitted in partial fulfillment of the ...

University of Nevada, Reno

Empirical Ground Support Recommendations and Weak Rock Mass Classification for Underground Gold Mines in Nevada, USA

A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Geo-Engineering

By Sean N. Warren Dr. Raj Kallu Dissertation Advisor May, 2016

© Copyright by Sean N. Warren 2016 All Rights Reserved

THE GRADUATE SCHOOL

We recommend that the dissertation Prepared under our supervision by

Sean N. Warren Entitled Empirical Ground Support Recommendations and Weak Rock Mass Classification for Underground Gold Mines in Nevada, USA

Be accepted in partial fulfillment of the Requirement for the degree of DOCTOR OF PHILOSOPHY

Raj R. Kallu, PhD., Advisor Jaak J.K. Daemen, PhD., Committee Member Ronald J. Breitmeyer, PhD., Committee Member Ramin Motamed, PhD., Committee Member Stephanie A. McAfee, PhD., Graduate School Representative David W. Zeh, PhD., Dean, Graduate School

May, 2016

i

Abstract Ground conditions at underground gold mines in Nevada range from good to extremely poor and implementing the most appropriate ground support can be challenging.

Existing empirical

ground support design methods were developed predominantly from experience in tunneling or more competent ground, making them less applicable to underground gold mining in Nevada. This research presents empirically derived support guidelines from experience at 5 underground gold mines in Nevada, including: discussions with engineers and miners, review of ground control management plans and consulting documents, and roughly 400 ground control casestudies. Support design recommendations are based on the Weak-Rock Mass Rating (W-RMR) which is a modified Rock Mass Rating (RMR) classification incorporating the Unified Soil Classification System (USCS) for very weak rock masses. Ground support recommendations include rock bolt pattern support pressure, rock bolt length category, excavation surface support, and excavation strategy.

ii

Acknowledgments Research presented in this dissertation was funded by the National Institute of Occupational Safety and Health (NIOSH) under research contract 200-2011-39965. Management from five underground gold mines in Nevada are thanked for providing access and logistical support for data collection. Engineers and mine crews at these mines are thanked for providing valuable information, transportation, time, and input during the data collection process and development of final conclusions presented in this dissertation. Fellow graduate Evan Keffeler assisted in data collection, and Rahul Thareja, Chase Barnard, and Jairo Usma provided time and feedback leading to the final results in this dissertation. Dr. Jaak Daemen provided valuable insight and advice during data analysis, and statistical analysis of field data was assisted by Dr. Ronald Breitmeyer and Dr. James Carr of the University of Nevada, Reno. My advisor Dr. Kallu is thanked for securing funding for this very interesting project, and for his support and guidance over the last four years. Finally, Harmony Ann Warren provided financial support and encouragement throughout this research and dissertation writing process, and is graciously thanked for her efforts.

iii

Table of Contents 1

2

3

Introduction .............................................................................................................................. 1 1.1

Background ...................................................................................................................... 1

1.2

Goals of Dissertation........................................................................................................ 4

1.3

Methodology and Sequence ............................................................................................. 4

Location and Conditions .......................................................................................................... 6 2.1

Geologic Setting............................................................................................................... 7

2.2

Geotechnical Conditions .................................................................................................. 9

2.3

Ground Instabilities ........................................................................................................ 11

2.4

Stress Conditions ........................................................................................................... 15

2.5

Hydrogeology ................................................................................................................ 15

2.6

Corrosive Conditions ..................................................................................................... 16

2.7

Mining Method and Excavation Longevity ................................................................... 18

Literature Review Empirical Underground Support Design .................................................. 19 3.1

Rules of Thumb.............................................................................................................. 21

3.2

Tunnel Quality Index (Q) Ground Support .................................................................... 22

3.3

Rock Mass Rating (RMR) Based Ground Support ........................................................ 27

3.3.1

RMR Extensions .................................................................................................... 33

3.3.2

Mining Rock Mass Rating (MRMR) ..................................................................... 34

3.3.3

Modified Basic RMR (MBR) ................................................................................ 41

iv 3.3.4

Mathis and Page MRMR Material Type Adjustment ............................................ 45

3.3.5

Critical Span Design Curve.................................................................................... 48

3.4

Geologic Strength Index ................................................................................................ 52

3.5

Engineering Rock Mass Classification Systems Used at Underground Gold Mines in

Nevada ....................................................................................................................................... 54 3.6

4

Comparison of Recommended Empirical Support Pressure .......................................... 54

3.6.1

Support Pressure Description ................................................................................. 54

3.6.2

Empirical Recommended Support Pressure ........................................................... 55

Field Work ............................................................................................................................. 65 4.1

Safety ............................................................................................................................. 65

4.2

Data Collection .............................................................................................................. 66

4.2.1 5

Ground Support Case-Study Data Collection ........................................................ 66

Weak-Rock Mass Rating (W-RMR) System ......................................................................... 85 5.1

Application of RMR in Weak and Highly Fractured Rock............................................ 87

5.1.1

Fracture Frequency (FF) Rating ............................................................................. 89

5.1.2

Rock Quality Designation (RQD) Rating .............................................................. 90

5.1.3

Intact Rock Strength Rating ................................................................................... 92

5.1.4

Condition of Joints and Groundwater Rating......................................................... 93

5.1.5

Suggested Numerical Formulas for Use in Weak Rock ......................................... 95

5.2

Soil Classification for Classification of very weak rock................................................ 97

5.2.1

Unified Soil Classification System (USCS) ........................................................... 98

v 5.2.2

Correlations between RMR and USCS classification systems ............................ 100

5.2.3

Derivation of the RMR-USCS Correlation .......................................................... 104

5.2.4

Use of the RMR-USCS Correlation ..................................................................... 104

5.3

Geo-Pick Strike Index (GPSI)...................................................................................... 105

5.4

W-RMR Summary ....................................................................................................... 109

5.4.1 6

W-RMR Conclusions ........................................................................................... 110

Empirical Based Support Recommendations for Underground Gold Mines in Nevada ...... 111 6.1

Current State of Practice: Ground Support at Underground Gold Mines in Nevada .. 112

6.1.1

Review of Ground Control Management Plans (GCMP) .................................... 113

6.1.2

Discussions with Engineers and Mine Crews ...................................................... 119

6.1.3

Consulting Documents and Internal Mine Memos .............................................. 121

6.2

Case-History Database Development and Analysis ..................................................... 122

6.2.1

Data Analysis ....................................................................................................... 122

6.2.2

Rib Support Pressure Requirements .................................................................... 128

6.2.3

Back/Roof Support Pressure ................................................................................ 132

6.2.4

Bolt Length .......................................................................................................... 137

6.2.5

Surface Support .................................................................................................... 141

6.3

Excavation Strategy ..................................................................................................... 146

6.3.1

Underground Blasting in Poor Ground Conditions .............................................. 146

6.3.2

Stand Up Time ..................................................................................................... 157

6.3.3

Ground Support Sequencing ................................................................................ 159

vi

7

6.3.4

Soft vs. stiff support ............................................................................................. 162

6.3.5

Excavation Strategy Summary ............................................................................. 163

Useful Correlations for Underground Gold Mines in Nevada ............................................. 165 7.1

Q vs. W-RMR76 .......................................................................................................... 165

7.2

GSI vs. W-RMR76....................................................................................................... 167

7.3

W-RMR89 vs. W-RMR76 ........................................................................................... 169

8

Ground Support Design and Excavation Strategy Field Use Figure.................................... 171

9

Conclusions and Recommendations .................................................................................... 173 9.1

Weak Rock Mass Rating (W-RMR) ............................................................................ 173

9.2

Ground Support Recommendations ............................................................................. 175

9.3

Rock Mass Classification Correlations ........................................................................ 176

9.4

Closing ......................................................................................................................... 176

References .................................................................................................................................... 178

List of Figures Figure 1-1. Non-fatal lost time injuries at underground mines in Nevada (MSHA, 2013) ............. 2 Figure 1-2. Fatalities at underground mines in Nevada by type. Note groundfall is the leading cause of death since 1990 (MSHA, 2013) ....................................................................................... 3 Figure 2-1. Study area in Northern Nevada, USA (Muntean and Davis, 2014) .............................. 6

vii Figure 2-2. Basin and Range extensional techtonics. (Earthscope.org, 2016) ................................. 8 Figure 2-3 Schematic showing extensional tectonics of the Basin and Range Provence (USGS, 2015) ................................................................................................................................................ 8 Figure 2-4: Poor ground conditions commonly encountered in Nevada mines. HQ core 2.5 inch diameter shown ................................................................................................................................ 9 Figure 2-5: Average to good quality ground by Nevada standards. HQ core 2.5 inch diameter shown ............................................................................................................................................. 10 Figure 2-6: Photograph of exceptional quality rock core is rare in Nevada or deposits. HQ core 2.5 inch diameter............................................................................................................................ 10 Figure 2-7: Geotechnical conditions at Nevada mines ranges from rock to soil like material ..... 11 Figure 2-8: Groundfall Occurrence ............................................................................................... 12 Figure 2-9: Raveling conditions. Note hollow shotcrete ribs ....................................................... 13 Figure 2-10: raveling ground (loose rocks) bagging in wire mesh ................................................ 13 Figure 2-11: Lost heading/chimney and laser survey to determine extent of cave ....................... 14 Figure 2-12: Squeezing ground conditions. Shotcrete cracking and bolt heads popping off or being pulled through wire mesh ..................................................................................................... 14 Figure 2-13. Copiapite stalactites in back (roof) of excavation in Nevada .................................... 17 Figure 2-14. Acidic water in excavation ........................................................................................ 18 Figure 3-1: Conceptual ground support based on Q. (Grimstad et al., 1986) ............................... 25 Figure 3-2: Q ground support (NGI, 2013) ................................................................................... 26 Figure 3-3: RMR76 Parameters and Ratings (Bieniawski, 1976)................................................. 29 Figure 3-4: Ground support recommendations based on RMR89 (Bieniawski, 1989) .................. 30 Figure 3-5: Overview of the MRMR system (Laubscher 1993) .................................................... 39 Figure 3-6: Intact Rock Strength Rating for MBR. From Kendorski et al., (1983) ..................... 42 Figure 3-7: RQD and Fracture Spacing Ratings for MBR. From Kendorski et al., (1983) ......... 42

viii Figure 3-8: Development ground support chart (Kendorski et al., 1983) ..................................... 44 Figure 3-9: Variation in support for similar ground conditions (Mathis and Page, 1995). Red dots represent case-studies ............................................................................................................. 46 Figure 3-10. Suggested Rock Support Material Property Correction (Mathis and Page, 1995) Red dots represent case-studies ............................................................................................................. 46 Figure 3-11: Critical Span Design Curve for Support Category A, pattern friction bolts (Ouchi, 2008) .............................................................................................................................................. 50 Figure 3-12: Critical Span Design Curve for Support Category B, Pattern friction bolts and spot rebar (Ouchi, 2008) ........................................................................................................................ 50 Figure 3-13: Critical Span Design Curve for Support Category C, Pattern friction bolts and pattern rebar (Ouchi, 2008) ............................................................................................................ 51 Figure 3-14: The Geologic Strength Index (GSI) (Hoek and Marinos, 2000) .............................. 53 Figure 3-15: Comparison of published empirical support guidelines converted to support pressure in tons/ft2 (tsf) ................................................................................................................................ 57 Figure 3-16: Estimated required support pressure (Barton et al., 1974) ....................................... 59 Figure 3-17: Estimated required support pressure (Barton et al., 1974) based on RMR. Note RMR76 was calculated using RMR= 9ln(Q)+44 (Bieniawski, 1976) ............................................ 60 Figure 3-18: Barton et al. (1974) based estimated Q support pressure vs. mapped RMR76 for the Nevada underground mine case-history dataset............................................................................. 61 Figure 3-19: Bhasin and Grimstad (1996) based support pressure vs. mapped RMR 1976 for the Nevada underground mining case-history dataset ......................................................................... 62 Figure 3-20: Goel and Jethva (1991) based support pressure vs. mapped RMR76 for the Nevada underground mining case-history dataset ...................................................................................... 64 Figure 4-1: Documentation of squeezing ground case-study. Note rock bolt head pulling through wire mesh and shotcrete ................................................................................................................. 68

ix Figure 4-2: Case-study documentation sheet ................................................................................ 69 Figure 4-3: Case-study ground support rehabilitation documentation sheet................................. 70 Figure 4-4: Mine site case-history distribution. R strength from Brown (1981) ......................... 73 Figure 4-5: Excavation use designation distribution..................................................................... 74 Figure 4-6: Case-study excavation depth histogram ..................................................................... 74 Figure 4-7: Case-study excavation span histogram ....................................................................... 75 Figure 4-8: Case-study excavation height histogram ..................................................................... 75 Figure 4-9: Case-study excavation RMR76 histogram .................................................................. 76 Figure 4-10: Excavation RQD histogram ...................................................................................... 76 Figure 4-11: Excavation fractures per foot histogram .................................................................. 77 Figure 4-12: Case-study excavation intact rock strength histogram ............................................. 77 Figure 4-13: Excavation groundwater condition distribution ....................................................... 78 Figure 4-14: Case-study rib bolt type Distribution ....................................................................... 78 Figure 4-15: Case-study excavation roof/back bolt type distribution ........................................... 79 Figure 4-16: Bolt capacity (tons) of bolts placed in the ribs .......................................................... 79 Figure 4-17: Bolt capacity (tons) of bolts placed in the roof/back ............................................... 80 Figure 4-18: Rib bolt spacing histogram....................................................................................... 80 Figure 4-19: Roof/back bolt spacing histogram............................................................................ 81 Figure 4-20: Rib bolt support pressure histogram ........................................................................ 81 Figure 4-21: Roof bolt support pressure ....................................................................................... 82 Figure 4-22: Shotcrete thickness on ribs histogram ...................................................................... 82 Figure 4-23: Shotcrete thickness on roof/back histogram.............................................................. 83 Figure 4-24: Longest bolt length in ribs histogram....................................................................... 83 Figure 4-25: Longest bolt length in roof/back histogram ............................................................. 84 Figure 4-26: Case-Study excavation primary instability distribution ........................................... 84

x Figure 5-1: Highly variable ground conditions at underground gold mines in Nevada ................ 85 Figure 5-2: Histogram of RQD plus fracture spacing ratings for the Nevada case-history dataset ....................................................................................................................................................... 88 Figure 5-3: Fracture frequency ratings after Bieniawski (1976 and 1989) .................................... 90 Figure 5-4: RQD ratings after Bieniawski (1976 and 1989) ......................................................... 91 Figure 5-5: Intact rock strength ratings Bieniawski (1976 and 1989) ISRM R strength according to Brown (1981) ............................................................................................................................. 93 Figure 5-6: Conditions of joint ratings (Bieniawski 1976 and 1989) ........................................... 94 Figure 5-7: Groundwater ratings Bieniawski (1976 and 1989) ..................................................... 94 Figure 5-8: Distribution of case-history RMR values. Eq-RMR76 is more sensitive for RMR 35 ........................... 96 Figure 5-9: HDistribution of case study RMR values. Eq-RMR89- is more sensitive for RMR 35 ....................... 97 Figure 5-10: Bulk samples of very weak rock taken from RMR mapped locations underground ..................................................................................................................................................... 101 Figure 5-11: Laboratory determination of USCS vs. Eq-RMR76 ................................................. 102 Figure 5-12: Laboratory determination of USCS vs. Eq-RMR89 ................................................. 102 Figure 5-13: Suggested correlation of USCS to Eq-RMR76 ........................................................ 103 Figure 5-14: Suggested correlation of USCS to. Eq-RMR89 ....................................................... 103 Figure 5-15: Photograph of a weak rock mass with a geo-pick strike index (GPSI) of approximately one........................................................................................................................ 106 Figure 5-16: Correlation of the Geo-Pick Strike Index to Eq-RMR76. ........................................ 107 Figure 5-17: Correlation of the Geo-Pick Strike Index to Eq-RMR89 ......................................... 107 Figure 6-1: End-spectrum attitudes toward ground support (Hoek, 2007) ................................. 111

xi Figure 6-2: Typical ground support from GCMP in Nevada. Left: 17 ft high x 24 ft wide development. Right: 17 x 17 ft development (GCMP Mine E) .................................................. 116 Figure 6-3: Photograph of 4x4ft bolt spacing over wire mesh and shotcrete .............................. 116 Figure 6-4: Support pressure from GCMPs in Nevada ............................................................... 118 Figure 6-5: Contoured minimum support pressure from GCMP in Nevada ............................... 119 Figure 6-6: Notes on bolt spacing and ground conditions from discussion with a mine crew leadman. Grid square equals one foot ............................................................................................... 120 Figure 6-7: Back/roof correlation coefficient matrix ................................................................... 123 Figure 6-8: Rib squeeze correlation coefficient matrix .............................................................. 124 Figure 6-9: Multivariate Regression of back/roof stability. Variables include depth, height, span, and W-RMR76 ............................................................................................................................. 126 Figure 6-10: Rib squeeze stability vs. bolt support pressure and W-RMR76 ............................... 129 Figure 6-11: Probability of rib stability success vs. W-RMR76 and support pressure ................ 130 Figure 6-12: 50% and 90% probability of success lines plotted over data presented in Figure 6-10 ..................................................................................................................................................... 131 Figure 6-13: Case-history dataset. Back stability vs. W-RMR76 and excavation span............. 133 Figure 6-14: Support pressure for span 24ft ............................................................................. 135 Figure 6-17: Estimated required roof/back support pressure from Figure 6-14 through Figure 6-16 .............................................................................................................................................. 135 Figure 6-18: Excavation stability vs. roof span and longest bolt length installed ....................... 138 Figure 6-19: Roof stability vs. W-RMR76 and Longest bolt for excavation spans 10% (by weight) and pH values ranging from 1.8 to 11.6. (Internal mine memo, 2013). Rock bolt pull tests, in-hole camera bolt inspections, and corrosion related fall of ground (FOG) incidents indicate that corrosion rates are highest in the 1 foot to 1.5 ft of the bolt closest to the excavation (experience and communication with engineers and miners). Because corrosion reduces the longevity and support capacity of the support system, it must be taken into account in the design process and excavation strategy. Further information regarding the design of ground support in corrosive conditions is available in Dorin and Hadjigeorgiou (2014).

Figure 2-13. Copiapite stalactites in back (roof) of excavation in Nevada

18

Figure 2-14. Acidic water in excavation

2.7 Mining Method and Excavation Longevity Underground gold mines in Nevada typically employ an underhand cut and fill or long-hole stoping method depending on ground quality and ore body dimensions/geometry. Temporary or production man-entry excavations are typically planned to be open anywhere from days to a year, and include tertiary development, top-cuts, undercuts and bottom or sill cuts. Long term or permanent openings are typically expected to be open more than a year and may have to last the life of mine (LOM). Long-term man-entry excavations include: Mine shafts, portals, primary declines, shops, crusher chambers, electrical stations etc. Support design can be influenced by the planned purpose and longevity of an opening, however in practice this is not always the case.

19

3 Literature Review Empirical Underground Support Design Requirements for greater mining efficiency and higher safety standards have made more reliable and effective support practices necessary (Lang, 1994). A comprehensive review of the history of empirical support design is beyond the scope of the dissertation; however, a brief discussion of early and currently used empirical design methods can provide insight into which factors are important to support design and empirical methodology development. According to Mark (2015), the first empirical design method that incorporates case history with rock mechanics principals appears to be Bunting (1911). Bunting addressed pillar sizing in coal mines of Pennsylvania because many pillars had failed causing roof cave and floor heave. Many empirical support design methods were subsequently developed for a variety of conditions and applications. Most of the commonly used support design tools have their roots in empirical data and are discussed subsequently. “Successful empirical methods are readily accepted because they are simple, transparent, practical, and firmly tethered to reality” Mark (2015). Major developments and methods relevant to underground support design are presented in Table 3-1.

Table 3-1 Historical and Recent Empirical Support Design Methods Name of Support Design Method First Developed Rock Load Theory

Stand-up Time

Author and First Version

Country of Origin

Terzhagi (1946)*

USA

Lauffer (1958)*

Austria

Description and Application

Prediction of rock loads carried by steel sets based on descriptive rock classification, depth and dimensions Stand-up time for unsupported span

Comment

Roots of NATM

20 New Austrian Tunneling Method (NATM) or Sequential Excavation Method (SEM)

Rabcewicz (1964)

Austria

More of a tunnel design philosophy than an empirical design method. Attempts to mobilize the self-supporting capability of the ground to achieve cost efficient support Basic support tunnel recommendations for 24 ft wide tunnel based on RQD

Rock Quality Designation (RQD) support design

Peck et al., (1969)** and Cecil (1970)**

USA

Rock Quality Designation (RQD) support design graph

Merritt (1972)

USA

Basic support tunnel recommendations based on RQD and tunnel width

Rock Structure Rating (RSR)

Wickham et al. (1972)*

USA

Tunnel Quality Index (Q)

Barton et al. (1974)

Norway/ Norwegian Geotechnical Institute

Rock Mass Rating (RMR) / Geomechanics Classification

Bieniawski (1973)

South Africa

Shotcrete, rock bolt and steel rib spacing for tunnels based on semi-quantitative rock mass classification Detailed support design based on quantitative ground classification, excavation use, and dimensions Originally detailed support recommendations for 30ft wide tunnel

Mine Rock Mass Rating (MRMR)

Laubscher, (1976)

South Africa

RMR modified for mines

MBR

Kendorsiki et al. (1983)

USA

RMR modified for mines

Critical Span Curve

Lang (1994)

Canada

Support of entry type excavations

Relies heavily on understanding of stand-up time and ground reaction to excavation and support. Continuous worldwide use

Based in Deere et al., (1967) RQD. Limited Applicability: Not sensitive to joint friction angle or intact rock strength Based in Deere et al., (1967) RQD. Limited Applicability: Not sensitive to joint friction angle or intact rock strength Multi-parameter classification- not widely used but laid groundwork for Q and RMR Based predominantly on tunneling case histories. Can be applied to mining. Continuous worldwide use Numerous empirical support recommendations developed since 1974. Worldwide use Predominantly from block cave mines in South Africa Predominantly from block cave mines in the western US Canada and US mines

21 Coal Mine Roof Rating

Molinda USA Support design for Specific to coal and Mark stratified coal roofs (1994) * As described in Hoek et at., (1995) ** As described in Deere and Deere (1988) Of the empirical support design and classifications systems described in Table 3-1, design guidelines based on the RMR system (or some variation) and to some extent the tunnel quality index (Q) has gained the most acceptance in Nevada.

3.1 Rules of Thumb Rules of thumb are the most basic of design tools and are applied to this day. Rule of thumb support engineering is based on experience and provides a useful check on support designs from more complex methods discussed subsequently. The minimum bolt length from Lang (1961) as cited in Hoek and Brown (1980) and US Army Corps of Engineers (1980) should be: •

Spans less than 20 ft, bolt length = ½ the span

•

Spans 60-100 ft, bolt length = ¼ the span

•

Height >60 ft, sidewall bolts 1/5 the height

•

Twice the bolt spacing

•

Three times the width of critical and potentially unstable rock blocks defined by the average joint spacing of the rock mass

Maximum bolt spacing from Lang (1961) as cited in Hoek and Brown (1980) should be: •

Half of the bolt length

•

one and a half times the width of critical and potentially unstable rock blocks defined by the average joint spacing of the rock mass

22 •

When weld mesh or chain-link mesh is used, bolt spacing of more than 6 ft makes attachment of the mesh difficult (but not impossible)

“In hard rock mining, the ratio of bolt length to pattern spacing is normally 1.5:1. In fractured rock it should be at least 2:1. (In civil tunnels and coalmines it is typically 2:1.)” (Lang and Bischoff (1982) as cited in Vergne (2008). “In mining, the bolt length/bolt spacing ratio is acceptable between 1.2:1 and 1.5:1” Bieniawski (1992) as cited in Vergne (2008). “In good ground, the length of a roof bolt can me 1/3 the span. The length of a wall bolt can be 1/5 the height. The pattern spacing may be obtained by dividing the rock bolt length by 1.5,” Gray (1999) as cited in Vergne (2008). The reader is cautioned against using these rules as a one size fits all design approach. Rule of thumb designs are a good reality check or starting point for support design, but cannot replace detailed engineering design, site-specific experience, and sound engineering judgement.

3.2 Tunnel Quality Index (Q) Ground Support The Tunnel Quality Index introduced by Barton et al. (1974) was based on roughly 200 tunnel case-studies. Since then, numerous updates have been made based on observations from additional case-studies, and improvements in excavation methods and ground support systems. Notably, Grimstad and Barton (1993) increased the case-studies to 1,050 and updated the Q support recommendations to reflect the Norwegian Method of Tunneling (NMT) which included rock bolt spacing, shotcrete thickness, and the use of rebar steel reinforced sprayed concrete ribs S(fr). Also, the Stress Reduction Factor (SRF) in heavy rock burst conditions was increased from a maximum of 20 to 200-400. Barton (2002) does not change any parameters of the Q system,

23 but does introduce multiple Q correlations to rock mass properties and Q based design tools to assist with tunnel design. Table 3-2 summarizes significant Q papers.

Table 3-2. Significant Q Papers Author’s and Year Barton, Lien, and Lunde (1974)

Name of Paper

Development

Support Design

Engineering Classification of Rock Masses for the Design of Tunnel Support

Introduced the Q system and explains its development.

Grimstad, Barton, Lien, Lunde, and Loset (1986)*

Classification of Rock Masses with Respect to Tunnel StabilityNew Experiences with the Q-System

Analyses of case histories and ground support types for various ground conditions.

Barton (1988)

Rock Mass Classification and Tunnel Reinforcement Selection using the QSystem Updating of the QSystem for NMT (Norwegian Method of Tunneling)

Comparison of current classifications, and description of Q. Case-history statistics are provided

Based ground and support categories. Charts and tables are not particularly user friendly Graph indicating appropriate support type for given ground conditions. No change in specific support designs Simplified support chart

Grimstad and Barton (1993)

Barton and Grimstad (1994)

Barton (2002)

The Q-System following Twenty Years of Application in NMT Support Application Some new Q-value correlations to assist in site characterization and tunnel design

Addition of 1,050 case histories. Updates support for NMT. Increases SRF in rockburst conditions. Introduced” modern rock support” design chart. Suggested estimates of deformation modulus. Adjustment for narrow weak zones Q-seismic velocity correlation discussed Q for TBM also discussed

User friendly support design graph

Numerous correlations presented: Q-seismic velocity, rock mass modulus, permeability. Friction and Cohesion Components (FF and FC) introduced and injection grouting discussed

No change from 1993

No change from 1993

24 Norwegian Geotechnical Institute (NGI) 2013

Handbook: Using the Q-System. Rock mass classification and support design

Formal description of how to apply the Q system.

Update of rebar reinforced ribs with shotcrete (RRS) category

* As stated in Barton and Grimstad (1994) The Tunnel Quality Index (Q) is based on the following formula: 𝑹𝑸𝑫 𝑱𝒓 𝑱𝒘 𝑸=( )∗( )∗( ) 𝑱𝒏 𝑱𝒂 𝑺𝑹𝑭

Equation 3-1

Where: Q = Tunnel Quality Index (.001-1000) RQD = Rock Quality Designation (Deer et al., 1967) JN = Joint Number Jr = Joint Roughness Ja = Joint Alteration Jw = Joint Water Reduction Factor SRF= Stress Reduction Factor

The first term (RQD/Jn) roughly represents the overall structure of the rock-mass and is a crude representation of block size. The second term (Jr/Ja) represents the roughness and degree of alteration of the joint walls with the inverse tangent (tan-1) of this quotient roughly approximating the friction angle of the joints in the rock-mass. The final term (Jw/SRF) represents stress parameters attempting to quantify effects of groundwater and in-situ or induced stress (Barton et al., 1974). Support requirements are dependent on the intended use of the excavation with temporary or low risk excavations and high importance/risk excavations requiring less and more support respectively. The Estimated Support Ratio (ESR) attempts to quantify this parameter and is required in the Q support design charts. ESR parameters are presented in Table 3-3 below.

25 Table 3-3: ESR for various excavation types (Barton et al., 1974)

Excavation Type

ESR

Temporary mine openings Vertical Shafts Permanent mine openings, water tunnels for hydro power (exclude high pressure penstocks), pilot tunnels, drifts and headings for large excavations Storage rooms, water treatment plants, minor road and railway tunnels, surge chambers, access tunnels, etc. Power stations, major road and railway tunnels, civil defence chambers, portals, intersections etc. Underground nuclear power stations, railway stations, sports and public facilities, factories etc.

3-5 2-2.5 1.6 1.3 1 0.8

Conceptual and detailed ground support based on the Q system are shown on Figures 3-1 and 3-2 respectively.

Figure 3-1: Conceptual ground support based on Q. (Grimstad et al., 1986)

26

Figure 3-2: Q ground support (NGI, 2013)

The Q system offers attractive support design figures however, the system is primarily intended to provide support requirements for long term-civil engineering projects and the case-history database is predominantly based on these type of excavations. Mining operations generally require much less support than tunnels because of limited life of openings, availability of fulltime geotechnical department, constant observation by mine personnel, and ability to quickly

27 repair or install additional support if required. Given these differences between underground environments and tunnel construction, the Q system provides a useful starting point for infrastructure and life of mine (LOM) openings. Case-history and literature documentation of the performance of the ESR ratio and Q design charts for temporary man-entry type mine openings is limited. The Q system is known for its sensitivity to changes in rock mass properties; however, as indicated by others including Bieniawski (1976) and Singh and Goel (2011), the system can appear complicated and daunting especially for the less experienced user. Experience indicates that because of the engineering knowledge and attention to engineering detail required to properly apply the Q system, only experienced engineering geologists or engineers should use the system.

3.3 Rock Mass Rating (RMR) Based Ground Support The Rock Mass Rating (RMR) system is a numeric (0-100) rock mass classification originally developed by Bieniawski (1973) and modified over the years as more case histories became available and to conform to international standards and procedures (Bieniawski, 1979). The classification is based on ratings of several rock mass properties (depending on version) including: 

Intact rock strength



Rock Quality Designation (RQD)



Joint spacing



Joint continuity



Joint condition



Joint orientation



Groundwater condition

28 

Weathering

Because the RMR system has evolved and changed over the years, it is important to reference which system is being used. Table 3.4 summarizes the evolution of the RMR system and allocated ratings associated with each parameter.

Table 3-4: RMR Parameter Ratings by RMR year Rock Mass Rating (RMR) Parameter Maximum Rating

Joint Condition

Groundwater

Joint Spacing

RQD

Joint Orientation

Joint Separation

Joint Continuity

Weathering

Bieniawski (1973) Bieniawski (1974) Bieniawski (1976) Bieniawski (1979/89) Lowson and Bieniawski (2013)

Intact Rock Strength

RMR year

10 10 15 15 15

15 25 30 30

10 10 10 15 15

30 30 30 20 40

16 20 20 20 -

15 15 adjustment adjustment adjustment

5 -

5 -

9 -

To apply the RMR classification system, the rock mass is divided into separate units such that rock mass properties are similar within each unit. The RMR rating parameters are applied and the sum of the ratings of each unit represents the overall rating of the rock mass associated with a rating class and description: 

RMR 0-20

= very poor rock



RMR 21-40

= poor rock



RMR 41-60

= fair rock



RMR 61-80

= good rock



RMR 81-100

= very good rock

29 Figure 3-3 and Figure 3-4 below show the RMR rating system and associated ground support recommendations, respectively.

–– Figure 3-3: RMR76 Parameters and Ratings (Bieniawski, 1976)

30

Figure 3-4: Ground support recommendations based on RMR89 (Bieniawski, 1989) Several modifications to the rating system have been developed for specific applications including the Slope Mass Rating (SMR) proposed by Romana (1985) and the Mine Rock Mass Rating (MRMR) proposed by Laubscher (1977). The RMR system has stood the test of time and benefited from extensions and applications by many authors around the world (Bieniawski, 1993). The RMR system has gained wide acceptance among mine and consulting companies in Nevada for both surface and underground geotechnical applications. Table 3-5 below lists significant papers associated with the development of the RMR system.

31 Table 3-5: Significant Papers and Development of the RMR System Author’s and Year Bieniawski (1973)

Name of Paper

Development

Support Design

Engineering Classification of Jointed Rock Masses

Support recommendations for 5-12m drill and blast, 25mm resin bonded bolts. Bolt length = ½ excavation diameter

Bieniawski (1974)

Geomechanics Classification of Rock Masses and its Application in Tunneling Unknown

Introduction of the Geomechanic’s Classification System. 8 original parameters. RQD, weathering, intact rock strength, spacing of joints, separation of joints, continuity of joints, groundwater, structural orientation. Introduction of stand up time based on system. Reduce parameters from 8 to 6: Dropped weathering and separation of joints

Adjustment of ratings

Reduction of recommended support requirements References Bieniawski 1973 for support design

Bieniawski (1975)* Bieniawski (1976)

Rock Mass Classification in Rock Engineering

Bieniawski (1979)

The Geomechanics Classification in Rock Engineering Applications

Bieniawski (1989)

Engineering Rock Mass Classifications. A Complete Manual for Geologists in Mining, Civil and Petroleum Engineering

Removed structure orientation from main classification and added as an adjustment factor. Comparison of various tunnel support systems. Further discussion and correlation of RMR and Q. Update of unsupported stand-up time graph Adoption of ISRM R (Brown 1981) strength for intact rock strength rating. Presentation of correlations between RMR and elastic modulus of rock masses where RMR>50. unsupported stand-up time graph Introduction of parameter rating graphs for intact rock strength, RQD, and joint spacing. Note: Parameter rating graphs to not go to zero.

Slight adjustments from 1973

32 Bieniawski (1993)

Classification of Rock Masses for Engineering: The RMR System and Future Trends

Bieniawski (2011)

Misconceptions in the Application or rock Mass Classifications and Their Corrections

Lowson and Bieniawski (2013)

Critical Assessment of RMR-Based Tunnel Design Practices: A Practical Engineer’s Approach

RQD and discontinuity spacing from Priest and Hudson (1976) May extensions of the RMR system are discussed. Add mine case-studies to Standup time graph Clarified that RMR can be zero. Incorrectly referenced his figures from 1989 showing that parameter ratings can be zero. In any case, the problem with RMR having a minimum value was addressed. Discussion of the use of RQD in weak ground is given but ambiguous. Further clarification that RMR can be 0. Bieniawski (2011) rating charts represented. Use of RQD at the face underground is formally “NOT Recommended.” RQD and fracture spacing are combined into fracture frequency and points are combined as well (40) New support charts developed.

Not discussed

Design charts incorporating variable dimensions and RMR are presented for: Rockbolt spacing, rockbolt length, rockbolt capacity, shotcrete strength, and shotcrete thickness. Review of these charts indicate they are too conservative for mining in Nevada

*As stated in Singh and Goel (2011) As can be seen in Table 3-4 and Table 3-5, the RMR system has a history of development and change. To complicate matters, various empirical formulas, support design charts, and extensions of the RMR system have their roots and case histories in their respective RMR versions. Therefore, it is important to state which version is used when RMR values are reported, and to apply the correct RMR version specific to the empirical tool being used. As a general rule, the 1976 and 1989 versions are the most commonly used systems and most RMR associated design tools are associated with one or both of these versions.

33 3.3.1

RMR Extensions

As stated previously, the RMR rating system has become a popular rock mass classification system for a variety of reasons including: 

Relative simplicity to apply



Representation of relevant rock mass properties



Simple 0-100 scale



Empirically speaking, it works

The wide acceptance of RMR has resulted in its application to a variety of rock engineering problems by various authors in a variety of geotechnical fields. The number of extensions is quite remarkable and is summarized in Table 3-6 below. It is likely that other extensions to the system have been developed of which the author is not aware. Table 3-6: Extensions of the RMR System

Author and Year

Country

Topic

Weaver (1975)* Laubsher (1976)** Olivier (1979)* Ghose and Raju (1981)* Moreno Tallon (1982)* Kendorski et al. (1983)* Nakao et al. (1983)* Serafim and Pereira (1983)* Unal (1983)

South Africa South Africa South Africa India

Ripability Hard rock mining Weatherability Coal mining

Spain USA

Tunneling Hard rock mining

Japan Brazil

Tunneling Dam foundations

USA

Gonzalas de Vallejo (1983)* Newman (1985)* Romana (1985)* Sandback (1985)* Smith (1986) Venkateswarlu (1986)* Robertson (1988)*

Spain

Underground coal mining roof control Tunneling

USA Spain USA USA India Canada

Coal mining Slope stability Borability Dredgeability Coal mining Slope Stability

34 Theil (1985) Poland Unal (1996) Turkey Lang (1994) Canada Mathis and Page (1995) USA Bieniawski et al. (2007) Spain *Bieniawski (1993) **Bieniawski (2011)

Carpathian flysch Weak rock, coal Underground mining Underground mining TBM tunneling

As shown in Table 3-6, the RMR system has been broadly adapted to many rock engineering situations. Several of these systems require special attention as they have specific application to underground support design in a mining environment. These include:

3.3.2



Mine Rock Mass Rating (MRMR) (Laubscher, 1976)



Modified Basic RMR (MBR) (Kendorski et al., 1983)



Critical Span Curve (Lang, 1994)



Drifting in very poor rock (Mathis and Page, 1995)

Mining Rock Mass Rating (MRMR)

The Mining Rock Mass Rating (MRMR) system was originally introduced by Laubscher (1977) as a support design system tailored to underground mining, and it has been modified and expanded several times since then. The MRMR system was originally based on experience at block cave mines in South Africa with updates addressing challenges encountered in block cave operations in Chile and Australia (Jarek and Esterhuizen, 2007). Table 3-7 lists important papers relevant to the MRMR system.

35 Table 3-7: Summary of important MRMR papers

Author and Year

Name of Paper

Development

Laubscher (1977)

Geomechanics Classification of Jointed Rock Masses Mining Applications Design Aspects and Effectiveness of Support in Different Mining Conditions A Geomechanics Classification System for the Rating of Rock Mass in Design Planning mass mining operations

MRMR

Laubsher (1984) Laubscher (1990)

Laubscher (1993)

Laubscher and Jakubec (2001)

The MRMR rock mass classification for jointed rock masses

Jarek and Esterjuizen (2007)

Use of the Mining Rock Mass Rating (MRMR) Classification: Industry Experience

Ground support recommendations

Explanation of the 1977 and 1984 systems

Further development of MRMR and mine predictive capabilities including caveability, stress estimation, etc. Dropping of RQD, accounting for healed and cemented joints, the concept of rock block strength/scale concept, and mining adjustments similar to Q Historical description and update of industry application

The parameters used to calculate RMR for MRMR are similar, but different to those used to calculate RMR in Bieniawski’s systems. This has led to confusion because Laubscher (MRMR) uses the term RMR, or “Basic RMR”, implying that the much more well-known and used Bieniawski RMR is used. MRMR was introduced as a co-development of Bieniawski’s RMR system catering to relatively more complex mining situations. The fundamental difference in MRMR was to adjust the RMR value (in-situ) according to the various mining environments so that the final MRMR could be used for mine design (Laubscher, 1990). Applications of MRMR include: 

Support design

36 

Caveability diagrams



Stability of open stopes



Pillar design



Determination of caveability



Extent of cave and failure zones

The following discussion is based on Laubscher (1990) except where noted. The MRMR system is a relatively complex rating system based on the concept of in-situ and adjusted ratings. Basic RMR or in-situ RMR is converted to MRMR by applying various adjustments based on the anticipated mining conditions or practices. In-situ RMR ratings are based on the physical properties of the rockmass and are independent of mine conditions. (Table 3-8)

Table 3-8: Summary of in-situ RMR. Laubscher (1990) and (2001)

In-situ RMR Parameter

Range of Parameter Ratings

RMR In-situ adjustments*

Intact rock strength/rock block strength

0-20 Laubscher (1990) 0-25 Laubscher (2001)

RQD and Fracture/joint spacing rating (RQD rating removed from Laubscher, 2001) Joint condition rating

0-20 Laubscher (1990) 0-25 Laubscher (2001)

1. size adjustment strength and % weak rock 2. %veins and Mohs’ hardness of vein infill (only Laubscher 2001) 1. 1, 2, or 3 joint sets 2. method to determine fracture spacing 3. borehole orientation to joint orientation 4. joint continuity 1. large scale expression 2. small scale expression 3. joint alteration 4. joint filling 5. groundwater

40 initial rating, reduced by adjustments

Note in-situ adjustments are independent of mining condition adjustments In-situ RMR = [IRS*(IRS adjustments)] + [FF or RQD*(adjustments)] + [JC*(adjustments)]

Equation 3-2

37 Where; IRS = intact rock strength FF= Fracture frequency RQD= rock quality designation JC= joint condition Once the in-situ RMR is determined, mine condition adjustments are made to give the MRMR. Adjustments are empirical and are based on numerous observations and require the engineer to assess the proposed mining activity in terms of its effect on the rock mass (Laubscher, 1990). Mining condition adjustments are summarized in Table 3-9. Table 3-9: Mine condition adjustments to MRMR (Laubscher, 1993)

Mining Adjustment

Adjustment Range

Description

Weathering

30-100%

Time dependent adjustment based on the grounds reaction to environmental changes caused by the excavation. I.e. air slacking, slaking, desiccation, decay to residual soil 1. Based on the number of joints that dip away from the vertical axis, 2. Based on orientation of shear zone, 3. Based on plunge of the intersection of joints on the base block Ambiguous adjustments based on maximum stress, minimum stress, and stress difference Ranging from mechanical excavation to poor blasting

Joint orientation

1. 70-90% 2. 76-92% 3. 70-80%

Mine-induced stress

60-120%

Blasting practices

80-120%

38 MRMR = in-situ RMR * (WA%)*(JOA%)*(MSA%)*(BA%) where

Equation 3-3

WA% = weathering adjustment JOA% = Joint orientation adjustment MSA% = Mine stress adjustment BA% = Blasting adjustment 3.3.2.1

Clarification and discussion

Much confusion over the use and complexity of the MRMR system has been expressed by geotechnical engineers in the Nevada mining community, and some clarification is due. The calculation of in-situ RMR as described in this paper is very similar to calculation of Bieniawski’s RMR with the added complication of in-situ correction factors for weak rock and veins. Users are recommended to initially ignore the in-situ adjustments which in the author’s opinion, are unnecessarily complicated. The application of the mine condition adjustments (weathering, joint orientation, stress, blasting practices) are the more important features that distinguish MRMR from Bieniawski’s RMR. Figure 3-4 below provides a useful flow-chart describing the MRMR system overall.

39

Figure 3-5: Overview of the MRMR system (Laubscher 1993)

3.3.2.2

Support Design Using MRMR

Support design using the MRMR system is given using Tables 3-10 and 3-11 below. Note that support design is based on both in-situ RMR and MRMR.

40 Table 3-10: Support guide for underground excavations (Laubscher 1993)

MRMR 91-100 81-90 71-80 61-70 51-60 41-50 31-40 21-30 11-20 0-10

In-Situ RMR 91-100

81-90

71-80

61-70

51-60

41-50

a a b b r

a b b r

a b c

a b c d

b c e

d d f f/p

31-40

21-30

11-20

0-10

d f f+f/p f+f/p

c+l h+f/l f/o

h+f/l t

t

Table 3-11: Support Techniques (Laubscher, 1993) Rock Reinforcement a Local bolting at joint intersections b Bolts 1m (3.3 ft) spacing c Bolts 1m (3.3 ft) spacing + straps (mesh if finely jointed) d Bolts 1m (3.3 ft) spacing + mesh + shotcrete e Bolts 3.3 ft spacing + [mesh + shotcrete] or [steel fiber reinforced shotcrete] f e + cable bolts h Spilling l Massive concrete o Yielding steel arches P Yielding steel arches + concrete or shotcrete r Bolts and rope-laced mesh. Spalling control t Avoid development if possible The previous discussion was an attempt to clarify a classification scheme which mine geotechnical practitioners in Nevada have considered cryptic and overly complex. Complicated empirical data input requiring experience and engineering judgement may be appropriate for important feasibility interpretations such as orebody caveability or stope stability; however, the routine estimation of support requirements in development or ore production headings by less experienced engineers requires relatively quicker and easier to apply empirical data input. In the opinion of many underground geotechnical engineers in Nevada (personal communication), the MRMR system is overly complex and provides over-generalized support recommendations for use in routine heading support design.

41 3.3.3

Modified Basic RMR (MBR)

The Modified Basic RMR or (MBR) was developed by Kendorski et al. (1983) to assist mine planners in support design for production drifts in block or panel caving mines. MBR closely follows Bieniawski’s RMR system and incorporates many ideas found in Laubschers MRMR system. Although the MBR can be modified to adapt to the ore block development process, it can also be used to predict support requirements for infrastructure located away from production areas and is the focus of this discussion. MBR is calculated similar to Bieniawski’s RMR system with the rock mass property ratings shown in Table 3-12.

Table 3-12: MBR Parameters and Maximum Ratings (Kendorski et al., 1983)

Parameter

Maximum Rating

Table or Figure

Intact Rock Strength RQD Fracture Spacing Discontinuity Condition Groundwater

15

Figure 3-6

20 20 30

Figure 3-7 Figure 3-7 Table 3-13

10

Table 3-14

To calculate MBR, the rock mass is mapped, or core is logged to estimate the parameters shown above. Once these parameters are determined, graphs and tables are used to determine a rating for each parameter which are summed to a MBR rating between 0 and 100.

42

Figure 3-6: Intact Rock Strength Rating for MBR. From Kendorski et al., (1983)

Figure 3-7: RQD and Fracture Spacing Ratings for MBR. From Kendorski et al., (1983)

43 Table 3-13: Joint Condition and Ratings for MBR (Kendorski et al., 1983) Discontinuity Description

Very rough, no separation or infill, unweathered

Rough to slightly rough, hairline separation, no infill, slightly weathered

Rating

30

25

Slightly rough, hairline to 2mm separation, minor clay infill, softened and strongly weathered 20

Smooth to slickensided, 2-6mm stiff clay gouge, softened and strongly weathered 10

Smooth, >6mm soft clay gouge, very soft and decompos ed 0

Table 3-14: Groundwater Condition Rating for MBR (Kendorski et al,. 1983) Water Condition Rating

Completely Dry 15

Damp

Wet

Dripping

10

7

4

Flowing 0

44 Once the MBR ratings have been determined, support estimates can be made using Figure 3-8. Note the wide range in support recommendations for the low and high reliability lines, and the use of steel sets for weak ground (RMR40

>30

>30

49 The Critical Span Design Curves produced by Ouchi (2008) are the most recent and include casestudies from weak ground in Nevada which is relevant to this study. The case-study database from 12 mines in the US and Canada was divided into the four support type categories described in Table 3-18 to compare similar support types/capacities. Several neural network analyses were performed on each category, and stability predictions for RMR = 20-60 and spans from 3.5 – 42 ft were created. Transition lines delineating stable, potentially stable, and unstable zones were created from these neural network predictions shown in Figures 3-11through 3-13 (Ouchi, 2008).

Table 3-18: Ground support categories

Support Category A B C D

Bolt Type Friction = split set or inflatable swellex type Friction bolts 3-4ft grid Friction bolts 3-4ft grid + resin rebar spot bolting Friction bolts 3-4ft grid+ Resin rebar 3-4 ft grid Friction bolts 3-4ft grid+ Resin rebar 3-4 ft grid + Cables spot and /or shotcrete > 3”

50

Figure 3-11: Critical Span Design Curve for Support Category A, pattern friction bolts (Ouchi, 2008)

Figure 3-12: Critical Span Design Curve for Support Category B, Pattern friction bolts and spot rebar (Ouchi, 2008)

51

–– Figure 3-13: Critical Span Design Curve for Support Category C, Pattern friction bolts and pattern rebar (Ouchi, 2008)

3.3.5.1

Discussion of Critical Span Design Curve

Examination of Figures 3-11 through 3-13 indicates relatively good fits of the case-study data. Ouchi (2008) reports satisfactory results for the Category A Critical Span Design Curve, however, caution should be used when near the lower end of the RMR scale where less data is available, and the transition between stable and unstable becomes sharp. Interestingly, support categories B and C indicate smaller stable spans for a given RMR as compared to Support Category A, which is relatively less supported. The reason for this is unknown but it has been observed that resin grouted rebar is difficult to install in weak rock (Ouchi et al., 2009) which can lead to poor quality installation or bond strength. “It would be imprudent to rely on results of Categories B and C” (Ouchi 2008). Based on the previous discussion, only the Category A (pattern friction bolts) should be applied (Figure 3-11).

52 The Critical Span Design Curve for Category A (Figure 3-11) indicates circumstances where pattern friction bolts with spacing of 3-4 feet will be effective; However, information regarding support design outside this bolt pattern is not given. In a sense, the chart is only useful for one bolt spacing design (3-4 ft), and is probably why this design chart is not widely used in Nevada. Examination of the case-history database from Ouchi (2008) indicates that the vast majority of case-histories are supported with either split sets or rebar, both of which have been almost completely phased out of routine support at underground mines in Nevada and replaced by inflatable (Swellex type) bolts. It is questionable to combine split sets and inflatable (Swellex type) bolts in a single category. While both of these bolts rely on friction with the rock-mass for support, the achievable bond strength, performance, and typical failure mode of these bolts is considerably different. Figure 311 is useful for some situations. However a design chart predicting ground support requirements for a given geotechnical condition would be useful and is the subject of this research.

3.4 Geologic Strength Index The Geological Strength Index (GSI) was introduced by Hoek et al. (1995) as a replacement for RMR in the Modified Hoek-Brown failure criterion (Hoek et al., 1992) because it had become increasingly obvious that RMR was difficult to apply to very poor quality rock masses (Hoek and Marinos, 2007). The GSI system has no rock-mass reinforcement or support design capabilities, and its only function is the estimation of rock mass properties (Marinos et al., 2005). The main advantages of using GSI for empirical support recommendations in Nevada include: 1) “Allows characterization of difficult-to-describe rockmasses” (Carter and Marinos, 2014), 2) Efficient procedure to collect geotechnical information, and 3) Easy to apply with minimal training. The main disadvantage of the GSI system is that it excludes intact rock strength and ground water condition variables, limiting its ability as a standalone classification system. The GSI system is

53 being used to document geotechnical conditions at headings at underground gold mines in Nevada and is therefore considered in the literature review. The GSI documentation figure from Hoek and Marinos (2000) is presented in Figure 3-14 below. Use of Figure 3-14 in Nevada support design is be discussed in Section 7.2.

Figure 3-14: The Geologic Strength Index (GSI) (Hoek and Marinos, 2000)

54

3.5 Engineering Rock Mass Classification Systems Used at Underground Gold Mines in Nevada Rock mass classification systems are tools commonly used by geotechnical engineers to quantify or “rate” a rock mass and compare it relatively to other rock masses. Commonly used classification systems used in Nevada include the Rock Mass Rating (RMR), Tunnel Quality Index (Q) and the Geologic Strength Index (GSI). Each of these systems was evaluated as a potential classification system for future empirical support design recommendations discussed in sections 3.2 to 3.4.

3.6 Comparison of Recommended Empirical Support Pressure Recommendations of specific support design based on Q and variations of RMR have been discussed in previous sections. These support recommendations were created at different times for different applications and with varying support elements. A direct comparison of these support design recommendations provides insight for future support design recommendations and is most appropriately carried out by comparing the equivalent support pressures of each support class, for each support system. 3.6.1

Support Pressure Description

Support pressure is the equivalent normal stress applied to the excavation boundary and is defined for rock bolts in Hoek and Brown (1980) as:

𝑃𝑠 =

𝐶𝑏𝑓 𝑆𝑟∗𝑆𝑙

Where: Ps = support pressure

Equation 3-4

55 Cbf = capacity at bolt failure or bolt slip Sr and Sl = radial and longitudinal bolt spacing respectively. Equations for calculating support pressure for steel sets are given in Hoek (1998). For the purpose of this research, bolt pattern and steel set support pressure is expressed as tons/ft2 or tsf. Support pressure from shotcrete can be calculated using thin-wall cylinder theory (Hoek and Brown, 1980) but is highly dependent on excavation shape. Variations in excavation design and drill/blast techniques result in highly non-uniform excavation shapes throughout Nevada, therefore, this study as many others, consider shotcrete as surface support only and is handled separately. 3.6.2

Empirical Recommended Support Pressure

Table 3-19 presents support guidelines evaluated for support pressure for comparison. Support pressures for each method were evaluated using procedures proposed by Hoek (1998).

Table 3-19: Empirical support guidelines converted to support pressure Author

System

Origination/ Purpose

Support Elements

Barton et al.(1974)

Q Converted to RMR using relations ship RMR= 9lnQ+44

Tunneling

Grouted rock bolts, fiber reinforced shotcrete, and steel sets

Kendorski et al. (1983)

MBR/RMR76

Block cave mines in western US

Grouted rock bolts, wire mesh, steel sets

Bieniawski (1989)

RMR89

Tunneling

Grouted rock bolts, wire mesh, steel sets

Mathis and Page (1995)

MRMR

Cut and fill mines weak rock Nevada, USA

6 ft split sets, straps, wire mesh shotcrete, cable bolts, lattice girders

56 Laubscher (1993)

MRMR

Block cave mines Africa

Rock bolts, wire mesh, fiber shotcrete, cable bolts, timber, steel arches, grouting, massive concrete

Ouchi (2008)

RMR76

Underground mines, Western US and Canada

Split sets, resin grouted rebar

Calculated support pressures from empirical systems in Table 3-19 where plotted on Figure 3-15 and show a wide variability in support pressure recommended in the literature. Even within a specific ground support recommendation system, particularly Kendorski et al. (1983) and Mathis and Page (1995), recommended support pressures can vary widely. With these wide ranges of recommended ground support, significant judgement is required by the engineer. Support guidelines produced by this research intend to narrow down the range of support pressure required to keep underground openings stable.

57

RMR based Rock Bolt, Lattice Girder and Steel Sets Support Pressure 15 to 30 ft Diameter Tunnel 35

Support Pressure TSF

30 25 20

15 10 5

0 0

10

20

30

40

50

60

70

80

90

RMR Barton et al. (1974) equation Laubscher (1993) Minimum Barton and Grimstad (1993) design chart no shotcrete Ouchi (2008) Potentially Unstable Power (Kendorski et al. (1983) High Reliability) Expon. (Mathis and Page (1995) Maximum)

Bieniawski (1989) Laubscher (1993) Maximum Ouch (2008) Unstable Ouchi (2008) Stable Power (Kendorski et al. (1983) Low Reliability) Power (Mathis and Page (1995) Minimum)

Figure 3-15: Comparison of published empirical support guidelines converted to support pressure in tons/ft2 (tsf)

100

58

3.6.2.1

Equation Based Support Pressure

Empirically derived required support pressure estimation equations are available in the literature and selected equations were applied to geotechnical properties acquired for the Nevada underground case-history database. Note that these equations are based on experience in tunneling which are typically larger diameter and have higher stability confidence requirements than for underground mines in Nevada. Table 3-20: Support pressure equations evaluated for the Nevada underground gold mine case history dataset Author Barton et al. (1974)

Equation

variables

Comment

Q = Tunnel quality index

Primarily developed for tunneling

Jr = Joint roughness number Jn = Joint set number Bhasin and Grimstad (1996)

Q = Tunnel quality index B = Tunnel size in meters

Goel and Jethva (1991)

RMR = Rock Mass Rating B = tunnel diameter

For use in weaker rock masses Dependent on diameter

Short term tunnel roof support pressure

H = depth in meters

The equations presented in Table 3-20 were applied to geotechnical data acquired for the casehistory data set and contoured for best fit using various methods including, inverse-distance weighing, polynomial equations, and local weighted scatterplot smoothing (LOWESS). This

59 procedure gives an idea of what support pressure each equation predicts for geotechnical conditions encountered in underground gold mines in Nevada. Note that for Q based equations the support pressure was calculated using Q properties but was plotted against W-RMR76 mapped at each site. 3.6.2.2

Barton et al. (1974)

The Q system (Barton et al., 1974) and the equation presented in Table 3-20 were originally developed for support design in tunneling applications based on roughly 200 case studies. The system retains common use in tunnel support design and, given its case-history base in tunneling, the system is expected to report more conservative support pressures than required for underground mining in Nevada. Estimated required support pressure based on Q and RMR is presented on Figure 3-16 and Figure 3-17. Figure 3-17 was created by converting Q values to RMR using the formula RMR = 9LN (Q)+44 (Bieniawski, 1976). A plot of support pressure calculated from Q parameters at each site vs. mapped W-RMR76 is presented on Figure 3-18.

Support Pressure vs. Q and and Jiont Roughness (JR)

Support Pressure (tsf)

45 40 35

Joint Roughness (JR)

30 25 20

0.5-Slickensides

15

1-Smooth and Planer

10

1.5-Rough and Planer

5

2-Smooth undulating

0

3-Rough and Undulating 4-Discontinuous Joints

Q Figure 3-16: Estimated required support pressure (Barton et al., 1974)

60

Support Pressure vs. RMR76 and Joint Roughness (JR) 45

Support Pressure (tsf)

40 35

Joint Roughness (JR)

30 25

0.5-Slickensides

20 1-Smooth and Planer

15 10

1.5-Rough and Planer

5

2-Smooth undulating

0

RMR76

3-Rough and Undulating 4-Discontinuous Joints

Figure 3-17: Estimated required support pressure (Barton et al., 1974) based on RMR. Note RMR76 was calculated using RMR= 9ln(Q)+44 (Bieniawski, 1976)

61

Mapped Q Recommended Support Pressure vs. Mapped RMR76 Q Estimated Required Support Pressure TSF

30.0

25.0

20.0 y = 2E-06x4 - 0.0005x3 + 0.0435x2 - 1.6577x + 26.145 R² = 0.74 15.0

10.0

5.0

0.0 0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

Mapped RMR76

Figure 3-18: Barton et al. (1974) based estimated Q support pressure vs. mapped RMR76 for the Nevada underground mine case-history dataset

Figure 3-18 shows wide variation in recommended support pressure for a given W-RMR76 at WRMR76