Bre-Anne Sainsbury. Itasca Australia Pty Ltd, Melbourne, Australia & The University of New South Wales, Australia. Britt-Mari Stöckel. LKAB, Kiruna, Sweden.
Historical Assessment of Caving Induced Subsidence at the Kiirunavaara Lake Orebody
Bre-Anne Sainsbury Itasca Australia Pty Ltd, Melbourne, Australia & The University of New South Wales, Australia. Britt-Mari Stöckel LKAB, Kiruna, Sweden.
ABSTRACT A numerical assessment of historical, caving induced subsidence at the Kiirunavaara Lake Orebody has been conducted. Established displacement and strain-based criteria have been used to accurately simulate the evolution of ground surface displacements during a numerical simulation of production from 2003-2010.
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INTRODUCTION
The Kiirunavaara mine, owned and operated by LKAB, is located in northern Sweden near the township of Kiruna, approximately 180 km north of the Arctic Circle. The orebody is 4 km long, 80160 m thick and the mineralization reaches a depth of at least 2 km (LKAB, 2012). It dips 50° to 60° to the east and plunges to the north-northeast as the orebody lens tapers out. The orebody has been divided into two main regions of mining; the Main Orebody and the Lake Orebody. The Lake Orebody defines the northern extent of the mineralization and is located immediately west of the township of Kiruna (Figure 1). The township is located on the hangingwall of the mine.
Figure 1.
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Location and extent of the Lake and Main Orebody at the Kiirunavaara Mine.
HISTORICAL MINING RECORD
Kiirunavaara is a transverse, sub-level caving mine that commenced underground operation during the early 1960s after initially being mined as an open pit since the start of the 20th century. Production from the non-daylighting Lake Orebody commenced in 2003 when workings in the Main Orebody were on the 792 Level (550 m below ground surface). During the years 2003-2010, the lowest level of production from the Lake Orebody was the 792 Level, and in the Main Orebody on the 935 m Level (693 m below ground surface) as shown in Figure 1.
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EVOLUTION OF SURFACE SUBSIDENCE
Since sub-level caving commenced at the Kiirunavaara Mine, the hangingwall has experienced surface displacements ranging from millimetres up to several metres in magnitude. The development of caving induced subsidence around the Lake Orebody has been monitored through routine ground surveys since production commenced in 2003. The following section provides a summary of the observations and measurements. The observations are made in reference to cave subsidence zones that are described in Figure 2 and detailed below.
Figure 2.
(a) Main behavioural regions of caving induced subsidence (b) Caving induced subsidence at the Lake Orebody at 2010
Mobilized Zone / Crater — The portion of the orebody that has moved in response to the production draw and may be recoverable. Previous back analyses show that the mobilized zone is consistent with numerical displacements that are ≥ 1-2 m (Pierce et. al., 2006; Sainsbury et al., 2008; Sainsbury et al., 2010; Sainsbury et al., 2011). Caved material consists of irregular blocks of rock, ranging in size from millimetres to several meters. Yield Zone / Limits of Large Scale Fracturing — consists of an area in which the ground surface is broken and has large open tension cracks, benches, and rotational blocks. The rock mass has lost all of its strength and provides minimal support to the overlying rock mass. Sainsbury et al., (2010) report that a total strain criterion of 0.005 (0.5%) can be used to assess the limit of the large-scale surface cracks at the abandoned Grace Mine. This total strain criterion also has been used to calibrate the limit of large-scale fracturing at the El Teniente block cave mine in Chile (Cavieres et al., 2003) and Palabora Mine in South Africa (Sainbury et al., 2008). Zone of Continuous Subsidence —Located between the large-scale surface cracking zone and the undisturbed surface. Continuous surface subsidence, as defined by Brauner (1973), is the response of the rock mass to a mined void, which results in the formation of a gentle surface depression. Surface buildings, roads, underground power lines, railroads and other structures can be impacted by continuous surface subsidence. Sainsbury et al. (2010) report that the limit of measured small displacements at the abandoned Grace Mine can be defined by all areas of horizontal strain > 0.002 (0.2%) and angular distortion > 0.003 (0.3%). These strain criteria are based on the surface subsidence required to cause damage to a masonry structure during active subsidence (Singh, 1993).
3.1
Crater/Mobilized Zone
Approximately three years after mining commenced in the Lake Orebody, a crater developed on the northern extents of the existing open pit - as illustrated in Figure 3. Initially developed as an isolated subsidence feature during 2006, additional production during 2007 and 2008 caused the enlargement of the crater towards the south. The development of this isolated crater can best be described as a chimney or plug cave. Lupo (1997) completed a detailed review of the chimney subsidence features that occur east of the Main Orebody at Kiirunavaara, and suggested that they are formed when the flow channel of a sub-level ring reaches the ground surface.
Figure 3.
3.2
Development of crater at northern extent of Lake Orebody.
Limits of Large-Scale Fracturing / Yield Zone
The progression of the large-scale fracture limits at the ground surface between 1997 and 2006 is presented in Figure 4. An angle of break of approximately 60o has previously been reported by Villegas et al. (2011), Lupo (1996) and Stephansson et al. (1978). Surface disturbances in this zone have previously been documented by Lupo (1997) and consist largely of surface cracks, and shear displacements.
Figure 4.
3.3
Fracture mapping (a) plan of fracture evolution immediately above Lake Orebody (b) Section through Main Orebody showing fracture advance (modified after Villegas et. al., 2011).
Limits of Continuous Deformation
GPS data shows that an area of continuous deformation extends approximately 150-200 m beyond the limit of large-scale fracturing (Villegas et al., 2011). The measured surface displacements from 2002 – 2010 above the Lake Orebody are provided in Figure 5. Surface displacements up to 250 mm were measured immediately above the Lake Orebody prior to the commencement of mining in 2003.
Figure 5.
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Evolution of surface total displacement profile above the Lake Orebody
NUMERICAL SIMULATION OF CAVING INDUCED SUBSIDENCE
In recent years a numerical approach to the assessment of cave propagation has been developed in conjunction with the Mass Mining Technology (MMT) project (Sainsbury et al., 2008, Board and Pierce, 2009, and Sainsbury, 2010). The approach simulates cave propagation as a function of the specified draw strategy, evolving induced stress conditions and the simulated constitutive behaviour of the rock mass, without assumptions having to be made regarding cave shape and/or the rock mass dilation (bulking) response. Production draw is simulated within the numerical model by applying a small downward-oriented velocity to all gridpoints in the back of the undercut. This velocity is set low enough to ensure pseudostatic equilibrium throughout the model. Volumetric changes to the mesh (induced by the applied velocity) result in bulking/dilation of the rock mass and changes to the density and deformation modulus. By summing the reduction in mass within the model, production draw from the cave can be calculated. The numerical approach has been applied successfully to a number of large-scale back-analyses at the Palabora Mine (Sainsbury et al., 2008), the abandoned Grace Mine (Sainsbury et al., 2010), Henderson Mine (Sainsbury et al., 2011) and Northparkes Lift 2 (Pierce et al., 2006). To further validate the modelling methodology, a three-dimensional model of the Kiirunavaara Mine and its surroundings has been developed using the numerical modelling code FLAC3D (Itasca, 2009). The model has been developed to assess the impact of the production schedule on the evolution of the surface displacement profile that is evident today. The model extents are provided in Figure 6.
Figure 6.
Regional extents of model
4.1 In Situ Stress The pre-mining in situ stresses at the Kiirunavaara Mine have previously documented by Sandström (2003). The major principal stress is estimated to be aligned perpendicular to the orebody and is approximately 1.28 times the vertical stress. Equations for deriving the principal stress components are provided below in Equation [1]. Where ym is equal to the depth below -100 m RL. [1]
4.2
σ Η = σ ΕW = −0.37 ym − 3.7
;
σ h = σ NS = −0.28 ym − 2.8 ; σ v = −0.029 ym − 2.9
Rock Mass Properties
Historically, material properties for the Lake Orebody have been developed based on a calibrated response of drive scale displacements and failure mechanisms in the Main Orebody. Previous analyses conducted by Perman et al. (2011) have derived a lower bound property set for the hangingwall domain in the Main Orebody that is defined by a UCS 130 MPa, GSI 58, mi 16 and Erm 15.8 GPa. This GSI value has been confirmed for the Lake Orebody by scanline mapping conducted during 2010. A bi-linear, Mohr-Coulomb, Strain-Softening constitutive model has been used to simulate the complex process of the progressive failure and disintegration of the rock mass from an intact, jointed material to a bulked state during the caving process. The Mohr-Coulomb criterion is used to define the peak strength properties (cohesion and friction angle) through a least-squares fit to the Hoek-Brown failure envelope. The low magnitude in situ stresses in relation to the strength estimates suggest a gravity driven caving mechanism is dominant at Kiirunavaara.
4.3
Production Schedule
In order to ensure an accurate induced stress state in the model prior to the simulation of mining from the Lake Orebody, simulation of the extents of open-cut mining was conducted during the development of the initial model state. Production from the Main Orebody has been simulated based on an elevation and tonnes basis. Production from the Lake Orebody has been scheduled on a drawpoint and tonnes basis consistent with the technique described in Sainsbury et al. (2008).
4.4
Simulation Results
Based on the displacement and strain criteria outlined in Section 3, the development of historical caving induced subsidence from the Lake Orebody has been assessed. The simulated crater and limit of large-scale fracturing is presented in Figure 7 compared to the ground surface observations.
Figure 7.
Simulated evolution of crater and limits of large-scale fracturing
Mobilization of the ground surface above the Lake Orebody during 2006 is observed in the numerical model. The location of this initial break-through is on the northern extents of the existing open pit and is consistent with the in situ observations. As the mining simulation continues beyond 2006, the crater is observed to advance towards the south and joins with the main orebody crater during the production years 2007 – 2008. The development and enlargement of this crater can be attributed to the draw schedule at the mine, since it occurred immediately above an area where draw was occurring on multiple sub-level levels that overlapped each other (vertically) at the same time (Figure 8). The crater has formed when the flow channels of the sub-level rings have combined. The rapid propagation to the surface suggests that the secondary and tertiary flow channels have also contributed to the formation of the isolated crater.
Figure 8.
Production rate versus simulated mobilized zone
Simulated displacements on the ground surface immediately above the Lake Orebody from 2002 – 2010 are presented in Figure 9. The initial pre-mining displacements along with the development of the crater location and shape are consistent with the measured displacements.
Figure 9.
Simulated evolution of surface total displacements compared to that observed
The 1 m displacement contour has been used to define the crater limits within the numerical model at the end of 2010 (presented in Figure 10). The simulated limits compare well with the in situ observations based on their comparison to the visual observations presented in Figure 7. A total strain criterion of 0.5% was used to confirm the limits of large-scale fracturing that have an angle of draw consistent with approximately 60o. The simulated extent and shape also compares well to the fracture limits defined by Stöckel et al. (2012) presented in Figure 11. The limits of continuous subsidence at the end of 2010 have been derived by generating a contour line that encompasses all the areas of horizontal strain >0.2% and angular distortion >0.3% (Figure 10). It extends approximately 200 m beyond the limits of large-scale fracturing which is consistent with previous observations documented by Villegas et al. (2011).
Figure 10.
Plan view of simulated total displacement and strain-based subsidence criteria at the end of 2010.
Figure 11.
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Plan view of simulated subsidence limits at the end of 2010 compared to observations
CONCLUSIONS
The observed limits of caving induced subsidence at the Kiirunavaara Lake Orebody have been accurately assessed by a numerical simulation of sub-level caving from 2003-2010. Established displacement and strain-based criteria have been used to successfully predict the evolution of caving induced subsidence. The successful assessment of the current subsidence limits at the Kiirunavaara Lake Orebody provides a robust base model for a predictive analysis of future mining induced subsidence at the mine.
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ACKNOWLEDGEMENTS
The authors wish to acknowledge the members of the MMT II project for sponsoring this case study validation. In addition, the authors would like to thank Dr. Jonny Sjoberg of Itasca Consultants AB (Sweden) and LKAB for their efforts in compiling data and reviewing this work. My thanks to the Mining Engineering Department at the University of New South Wales and Professor Bruce Hebblewhite and Dr Rudrajit Mitra for supporting this research.
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REFERENCES
Board, M. and Pierce, M. (2009). A review of recent experience in modelling of caving. International Workshop on Numerical Modelling for Underground Mine Excavation Design, June 28, 2009. Asheville, North Carolina in conjunction with the 43rd US Rock Mechanics Symposium. Brauner, G. (1973) Subsidence Due to Underground Mining, U.S. Bureau of Mines, IC 8571. Cavieres, P., S. Gaete, L. Lorig and P. Gómez. (2003) Three-Dimensional Analysis of Fracturing Limits Induced by Large Scale Underground Mining at El Teniente Mine, in Soil and Rock America 2003 (Proceedings of the 39th U.S. Rock Mechanics Symposium, Cambridge, Massachusetts, June 2003), pp. 893-900. P. J. Culligan, H. H. Einstein and A. J. Whittle, Eds. Essen: Verlag Glückauf. Itasca Consulting Group, Inc. (2009) FLAC3D – Fast Lagrangian Analysis of Continua in 3 Dimensions, Ver. 4.0. User's Manual. Minneapolis: Itasca. LKAB (2012) accessed January 31, Lupo, J.F. (1996) Evaluation of Deformations Resulting from Mass Mining of an Inclined Orebody. Ph.D. thesis (unpublished), Colorado School of Mines, Golden, Colorado. Lupo, J.F. (1997). Progressive Failure of Hangingwall and Footwall at the Kiirunavaara Mine, Sweden. Int. J. Rock Mech. & Min. Sci. 34:3-4, paper No. 184. Pierce, M., P. Young, J. Reyes-Montes and W. Pettitt. (2006) Six Monthly Technical Report, Caving Mechanics, Sub-Project No. 4.2: Research and Methodology Improvement and Sub-Project 4.3, Case Study Application, Itasca Consulting Group Inc., Report to Mass Mining Technology Project, 2004–2007, ICG06-2292-1-T3-40, September. Perman, F & Sjoberg, J (2011) Numerisk analys av brytningssekvenser i block 19. LKAB Utredning 11-776, 2011-05-20 (in Swedish). Sainsbury, D., Sainsbury, B., Board, M. and Loring, D. (2011) Numerical back analysis of structurally controlled cave initiation at propagation at the Henderson Mine. ARMA, 11-321, San Francisco, USA. Sainsbury, D., Sainsbury, B. and Lorig, L. (2010) Investigation of caving induced subsidence at the abandoned Grace Mine. Mining Technology 2010 VOL 119 NO 3. Sainsbury, B., Pierce, M. & Mas Ivars, D. (2008) Analysis of Caving Behaviour Using a Synthetic Rock Mass —Ubiquitous Joint Rock Mass Modelling Technique, In Proceedings of the 1st Southern Hemisphere International Rock Mechanics Symposium (SHIRMS), Y. Potvin, J. Carter, A. Dyskin and R. Jeffrey (eds), 16–19 September 2009, Perth, Australia, Australian Centre for Geomechanics, Perth, Vol. 1 – Mining and Civil, pp. 243–254. Sandström, D. (2003) Analysis of the Virgin State of Stress at the Kiirunavaara Mine. Licentiate thesis 2003:02, Luleå University of Technology, Luleå, Sweden. Singh, U.K., O.J. Stephansson and A. Herdocia. (1993) Simulation of Progressive Failure in HangingWall and Footwall for Mining with Sub-Level Caving, Trans. Instn. Min. Metall., Sect A: Min. Industry, 102, A188-194. Stephansson, O., Borg, T. & Bäckblom, G. 1978. Sprickbildning i norra Kiirunavaaras hängvägg. Teknisk rapport 1978:51T, Avdelningen för Bergmekanik, Högskolan i Luleå. Stöckel, B-M., Sjoberg, J., Makitaavola, K. & Savilahti, T. (2012) Mining-induced ground deformations in Kiruna and Malmberget, EUROCK 2012, 28-30 May, Stockholm, Sweden. (in press). Villegas, T., E. Nordlund and C. Dahnér-Lindqvist. (2011) Hangingwall Surface Subsidence at the Kiirunavaara Mine, Sweden, Eng. Geol., doi:10.1016/ j.enggeo.2011.04.010.
