TRANSACTION PAPER

 

Engineering application of thrust block analysis in slope stability problems in open pit mines

 

 

K.N. Karparov^I; M.F. Handley^II

^IAgricultural Engineering Services, National Department of Agriculture
^IIDepartment of Mining Engineering, University of Pretoria

 

 

---

SYNOPSIS

The classical limit equilibrium methods are suspected to be ineffective in predicting the potential for highwall failure in many instances in coal mines around the world. Slope engineers have for many years recognized a block thrust failure mechanism for slope failures, but little work had been done before 2000 to explain the actual mechanisms which must be responsible for the failure. The main reason for this is that limit equilibrium methods implicitly assume rigid blocks, and the resulting force equations must be satisfied everywhere simultaneously for them to have any meaning. This paper will show that the material involved in the slope failure is not rigid; indeed it undergoes considerable permanent deformation during failure. This observation allows the authors to treat the block boundaries independently, because they need not maintain a constant spatial relationship with one another, as is assumed in other methods. To enable analysis of this type the authors assume that the weight of the blocks is evenly distributed. This is reasonable, because the slope material is not strong enough to be self-supporting without some sort of constraint, or strong enough to be able to apply point loads to the surrounding material. The purpose of this paper is to extend the practical application of the analytical method developed from mechanism studies to provide an objective assessment of the risk of slope failure, and therefore guidelines for more stable slope designs. This work is based on studies carried out on two failures in an open pit coal mine in South Africa, and provides a methodology to assess the potential for failure more objectively than is possible with currently accepted methods, while at the same time remaining sufficiently simple to allow a 'back of the cigarette box' assessment by geotechnical engineers on site.

---

 

 

“Full text available only in PDF format”

 

 

References

1. HOEK, E. and BRAY, J.W. Rock slope Engineering. The Institute of Mining and Metallurgy, London, 1981.         [ Links ]

2. CANMET. Pit Slope Manual, Mining research Laboratories, CANMET, Canada 1997.         [ Links ]

3. HARRIES, N. Personal communication at the International Symposium on Slope Stability, Cape Town, 4-7 March 2006.         [ Links ]

4. STURMAN, J.M. Influence of slope stability on economics of opencast coal mining in the east and north midlands of England. Second International Surface Mining and Quarrying Symposium, 4-6 Oct. 1983, Bristol, UK, 1984. pp. 217-224.         [ Links ]

5. SINGH, T.N. and SINGH, D.P. Assessing stability of voids in multi seam opencast mining. Colliery Guardian, July 1992, pp. 159-164.         [ Links ]

6. MALGOT, J., BALIAK, F., and MAHR ,T. Prediction of the influence of underground coal mining on slope stability in the Vtacnik mountains. Bulletin of the International Association of Engineering Geology, vol. 33, 1986. pp. 57-65.         [ Links ]

7. BOYD, G.L. Geomechanics research applied to open strip coal mining in Australia. Second International Surface Mining and Quarrying Symposium, 4-6 Oct. 1983, Bristol, UK, 1983. pp. 193-204.         [ Links ]

8. SKEMPTON, A.W. and HUTCHINSON, J.N. Stability of natural slopes and embankment foundations, State-of-the-art report, Proc 7th Int Conf SMFE, Mexico City, 2, 1969. pp. 294-335.         [ Links ]

9. STEAD, D. and SCOBLE, M.J. Rock slope stability assessment in British surface coal mines. Second International Surface Mining and Quarrying Symposium, 4-6 Oct. 1983, Bristol, UK, 1983. pp. 205-216.         [ Links ]

10. ANDERSON, M.G. and RICHARDS, K.S. Slope stability-geotechnical engineering and geomorphology. Reprinted, John Wiley & Sons. 1992.         [ Links ]

11. BROMHEAD, E.N. The Stability of Slopes. Surrey University Press, Chapman and Hall, New York, 1992. pp. 109-165.         [ Links ]

12. WRIGHT, S.K., KULHAWY, F.H., and DUNCAN, J.M. Accuracy of equilibrium slope stability analysis. J. of the Soil Mech. and Found. Div., ASCE, vol. 99, no. SM10, 1973. pp. 783-791.         [ Links ]

13. FELLENIUS, W. Calculation of the stability of earth dams. Trans. 2nd Congr.on Large Dams, Washington, vol. 4, 1936. pp. 445-459.         [ Links ]

14. BISHOP, A.W. The use of the clip circle in the stability analysis of earth slopes. Geotechnique, vol. 5, 1955. pp. 7-17.         [ Links ]

15. LOWE J. and KARAFIATH L. Stability of earth dams upon draw down. Proc.1st Pan-Am. Conf. Soil Mech. Found. Eng., Mexico, vol. 2, 1960. pp. 537-560.         [ Links ]

16. JANBU, N. Stability analysis of slopes with dimensionless parameters. Harvard University Soil Mech. Series, no. 46. 1957.         [ Links ]

17. MORGENSTERN, N.R. and PRICE, V.A. The analysis of the stability of general slip surfaces. Geotechnique, vol. 15, 1967. pp. 79-93.         [ Links ]

18. SPENCER, E.E. A method of the analysis of the stability of embankments assuming parallel inter-slice forces. Geotechnique, vol. 17, 1967. pp. 11-26.         [ Links ]

19. TURNBULL, W.J. and HVORSLEV, M.L. Special problems in slope stability, ASCE, J. Soil Mech. Fdnt. Div., 93, (SM4), 1967. pp. 499-528.         [ Links ]

20. WHITMAN, R.V. and BAILEY, W.A. Use of computers for slope stability analysis, ASCE, J. Soil Mech. Fdnt. Div., 93, (SM4), 1967. pp. 475-498.         [ Links ]

21. SRK. Failure analysis of the North Pit Failure, Colliery A-1. Unpublished internal mine report. 1995.         [ Links ]

22. CANADY, I.I. Personal communication. Head of failure slope profiling team, 1999. 2006.         [ Links ]

23. MATTUSHEK, M. Personal Communication. Divisional Geologist, New Vaal Colliery, South Africa. 2005.         [ Links ]

24. CAIRNCROSS, B. Paleodepositional environments and tectono-sedimentary controls of the postglacial Permian coals, Karoo Basin, South Africa. International Journal of Coal Geology, vol. 12, 1989. pp. 365-380.         [ Links ]

25. SNYMAN C.P., and BARCLAY J. The coalification of South African Coal. International Journal of Coal Geology, vol. 13, 1989. pp. 375-390.         [ Links ]

26. VOIGHT, B. Beziehung zwischen grossen horizontalen Spannungen im Gebirge und der Tektonik und der Abtragung. 1st Congress Int. Soc. Rock Mech., Lisbon, 1966, vol. 2, pp. 51-56.         [ Links ]

27. GAY, N.C. In situ stress measurements in Southern Africa, Tectonophysics, vol. 29 1975. pp. 447-459.         [ Links ]

28. GAY, N.C. The state of stress in a large dyke on E.R.P.M., Boksburg, South Africa. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., vol. 16, 1979. pp. 179-185.         [ Links ]

29. STACEY, T.R. and WESSELOO, J. In situ stresses in mining areas in South Africa. Jour. S. Afr. Inst. Min. and Metall. vol. 98, no. 7, 1998. pp. 365-368.         [ Links ]

30. SELLERS, E.J., COETZER, S.J., and KAMSTRA, R. Understanding and Determining the Variability of the Primitive Stress Environment. SIMRAC Research Project GAP707, Department of Minerals and Energy, Pretoria, March 2002.         [ Links ]

31. VAN DER MERWE, J.N. Personal communication, Department of Mining Engineering, University of Pretoria. 2002a.         [ Links ]

32. VAN DER MERWE, J.N. Horizontal stress: The root of all evil?, Proc. 19^th Conf. on Ground Control in Mining, Teheran, Iran. 2002b.         [ Links ]

33. HOEK, E. and BROWN, E.T. Underground Excavations in Rock. The Institution of Mining and Metallurgy, London, 1980. p. 100.         [ Links ]

34. KARPAROV, KN. Slope Stability Analysis in Complex Geotechnical Conditions-Thrust Failure Mechanism. PhD Thesis submitted for examination to the University of Pretoria, Pretoria, South Africa, August 2006.         [ Links ]

35. ITASCA CONSULTING GROUP. FLAC2D: Fast Lagrangian Analysis of Continua. Minneapolis, Minnesota, USA. 1999.         [ Links ]

36. BARTON, N.R. A Model Study of the Behavior of Excavated Slopes. Ph.D. Thesis, University of London, Imperial College of Science and Technology. 1971.         [ Links ]

37. RIEDEL, W. Zur mechanik geologisher brucherscheinungen. Zentralblatt fur Mineralogie, Geologie und Paleontologie B, (Abhandlung). 1929. p. 354-368.         [ Links ]

38. LADE, P.V., COLE, D.A., and CUMMINGS, D. Multiple failure surfaces over dipslip faults. Journal of Geotechnical Engineering, vol. 110, no. 5, May 1984, 1984. pp. 616-627.         [ Links ]

39. SYLVESTER, A.G. Strike-slip faults. Bulletin of the Geological Society of America, vol. 100, 1988. pp. 1666-1703.         [ Links ]

40. VERMEER, P.A. and DE BORST, R. Non-associated Plasticity for Soil, Concrete and Rock, Heron, vol. 29, no. 3, Delft University of Technology, Delft, The Netherlands. 1984.         [ Links ]

41. ORTLEPP, W.D. Rock Fracture and Rockbursts: an illustrative study. Monograph Series M9, South African Institute of Mining and Metallurgy, Johannesburg. 1997.         [ Links ]

42. GAMMOND, J.F. Displacement features associated with fault zones: a comparison between observed examples and experimental models. Journal of Structural Geology, vol. 5, 1983. pp. 33-45.         [ Links ]

43. JAEGER, J.C. and COOK, N.G.W. Fundamentals of Rock Mechanics, 3rd Ed. New York, Chapman and Hall. 1979.         [ Links ]

44. KOVARI, K. AND FRITZ, P. Special contribution: Slope stability with plane, wedge and polygonal sliding surfaces. Symposium on Rock Mechanics Related to Dam Foundations, Rio de Janeiro. 1978.         [ Links ]

45. BAER, G. Mechanisms of dike propagation in layered rocks and in massive porous sedimentary rocks. Journal of Geophysical Research 96, vol. 11, 1991. 911-11, 929.         [ Links ]

46. NARR, W. and SUPPE, J. Joint spacing in sedimentary rocks. Journal of Structural Geology, vol. 13, 1991. pp. 1037-1048.         [ Links ]

47. GROSS, M., FISCHER, M., ENGELDER, T., and GREENFIELD, R. Factors controlling joint spacing in interbedded sedimentary rocks; integrating numerical models with field observations from the Monterey Formation, USA. Ameen, M.S. (ed.). Fractography. Geological Society Special Publication, Geological Society, London, 1995. pp. 215-233.         [ Links ]

48. BECKER, A. and GROSS, M. About the Dugdale crack under mixed mode loading. Int J Fracture, 37. 1988.         [ Links ]

49. JI, S. and SARUWATARI, K. A revised model for the relationship between joint spacing and layer thickness. Journal of Structural Geology, vol. 20, 1998. pp. 1495-1508.         [ Links ]

50. HELGESON, D.E. and AYDIN, A. Characteristics of joint propagation across layer interfaces in sedimentary rocks. Journal of Structural Geology, vol. 13, 1991. pp. 897-991.         [ Links ]

51. TSANG, Y.W. The effect of tortuosity on fluid flow through a single fracture. Water Resources Research, vol. 20, 1984. pp. 1209-1215.         [ Links ]

52. ITASCA CONSULTING GROUP. FLAC2D. Fast Lagrangian Analysis of Continua, 1999.         [ Links ]

53. HOEK, E. Slope stability analysis. Lecture at Santiago Technical University, Chapter 7-A: Slope stability problems in Hong Kong, 1986. pp. 92-104.         [ Links ]