Deterministic and probabilistic treatments of multiplanar pitwall failuresat Himmetoglu (Turkey) coal mine

This paper describes the investigation of a large and progressive slope fai lure in the south sidewall of the Himmetoglu coal mine in northwest Turkey. A number of slope instabilities leading to interruptions in mining have be en experienced at the mine. Geotechnical investigations consisted of struct ural mapping, observations of instabilities, review of detailed groundwater information, long-term monitoring of slope movement, and back-analysis of the various failure modes. Deterministic and probabilistic approaches based on the limiting-equilibrium method were employed to examine various slope profiles and excavation sequences for achieving better stability conditions . Analysis of the movement monitoring records and geotechnical information indicated that the failure of the south sidewall slope developed by a combi nation of sliding along the bedding planes in the hanging wall and faults i n the mode of multiplanar sliding. The movement history and the results fro m the back-analysis of the failures revealed that the stability of the slop e was highly sensitive to changes in the length of the lower part of the sl iding surface, and the shear strength reduced to residual values along the discontinuity surfaces 8-10 m above the coal seam at the time of failure. T he results of the analysis both from deterministic and probabilistic approa ches suggest that an excavation sequence consisting of a number of strippin g stages from the uppermost benches and advancing to the toe of the slope m ay provide the resistance to stabilize the overall slope. Simple and prelim inary economic assessments based on the overburden costs in conjunction wit h the deterministic and probabilistic approaches are briefly discussed.