Engineering geological characterization of coal mine waste material and an evaluation in the context of back-analysis of spoil pile instabilities in a strip mine, SW Turkey

Coal mine waste material, which usually consists of a mixture of coarse-grained particles to rock fragments grading to fine-grained particles, causes geotechnical and environmental problems on disposal. A study of the geotechnical aspects of the problem is, therefore, important in rational planning for the disposal, reclamation, treatment and use of such material. pile instability has been a continuing problem in the Eskihisar strip coal mine of southwestern Turkey. Particularly shallow-seated circular failures appearing along the haul road and instabilities covering large agricultural areas outside the dumping yard increase the importance of spoil pile stability. The fact that the height of spoil piles dumped by trucks on both sides of the haul road will increase much more in the future has focused the attention of the miners to this problem. This study outlines the geotechnical characteristics of waste material from the Tertiary coal-bearing rocks derived from field and laboratory investigations, and describes the causes and mechanism of spoil instabilities which threaten the safety of the haul road and the agricultural areas in the vicinity of the pit. ion results suggest that fines and coarser material were approximately equally represented in the spoil. The percentage of fines is indicative of material degradation as evidenced by slaking tests. Site observations and numerous back analysis of investigated failures reveal that the failures do not penetrate the foundation material and only occur along circular surfaces through the spoil material. It is also noted that surface water infiltration, reducing the effective shear strength of the spoil material, contributes to failures. Shear test results indicate that the spoil is a nearly cohesionless material with a residual internal friction angle of about 33°, which confirms the values derived from standard penetration tests. It is also noted from back analyses that the spoil material exhibits a shear strength at or approaching the residual value at the time of failure. The use of shear strength parameters defined by the linear Mohr-Coulomb envelope yields results that are comparable with those obtained when the non-linear failure envelope is used, provided that such strength parameters were defined at the operating normal stress levels on the failure surfaces.