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 coars e-grained particles to rock fragments grading to fine-grained particle s, causes geotechnical and environmental problems on disposal. A study of the geotechnical aspects of the problem is, therefore, important i n rational planning for the disposal, reclamation, treatment and use o f such material. Spoil 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 ins tabilities covering large agricultural areas outside the dumping yard increase the importance of spoil pile stability. The fact that the hei ght of spoil piles dumped by trucks on both sides of the haul road wil l increase much more in the future has focused the attention of the mi ners to this problem. This study outlines the geotechnical characteris tics of waste material from the Tertiary coal-bearing rocks derived fr om field and laboratory investigations, and describes the causes and m echanism of spoil instabilities which threaten the safety of the haul road and the agricultural areas in the vicinity of the pit. Gradation results suggest that fines and coarser material were approximately equ ally represented in the spoil. The percentage of fines is indicative o f material degradation as evidenced by slaking tests. Site observation s 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 s urface water infiltration, reducing the effective shear strength of th e spoil material, contributes to failures. Shear test results indicate that the spoil is a nearly cohesionless material with a residual inte rnal friction angle of about 33 degrees, which confirms the values der ived from standard penetration tests. It is also noted from back analy ses that the spoil material exhibits a shear strength at or approachin g 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 e nvelope is used, provided that such strength parameters were de fined at the operating normal stress levels on the failure surfaces.