CHARACTERISTICS OF BIPLANAR WEDGE SPOIL PILE INSTABILITIES AND METHODS TO IMPROVE STABILITY

The stability of spoil piles in surface coal mines is an important fac tor in the successful exploitation of coal reserves. Spoil pile instab ility must be avoided not only to ensure negligible risk to both perso nnel and equipment but also to prevent increased production costs. Sin ce mining began at the Eskihisar strip coal mine, southwest Turkey, st ability problems have been experienced in both spoil piles adjacent to highwalls (slices) and outside dumping areas. This paper outlines the results of field and laboratory investigations performed to describe the causes and mechanism of pile instabilities threatening production along the highwalls. Back-analysis of failures and monitoring data hav e revealed that failures occur along planar surfaces that define two-w edge shaped blocks. In addition, shear strength of the moist underclay at the base of piles and changes in the floor dip are the controlling factors in the development of failures. It is also noted that sharp d ecreases in shear strength after peak values for the underclay indicat ed that weak basal planes exhibit a shear strength at or approaching t he residual value at the time of failure. Results show that failures a re most likely initiated along the spoil - natural ground interface an d propagate into the spoil mass. Three possible remedial measures have been evaluated by carrying out two-dimensional stability analyses, an d the most effective method for improving spoil pile stability appeare d to be spreading of a rockfill material on the floor after coal produ ction.