CHARACTERISTICS OF A MULTIPLE RETROGRESSIVE FAILURE IN A COAL-MINE INSOUTHWEST TURKEY

This paper describes the geotechnical investigations and the stability analysis of a multiple retrogressive circular failure developed throu gh a few benches cut in deposits of a weak transition zone at a strip coal mining area, southwest Turkey. The instability was analyzed to un derstand the possible causes of failure and to determine mobilized she ar strength at the time of failure through the use of back analysis. G eotechnical investigations comprised geological mapping, review of dri ll core and groundwater data, sampling and laboratory testing, and bac k analysis of the failure. The failed material is essentially composed of poorly consolidated claystone, which is a clay in engineering term s, and minor amounts of clayey silts. To evaluate the shear strength p arameters mobilized during failure, drained soil shear tests were carr ied out on specimens taken from potential failure surface materials. B ased on the results of the back analysis performed for various conditi ons and using the held observations, actual failure surfaces were defi ned by successive slip surfaces developing from the toe and advancing to the crest of the slope. The analysis also indicated that the instab ility was initiated by the construction of steep and high benches in t he transition zone deposits. It was further noted that there is a good agreement between the laboratory-derived residual shear strength para meters and the results of back analysis. The results of the stability analyses performed to determine safe side slopes in the transition zon e are also presented.