Assessment of design parameters and the slope stability analysis of weak clay-bearing rock masses and associated spoil piles at Tuncbilek basin

Tuncbilek lignite basin situated in western part of Turkey has been increasingly excavated by open-pit and underground mining methods since 1940 to meet Turkey's energy demand. The blasted and then the excavated stratified rock masses in this basin consist mainly of clay-bearing rocks such as claystone, mudstone, siltstone, and marl. It was found that the strength and deformation properties of these clay-bearing rocks decrease with the increase in water content. In addition, time-dependent attenuations were also recorded in the mechanical properties due to disintegration caused by physical weathering processes. Therefore, considerable time-dependent instabilities were observed in the slopes left after open-pit mining activities. In this study, it was aimed to determine the design parameters of weak rock masses and spoil piles at Tuncbilek basin and then to recommend safe slope geometry for open-pit mining activities. For this purpose, the geotechnical properties of rock masses and spoil piles were obtained based on field tests as well as characterizations and back analyses of the instabilities. This study reveals that shear strength parameters of undisturbed clay-bearing rock masses are 200 kPa and 30 degrees. In addition, a cohesion of 150 kPa and an internal friction angle of 25 degrees should be used for slope stability analyses of both weakly cemented conglomerate-sandstone and disturbed clay-bearing rock masses.