LAUSR

Abstract In February 2011, two landslides in which 11 people were killed, occurred in the Collolar open pit mine, situated in the Elbistan Basin in eastern Turkey. In the coalfield, a thick sequence of clay (>100 m) overlies karstic limestone bedrock. These are overlain by deposits consisting of a 20–50 m thick lignite sequence and a 20–50 m thick gyttja sequence. The gyttja is in turn overlain by Quaternary deposits. In this study, we aimed to shed light on the geologic factors that led to the instability and on the precise trigger for the failure. Therefore, we focused on the above Plio-Quaternary lacustrine and fluvial sediments and applied geomorphological, sedimentological, and geotechnical analyses. After establishing the geomorphology of the coalfield and the landslides, we analyzed 38 samples from existing drill cores, 10 surface samples, and 35 undisturbed samples from new boreholes. Our results indicate that the February 2011 landslides were caused by the liquefaction of the organic-rich “seekreide” (low plasticity, Ca-rich marls) layers at the transition between gyttja and unconsolidated, fine-grained Quaternary sediments. We inferred that the trigger for the landslides might be related to changes in the groundwater flow dynamics in and around the coalfield, and that the shaking caused by the first landslide might have contributed to triggering the second landslide. In brief, we conclude that the second landslide at the Collolar coalfield was a unique example of a landslide that did not have typical rotational, spoon-shaped failure planes and retrogressive progression, as previously proposed.