Assessment of slope stability and monitoring of a landslide in the Koyulhisar settlement area (Sivas, Turkey)

The Koyulhisar settlement area is located in the seismically active zone of Kelkit valley. The settlement area has been adversely affected by active landslides as a result of which some of the houses have been evacuated. In this study, areas within the Koyulhisar settlement area where the landslides exist were delineated with a field mapping technique, and active landslides were investigated by means of field observations, drilling, sampling, field and laboratory testing, and in situ monitoring using an inclinometer. Based on the field studies, it is observed that a flyschoidal sequence of bedrock and colluvium consisting of clay and silt with some gravels comprise the main lithological units. Two areas affected by the landslides were identified. One of the landslide areas is local and there were no adverse effects on the houses. However, the other one was investigated in detail due to its adverse effect on some important governmental buildings. This is a slide with a non-circular failure surface caused by the existence of the flyschoidal sequence that forms a resistant bedrock below the colluvium. The inclinometer measurements reveal that the displacements are local and their velocities are generally less than 14 mm/year, indicating that the landslide is extremely slow. In addition, a high groundwater table with slow snow melt was observed as one of the major environmental parameters in the occurrence of the landslide. Nevertheless, the slope stability analyses reveal that groundwater lowering alone is not an effective measure in increasing stability and therefore either reinforcement should be applied to the entire active landslide area or the area should be abandoned.