Abstract
A dam is a construction built to hold the flow of water and creating a reservoir, one of which is the Tukul dam, administratively located in Karanggede Village, Arjosari District, Pacitan Regency, East Java Province. Morphologically, the study area consists of hills that are flanked by steep valleys, the Brungkah watershed, with slope classified as steep to very steep. Based on Map of Forecast of Potential Areas for Soil Movement in Pacitan Regency, East Java Province in February 2017 [1], the Tukul dam is included in the medium to high ground potential movements zone. It indicates that in the study area, there is a potential for ground movement if slope disturbance occurs in case the slopes are cut. Therefore, in the planning of slope stability in dam construction, it is necessary to analyze and investigate the slope geometry stability in dam reservoirs in detail so the movement of soil that is harmful to Tukul Dam construction can be avoided. Analysis of the stability of the dam reservoir slope is performed using the limit equilibrium analysis method of the bishop simplified method. The result of geological research on the surface shows that the morphology of the slope is in tilted to very steep conditions. The constituent lithology in the study area consists of volcanic breccia, tuffaceous sandstone, and andesite porphyry intrusion. Geological structures found in the field are dextral strike-slip faults and joints. Poor rock mass quality with GSI value of 21-40 dominating the quality of rock mass in the study area, and in several locations, landslides were also discovered. The reservoir stability analysis of the slope geometry was carried out in the spillway, main dam, and inundation areas to determine the safety factor (Fs) value. Under normal conditions, the safety factor (Fs) of each analysis has a safety factor value of > 1.5, which indicates that the slope is in a stable (safe) condition. However, the slope in the spillway area has a safety factor value <1.5 due to the influence of landslides or slope collapse in the zone. This value signifies that the stability of the said slope is in an unstable (unsafe) condition.
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