VOLCANIC ERUPTION PRODUCTS AND ITS SLOPE STABILITY IN MERAPI VOLCANO, YOGYAKARTA

Banjarsari Village, of Cangkringan Sub District, Sleman Regency, Special Region of Yogyakarta (110°28’04” E, 7°39’11” S), in the geomorphological system, is known as the middle volcanic slope located in the southern area of Merapi Mount, as it is formed by the pyroclastic and lahar deposits. Such condition is attributed to the emergence of sand and stone quarries at many points. On this account, this research aims to identify the potency and type of mass movement on the sand-stone quarry deposits, in order to determine the best mitigation endeavor against the mass movement disaster. Methods of research in use are direct field observation, measurement of the type and position of the stratification, and slope dimension. Furthermore, soil mechanic laboratory analysis was done to the soil sample, consisting of bulk density of the soil (γ), cohesion parameter (c) and friction angle (Ø). The analysis of Safety Factor (FS) was done by using Geo-Slope version 9.0.3 software, aiming to define its stability class. This research resulted in 2 potential locations of a mass movement with debris slide type. According to the analysis of slope stability on the two location, it was revealed that these slopes were categorized into the unstable class with FS= 0.799, in the pyroclastic deposits and for the other location the slope was categorized into the critical classes with FS value = 1.117 in the lahar deposits. The main driving factor of mass movement was the increasing soil mass due to the effect of rainfall.