Slope movement characteristic and models based on engineering-geological properties of the Menoreh Hills, Purworejo Regency, Central Java, Indonesia

Abstract

The research area is situated in Menoreh Hills of the Kulonprogo Zone. Geologically the zone is mainly developed by volcanic rocks, of Tertiary era. The stratigraphy from the oldest rock unit comprises Kaligesing and Dukuh formations, overlain by Sentolo Formation, and Quaternary deposits. Sentolo Formation is mainly composed of bedded limestone, while the Quaternary deposits include alluvial of fluvial sedimantations. Volcanic rocks of Kaligesing and Dukuh formations consist of lava, andesite pyroclastic breccia, tuffaceous breccia, as well as laharic breccia, and andesite intrusion. In general, the volcanic rocks are subjected to quite intensive joint extension. Some lateral slip faults are discovered with an over-all pattern of Northeast - Southwest and Northwest - Southeast. The lithology developing the Menorh Hills, especially volcanic rocks are undergone a very massive weathering. This process forms soil with a thickness that can be greater than 4 meters. Physical properties of the fresh volcanic rocks, described in the value of friction angle (φ) ranges 40° - 65°, cohesion force (c) 15 kg/cm2 - 40 kg/cm2. On the other hand, friction angle, φ of the soil varies 3o - 24°, with c=2.5 kg/cm2 - 8 kg/cm2. The geological setting and rock properties of the Menoreh Hills generate particular characteristics of landslides. Slope movements in the Menoreh Hills are in general characterized by involvement of soil or debris, saturated water conditions, value of friction angle, φ less than 25°, inclination slope is greater than 20° (36%), and thickness of soil is >3 meters. Finally, mass movements of the Menoreh Hills can be classified into three types, containing rotational mass movement, soil slide through the boundary plane between fresh and weathered rocks, and wedges failure alongside joint intersections act as the slip plane.