Analysis of ground shear strain and site class on Wonogiri Dam and surrounding area, Central Java

Abstract

Wonogiri Dam is one of the dams in Indonesia, located about 2 km south of the Wonogiri City, in the province of Central Java. Wonogiri Dam has a volume±400 million m3 with a surface area of 73,6 km2 which has many benefits such as for agriculture, tourism, and power plant. Wonogiri is a part of the East Java Southern Mountain Zone. Based on the geological map, there are many rocks types found at Wonogiri, such as volcanic breccias, andesite lavas, tuffs, and many igneous intrusive rocks. This research aimed to determine Ground Shear Strain (GSS) value along the main dam and the surrounding area and to predict phenomena based on GSS values. From the dominant period (TG) value, we can predict site class based on the classification of Zhao. The microtremor data was taken on 31 observation points which were divided from 14 observation points with 100 m spacing in the main dam and 17 observation points with 1.000 m spacing in the surrounding area from Wonogiri Dam. Microtremor data was analyzed using Horizontal to Verctical Spectral Ratio (HVSR) method to get the amplification factor and dominant frequency. Those data were used to determine the seismic vulnerability index (Kg). Kanai empirical equation was used to calculate Peak Ground Acceleration (PGA) value based on earthquake event on 27th of Mei, 2006 with a moment magnitude of 6,3 and 11,8 km depth. Seismic vulnerability index and peak ground acceleration value are used to determine the GSS value. The results of this research showed that GSS value in the research area was about 3,4 × 10−4 to 7,2 × 10−3. The phenomena that can happen were ground movement as the form of surface cracking and settlement. Seismic vulnerability index was below 20 (0,35 to 16,97) which means the surrounding area from Wonogiri Dam and main dam have low-risk, but the potential for damage to the building is still possible. Along the main dam area, we need to notice the point at 400m and 600m. The value of the dominant period in the research area ranged from 0.07 to 1.11 second. Based on the classification of Zhao, there are three classes of site class in the research area, that are site class I which consist of rock/ stiff soil, site class II which consist of hard soil, site class IV which consist of soft soil.