Reconstruction of Retrogressive Landslides Using Integrated Technique of Geoelectric Resistivity and Borehole Logging in Sumampir, Purbalingga Regency, Central Java

Abstract The landslide occurred in Sumampir of Purbalingga Regency has reached 120 m in length and 50 m in width. This landslide is suspected to be a retrogressive landslide, which was manned by a landslide associated with the cliffs of the Brangkal River. This landslide is interpreted as progressive development of a landslide associated with Brangkal river bank failures, which continues to enlarge to the dimensions as measured currently. The main active landslide has a steep scarp of along 60 m with a height of up to 8 m, with several transverse tensile cracks were found in between the scarp and above the crown. These tensile cracks have caused partial subsidence on the side of the village road that crosses it. In order to determine the existence of slip surface that have developed since the first landslide formed until the last one, an integrated technique of geoelectric resistivity and borehole logging has been applied. This integrated technique was then used to reconstruct the landslide progression retrogressively towards the top. The results of the geoelectric resistivity analyses indicated that there are four stages of slip surface formation. The first slip surface is located at 80 m south of the village road, which is defined to be the initial of a retrogressive landslide. The second slip surface located at 40 m from the village road is possible to be a former landslide damaged the prior village road. The third slip surface is formed at a distance of 15 m from the village road, which is a landslide considered to have caused the bridge to collapse. The fourth slip surface is located at the side of the village road, which is the last landslide causing the concrete slab of the bridge moved 15 m to the south. The average depth of the slip surface is about 8 m. This depth has been confirmed using borehole logging data, by obtaining a relatively hard weathered claystone layers at a depth of 6-8 m.