INTERDISCIPLINARY INVESTIGATION OF DEEP-SEATED COMPLEX LANDSLIDE ON THE PONOROGO – PACITAN ROAD, EAST JAVA PROVINCE, INDONESIA

Abstract

Road transportation plays an important role in driving economic growth in remote areas. Pacitan Regency, located in the southwest corner of East Java Province Indonesia, is an area with low economic growth. One of road transportation infrastructures that reach Pacitan Region is the Ponorogo - Pacitan Roads. However, along this route, there are some mass movements. Previous investigations have successfully identified dozens of landslide and rockslide points, which can be classified into three types: planar rockslides, wedge rockslides, and complex landslides. One significant landslide has been observed at 226 km, with the potential to shift the highway. Geodetic survey in 2021 using aerial mapping methods showed that the landslide zone had moved by 20 cm over 3 years. This landslide not only buried the road under debris but also shifted a 200-meter segment of the road. Later investigations in 2023 utilized the resistivity geoelectric method to determine the slip plane of the landslide. The results of the geoelectrical investigation concluded that the slip plane is located approximately 30 meters beneath the existing road. Based on the initial and follow-up investigations, that landslide can be classified as a deep-seated complex landslide. Numerical stability analysis, both using limit equilibrium and finite element method, showed that the number of safety factors of slope stability was low to very low. Landslide mitigation using retaining pile methods has only a small effect on the factor of safety number and is very costly. Therefore, non-conventional methods are needed to mitigate the landslide.