Geological investigation of the excavation-induced landslide in a geothermal area in Sumatra, Indonesia

Abstract

A geothermal power plant was planned to be constructed in mountainous and geothermal area in South Sumatra, Indonesia. The site is underlain by volcanic rocks and their decomposed soils in the Quaternary period. High cut slopes with a maximum height of about 80 m were required for the site formation works on a mountain ridge. The cut slopes were designed based on limited soil investigation data in the thick tropical rainforest. A small scale landslide took place immediately after the excavation started on one of the slopes. To find out the cause of the landslide, an additional soil investigation was carried out. The removal of the vegetation from the slope made an examination of rock conditions by a geologist possible. The additional investigation and examination revealed that there exist thick residual soils, expansive clay minerals related to hydrothermal alteration, discontinuities/faults with slickenside planes irregularly on the slopes, typically formed by geothermal activities under tropical conditions. A few potential landslide masses were also recognized on the slopes. A remedy slope design was carried out using newly obtained information for the entire slopes including rectification of landslides. The rectification work includes soil removal and installation of rock anchors at the upper part and foot of landslides respectively based on results of stability analysis. Because of complex geological features, we decided to resume the slope excavation after examination of the slopes and guidance by an on-site geologist and monitoring of ground using geotechnical devices. The slope design including slope angle and protection was altered in a timely manner once the adverse geology was encountered. After downtime for 9 months, the remaining slope excavation was safely completed. This paper presents the landslide feature related to geothermal geology and how it was rectified.