Slope Failures in the Main Boundary Thrust Zone along Siang Valley between Pasighat and Rengging, East Siang District, Arunachal Pradesh

The low lying hilly terrain between Pasighat and Rengging along Siang Valley in Arunachal Himalaya has been affected by various types of slope failures. This region receives heavy rainfall during prolonged rainy season from May to August, when most of the failure takes. To address this problem wedge failure analysis has been carried out for various types of the rocks to determine the potential mode of failure. Wedge analysis shows that sandstones of Siwalik Group have three to four wedges while quartzites and sandstones of Miri Formation and Gondwana rocks have three and four wedges respectively. The rocks of Main Boundary Thrust zone are highly crushed and fractured with extensive development of slickenside. Deep cut erosion by small tributaries and Siang River has made the steep slopes more vulnerable to mass movement. The main slope failures are rock fall, debris fall, sliding and complex landslides.