An alternative mitigation method for Gerandi-Ella reactivated landslide

Abstract

Gerandi-Ella, a former landslide area is currently under threat of reactivation. National Building Research Organization has implemented a surface and vertical well drainage system to stabilize it. However, vertical drainage system has failed,and an unexpected behavior of groundwater was observed due to presence of clay layers in the colluvium. An effective and economical alternative mitigation method is required to stabilize the area. A particular portion of the site,positioned such that it keeps the rest of the landslide mass intact, was chosen as the study area. Implementing a proper alternative mitigation methodto the study area may help to stabilize the landslide along with existing surface drainage system.Piezometric levels at two boreholes and rainfall data at the study area were analyzed. Stability analysis through the software SLOPE/W gave the current factor of safety as 0.965. A horizontal drain network with suitable design parameters was proposed to achieve a factor of safety of 1.206 by considering the effect of the matrix suction. A series of sand and gravel filled vertical holes capped with 100 cm clay layer are also proposed to expedite the water drawdown through impervious strata to horizontal drains.