INCORPORATING SIDE RESISTANCE IN 3D STABILITY ANALYSIS OF TRANSLATIONAL LANDSLIDES

Abstract

Most stability analyses are two dimensional and do not consider 3-D side resistance of the slide mass. Using 3-D analyses makes the stability problem closer to ground conditions and is critical for complex slope geometries. 2-D analyses result in favorable and conservative results while designing, but works contrariwise for back calculations, where back calculated shear strength parameters are overestimated. Hence considering the 3-D side resistance is essential for back calculation of shear strength and design of remedial measures for failed landslides. Currently commercially available softwares do not consider the side resistance offered by the sides of the translational slide mass (Stark and Eid 1998). The factor of safety is same for 3D analysis as it is for 2D analysis. Hence side resistance offered by the sides of the slide mass needs to be incorporated in slope model. Different researchers have proposed different methods. Stark and Eid (1998) proposed an imaginary layer that is covering the slide mass. This layer has friction angle of zero and cohesion given by earth pressure at rest. Arellano and Stark (2000) proposed the application of horizontal and vertical force acting on sides at centroid location. This force being equal to the earth pressure force at-rest. But these procedures are difficult to use in modelling. Akhtar and Stark (2011) proposed a simpler procedure, where a slight inclination of the sides of the slide mass was suggested to incorporate the side resistance of the slope. The software in this way considers the sides of slope using the base of columns in the inclined sides of the slide mass. This approach is further supported by this research which advocates that slight inclination of slide mass, 3-degree ~ 8-degree, provides sufficient resistance that takes account of the effect of side resistance provided by the slide mass. This value also conforms to results of finite element and finite difference approach. Hence slight inclination applied to sides of the slide mass is easier approach and can be calculated using the relation suggested in this research. The required resistance to the slide mass in analysis calculations is provided by the area of the inclined sides. Steeper slopes require more side resistance area to incorporate the side resistance of the slide mass and thus steep angles of the sides and gentler slopes require less side resistance area and hence gentle angles of the sides.