Wedge Failure Analysis of Slopes in Highly Jointed Rock Masses in the Zones of High Seismicity

Wedge failure is the predominant failure mode in slopes with highly jointed rock mass. The orientation of the joints of the rock mass governs the stability of the slopes depending upon their orientation relative to the orientation of the slope. Extensive slope stability analyses were carried out for the slopes of Chenab bridge. These analyses pointed out that the stabilization measures need to be designed against the wedge failures. This paper presents the various special considerations of the wedge failure analyses like application of the seismic accelerations, load cases considered for analysis and factors of safety required. A summary of the various analyses and important conclusions drawn there from are presented. Some of the observations like the load case with higher factor of safety demanding higher anchor force and foundation loads improving the factor of safety are counter intuitive. Since sliding of the wedge either along a joint plane or along the line of intersection of two joint planes is like a block sliding on a plane, using the sliding block analogy, the reasons for the specific observations noted during the detailed wedge failure analyses of Chenab bridge slopes are explained. The study lays emphasis on the anchor force or stabilizing force required in the analysis and design over factor of safety since the design is always governed by the former. The study recommends special considerations for the wedge failure analysis in highly seismic areas like the direction of seismic accelerations, designing for MCE condition, analysis during construction stage, etc.