3.3 Wedge Failure:

Wedge failure of rock slope results when rock mass slides along two intersecting discontinuities, both of which dip out of the cut slope at an oblique angle to the cut face, thus forming a wedge-shaped block (Figure 5 & 6). Wedge failure can occur in rock mass with two or more sets of discontinuities whose lines of intersection are approximately perpendicular to the strike of the slope and dip towards the plane of the slope. This mode of failure requires that the dip angle of at least one joint intersect is greater than the friction angle of the joint surfaces and that the line of joint intersection intersects the plane of the slope.

Figure 5 : Typical view of wedge failure (A = wedge block)

Figure 6: 3D figure of wedge failure with dip and dip direction.

 

Depending upon the ratio between peak and residual shear strength, wedge failure can occur rapidly, within seconds or minutes, or over a much longer time frame in the order of several months. The size of a wedge failure can range from a few cubic meters to very large slides from which the potential for destruction can be enormous. The formation and occurrence of wedge failures are dependent primarily on lithology and structure of the rock mass (Piteau, 1972). Rock mass with well defined orthogonal joint sets or cleavages in addition to inclined bedding or foliation are generally favorable situations for wedge failure. Shale, thin-bedded siltstones, claystones, limestones, and slaty lithologies tend to be more prone to wedge failure development than other rock types. However, lithology alone does not control development of wedge failures. Figure 7 shows a photograph of wedge failure obtained in field. The necessary structural conditions for this failure are summarized as follows:

         The trend of the line of intersection must approximate the dip direction of the slope face.

         The plunge of the line of intersection must be less than the dip of the slope face. The line of intersection under this condition is said to daylight on the slope.

         The plunge of the line of intersection must be greater than the angle of friction of the surface.

 

Figure 7: Occurance of Wedge failure in a slope