3.2 Plane failure

A rock slope undergoes this mode of failure when combinations of discontinuities in the rock mass form blocks or wedges within the rock which are free to move. The pattern of the discontinuities may be comprised of a single discontinuity or a pair of discontinuities that intersect each other, or a combination of multiple discontinuities that are linked together to form a failure mode.

A planar failure of rock slope occurs when a mass of rock in a slope slides down along a relatively planar failure surface. The failure surfaces are usually structural discontinuities such as bedding planes, faults, joints or the interface between bedrock and an overlying layer of weathered rock. Block sliding along a single plane (Figure 2) represents the simplest sliding mechanism. Figure 3 shows a three dimension representation of such a type of failure. In case of a plane failure, at least one joint set strike approximately parallel to the slope strike and dips toward the excavation slope and the joint angle is less than the slope angle.



Figure 2: Typical view of Plane failure (A = Sliding plane, B = Slope face)

Figure 3: 3D view of plane failure along single discontinuities




Figure 4: Plane failure with condition of failures


The favorable conditions of plane failure are as follows:


The study of planar failure mechanism provides insight knowledge of the behaviour of rock slopes, and is particularly valuable for investigating the sensitivity of slope behaviour to variations in parameters such as shear strength of failure surfaces and groundwater conditions.