4.1 Planar Failure Analysis

Planar failure of rock slope occurs when the mass of rock in a slope slides down along a weak plane. There are number of conditions that must exist in a rock slope for planar failure to occur. First, the failure plane must be sub-parallel to the slope face (strike within 20 degrees to the slope face). Secondly, the failure planes must daylight into the slope face (dip than the slope angle of face) and intersect the face above the toe of the slope. Stability analysis for planar failure requires the resolution of forces perpendicular to and parallel to the potential failure surface.  This includes the shear strength along the failure surface, the effects of pore-water pressures, and the influence of external forces such as reinforcing elements or seismic accelerations. The different condition of planer failure is explained in section 4.1.1 to 4.1.7.

4.1.1 Sliding analysis of a block

Figure 1 shows a solid block resting on a sliding plane. The block will try to move due to gravity along the slope plane inclined at an angle θ from horizontal. However, resistance between base of the block and surface of the slope will prevent slippage of the solid block. Weight of the block W is acting vertically downward (figure 1), which can be resolved in normal as Wcosθ and in shear as Wsinθ. The stability of block against slippage is defined as ratio of shear strength to shear stress. The block is considered to undergoes slippage along the plane for the value of ratio < 1, else it is stable.

Figure 1: Geometry of slope, a block is rest on a slope having angle θ