**
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