4.1.6 The tension crack is filled with water with upper slope angle
Compound slopes have appreciable angle with the horizontal () (figure 5). High slope formation has in generally a positive upper slope angle while the shorter slope has a negative slope angle. These types of slope are very common in complicated hilly terrains controlled by tectonics and geomorphic agents.
Figure 5: Compound slope with a positive upper slope angle
A simplified model consisting of a measured depth of water in the tension crack is shown in figure 6. It is assumed that the tension crack is vertical and is filled with water to a depth zw. Under this condition, the water pressure decrease linearly toward and exit at the toe of the slope. This pressure distribution results in a force V due to water filling in the subvertical discontinuity and an uplift force U due to water flowing at the surface between the block and its base.
It is considered that water enters the sliding surface along the base of the tension crack and seeps along the sliding surface, escaping at atmospheric pressure where the sliding surface daylights in the slope face. For slope stability analysis, a unit thickness of the slice is considered at right angle to the slope face.
Figure 6: Geometry of slope with tension crack in upper slope angle
Depth of tension crack,
Weight of unstable block,
Area of failure surface,
Driving water force,
Uplift water force,
Factor of safety =