Various modern methods for testing soils are developed in recent years and as well stability analysis techniques improved. Various dynamic testing facilities invented that help us to take decision a soil slope either it is safe or unsafe under both static and dynamic loading. Before learning analysis techniques we have to realize the factors that lead to failure of a soil mass.
Failure of soil mass may occur along curved or plane surface. When a soil mass (considerably large) slides relative to remaining mass, failure is commenced. Usually an outward and essentially downward movement of soil mass is observed.
A slope is failed when forces acting on soil mass that result failure are greater than shearing resistance developed along critical failure surface. The factors that lead failure to slope of soil mass are of two categories:
a. Factors increasing shear stress
b. Factors reducing shear strength
a. Factors increasing shear stress:
The factors that increase shear stress exceeding shear strength of soil mass results slope failure. The stress may be increased due to following cases
1. Stresses exerted by water under saturation of soil mass
2. Surcharge loads
3. Seepage pressure
4. Steepening of earth slopes due to natural erosion or excavation
b. Factors reducing shear strength:
If the stresses applied are same but soil mass lose its shear strength, the slope is also failed. The shear strength can be lost due to
1. Increase in moisture content and pore water pressure
2. Cyclic loading or shock
The factors discussed above (two types) actually are of same sources i.e. when water content increases, stress is increased with simultaneous decrease in strength of soil mass and due to this natural slopes are commonly failed in rainy season.