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failure of a slope certainly leads to big and undesired loss of life and
property. Thus it is extremely important to check and analyze the stability of
proposed slopes. With the development of modern efficient techniques of testing
of soils and stability analysis, a safe & economical design of slope can be
successfully made. Hence, a geotechnical engineer efficiently should have
through and reliant knowledge/ideas of all those existing accessible methods for
checking the stability of slopes and their limitations.Basis of slope failure
analysis

The soil mass must be safe against slope failure on any conceivable surfaces
across the slope. Even though the methods using the theory of elasticity or
plasticity are also being increasingly used, the most common methods are based
on limiting equilibrium in which it is assumed that the soil is at the verge of
failure. The methods of limiting equilibrium are statically indeterminate. As
the stress-strain relationships along the assumed surface are not known, it is
necessary to make assumptions so that the system becomes statically determinate
and it can be analyzed easily using the equilibrium equations.

The general assumptions to be made are as follows;

1. The stress system is assumed to be two-dimensional. The stresses in the
third direction (perpendicular to the section of the soil mass) are taken as
zero.
2. It is assumed that the coulomb equation for shear strength is applicable and
the strength parameters c and phi* are known.
3. It is further assumed that the seepage conditions and water levels are
known, and the corresponding pore water pressure can be estimated.
4. The conditions of plastic failure are assumed to be satisfied along the
critical surface. In other words, shearing strain strains at all points of
the critical surface are large enough to mobilize all the available shear
strength.
5. Depending upon the method of analysis, some additional assumptions are made
regarding magnitude and distribution of forces along various planes.
6. Thus, in the analysis the resultant of all the activating forces trying to
source the failure is determined. Also the estimate of shear strength has to
be made. For factor of safety, the available resisting forces and the
actuating forces shall be used.