We need to make certain assumptions in analysing slopes using limit equilibrium:-

• Assume a Failure Mechanism: We assume the shape and location of a failure surface rather then determining it by analysis.
• Assume Plane-Strain (2-D):We ignore 3-D effects (although of course in reality slopes are in three dimensions). We are being conservative by neglecting these effects, ie. we would achieve higher FoS's by taking them into account.

• Assume Rigid Block Movement: We assume the soil mass to moove as a rigid block, with the movement only taking place on the failure surface itself.
• Assume Uniform Localisation of Shear Stresses: As previously discussed in "Progressive Failure", the shear stresses are not usually uniformly mobilised over the whole length of the failure surface. However, for the purpose of our analysis, we assume they are.

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