Material properties
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The MohrCoulomb constitutive model is used to describe the soil material behavior. The MohrCoulomb criterion relates the shear strength of the material to the cohesion, normal stress and angle of internal friction of the material. The failure surface of the material has been already presented and is given by the equation (63). For MohrCoulomb material model, six material properties are required. These properties are:
• The friction angle ϕ
• Cohesion C
• Dilation angle ψ
• Young's modulus E
• Poisson's ratio ν
• Unit weight of soil γ
Young's modulus and Poisson's ratio have a important influence on the computed deformations prior to slope failure, but they have little influence on the predicted factor of safety in slope stability analysis. Dilation angle ψ affects directly the volume change during soil yielding. If ψ=ϕ the plasticity flow rule is known as "associated", and if ψ≠ϕ nonassociated plastic flow rule is assumed. The change in the volume during the failure is taken into account through the coefficient ψ.

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