Sliding between the particles during loading is a major factor for deformation of soil. The resistance that the soil offers during deformation is mainly due to the shear resistance between the particles at their contact points. Figure 9.1 shows the two particles in contact which is similar to the contact between two bodies. The normal force (N) is perpendicular the contact surface and shear force (t) is tangential force parallel to the contact surface. During sliding between two bodies, the maximum shear force can be written as:
where μ is the coefficient of friction. In case of soil particles, the maximum shear force can be written as:
where ø is the angle in internal friction of soil.