Angle of internal friction 

         the resistance in sliding of grain particles of a soil mass depends upon the angles of internal friction. It is usually considered that the value of the angle of internal friction is almost independent of the normal pressure but varies with the degree of packing of the particles,i.e. with density.

The soil subjected to the higher normal stresses will have lower moisture content and higher bulk densities at failure than those subjected to lower normal stresses and the angle of internal friction may thus change.
the true angle of internal friction of clay is seldom zero and may be as much as 26 degrees.