The internal angle of friction is essentially the angle at which the material will slip on its own surface, just like the angle of wall friction is the inclination at which the material will slip on a sloping surface of material as a confining wall. However, when construction a graph of wall friction account has to be taken of the surface cohesion of some materials, which requires an increased force to promote slip. This extra effort is not as significant at higher surface pressures, but has a bigger influence at low normal forces on the contact surface, so the 'effective' angle of wall friction reflects the apparent wall friction angle at a given normal load. This 'friction' angle can be amazingly high with cohesive materials at low contact pressures

A similar effect pertains on internal friction, so the 'effective' angle is taken from the graph of results to give a steeper slope than might at first appear valid. It is defined as 'The angle between the axis of normal stress and the tangent to the

Mohr envelope at a point representing a given failure stress condition for the solid material', which sounds a bit gobbledegook unless you are familiar with yield locus and Mohrs circles, but the principle is fairly simple. ■