Angle of Internal Friction of Soils

The resistance in sliding of grain particles of a soil mass depends upon the angle 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 the density. The soils subjected to the higher normal stresses will have lower moisture contents 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°. The angles of internal friction for granular soils are done in “Shear Tests”.

Capillarity is the ability of the soil to transmit moisture in all directions regardless of any gravitational force. Soils possess capillary action similar to a dry cloth with one end immersed in water. Water rises up through soil pores due to capillary attraction. The maximum theoretical height of capillary rise depends upon the pressure which tends to force the water into the soil, and this force increases as the size of the soil particles decreases. The capillary rise in a soil when wet may equal as much as 4 to 5 times the height of capillary rise in the same soil when dry.

Coarse gravel has no capillary rise ; coarse sand has up to 30 cm fine sands and silts have capillary rise up to 1.2 m but dry sands have very little capillarity. Clays may have capillary rise up to 0.9 to 1.2 m but pure clays have very low value. In coarse grained soils, the time required to reach the limit of the rise is much less than in fine textured soils.