Internal Friction of Soils
Internal friction of soils is due to the resistance of grains to sliding over each other and is the characteristic of the coarse materials of particle-size larger than about 0.002 mm. The magnitude of the internal friction of a granular mass depends on the grading, shape, and surface texture of the particles, the degree of compaction and moisture content of the mass, and the load to which it is subjected. Frictional resistance is highest with angular grains having a rough surface and of varied size and shape, and increases with increasing load and is reduced in the presence of a lubricant such as water, present in excessive proportions. For the coarse material it is usually assumed that the particle-size distribution giving the greatest dry density has the greatest internal friction. The strength of a non-cohesive soil depends entirely on internal friction.