Shear strength of Cohesionless Soil
- Shear strength of cohesionless soils is only developed with the presence of effective stress
- Shear strength increases with increasing effective stress
- The increase of shear strength of cohesionless soil depends upon the internal friction angle of the soil
- The type of soil most susceptible to liquefaction is one in which the resistance to deformation is mobilized by friction between particles. If other factors such as grain shape, uniformity coefficient and relative density are equal, the frictional resistance of cohesion less soil decreases as the grain size of soils becomes smaller.
Other factors explained in detail are as follows:
Water Content in Soil
Sand and gravel mixtures have a higher effective friction angle than nonplastic silts Soil density: For a given cohesionless soil, the denser the soil, the higher the effective friction angle. This is due to the interlocking of soil particles. It has been observed that in the ultimate shear strength state, that the shear strength and density of a loose and dense sand tend to approach each other.
A well graded cohesionless soil will usually have a higher friction angle than a uniform soil. With more soil particles to fill in the small spaces between soil particles, there is more interlocking and frictional resistance developed for a well graded than for a uniform cohesion less soil.
Mineral type, angularity, and particle size:
Soil particles composed of quartz tend to have a higher friction angle than soil particles composed of weak carbonate. Angular soil particles tend to have rougher surfaces and better interlocking ability. Larger size particles, such as gravel size particles, typically have higher friction angles than sand.
Because of variations in soil types, gradations, particle arrangements, and dry density values, the effective friction angle is rarely uniform with depth. It takes considerable judgment and experience in selecting an effective friction angle.
- In general the shear strength of cohesive soils tend to be lower than the shear strength of cohesion less soils. As a result, more shear induced failures occur in cohesive soils, such as clays, than in cohesion-less soils.