Components of shear strength in soils

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The angle of internal friction (ϕ) and cohesion (c) are the two important parameters that determine the shear strength of soils.

The cohesion measures the ionic attraction and chemical cementation between soil particles, and the angle of internal friction shows the amount of friction and interlocking that exists among soil particles.

The shear strength of a soil mass can be calculated by using the Mohr-Coulomb shear strength equation as follows:

τ = σ tan(ϕ) + c ————(1)

Mohr’s circle for shear strength of soils

i) Angle of internal friction (ϕ)

The friction angle for a particular soil is the angle on the graph (Mohr’s Circle) where shear failure takes place. Internal friction is the grading-based resistance that two planes experience when they move in opposition to one another.

If the soil specimen given time to solidify, friction increases as the typical load increases.

Sand-containing gravels usually have a friction angle of 34° to 48°. Loose to dense sand has a friction angle of 30° to 45°. Silts have a friction angle of 26° to 35°, and clay has a friction angle of about 20°.

All well-graded soils have a high angle of internal friction values. Particle size, compaction force, and applied stress level are factors that affect friction angle. Some research has made it clear that the friction angle does increase with an increase in particle size. With an increase in surface angularity and roughness, friction angle has also been increase.

ii) Cohesion (c)

It is possible to define cohesion as the specific portion of shear strength that results from the forces of attraction that exist between the clay minerals. The ability of soil to act like glue and binding the grains together, is known as cohesion. It is an important element of shear strength, especially for fine-grained soils.

The soil grains will tend to fuse together due to heat fusion and sustained overburden pressure, resulting in substantial cohesion.

At the start of the stress condition, cohesion mobilizes and achieves its peak levels around the plastic limit, or at the start of structural collapse. Cohesion increases as one approach the shrinkage limit and diminishes as one approaches the liquid limit. With the exception of clayey soils, where a rise in stress induces an increase in molecular bonds, cohesion often does not increase as stress increases.

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Arbaj Demrot is the founder of VideRime Online Learning, a leading engineering website. He did his BE Civil and M.Tech Structure from RGPV University, Bhopal and has been working as an Assistant Professor in a reputed college.

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