What is the meaning of shear strength in soil?

Generally, shear strength is a term used in soil mechanics to describe the magnitude of the shear stress that a soil can sustain. The shear resistant of soil is a result of friction and interlocking of particles, and possibly cementation or bonding at particle contacts. Due to interlocking, particulate material may expand or contract in volume as it is subject to shear strength. If soil expands its volume, the density of particles will decrease and the strength will decrease; in this case the peak strength would be followed by a reduction of shear stress.

The shear strength is the maximum resistance or stress that a particular soil can offer against failure over its improper surface loading.

The main constituents of shear strength are :

• the structural resistance offered by interlocking of particles
• the frictional resistance offered by particles due to their shape and surface
• the cohesion and adhesion

then comes Mohr-Coulomb theory (in 1776) to explain shear strength of soil….

The theory is firstly predicted by Coulomb and it says that a proper relation exists in between shear at failure and normal stress and hence relation be linear and gave expression

𝜏f = F (σ)

Where, σ = normal stress, 𝜏f = shear stress at failure

as per this the shear stress is the function of normal stress.

And coulomb says that the relation is linear and added a relation I.e.

S = C + σ tan θ

Where, θ = friction angle, C = cohesion

θ & C depends on material property

And further, Mohr came and proposed that the shear stress depends on cohesion, friction, type of soil along with normal stress.

for clays, cohesive forces are the main forces that cause shear strength of soil, and the friction is zero in plastic soils.

for granular soils like sand, cohesion is zero and the shear strength is just because of friction only.

THEREFORE various tests have been carried out for sand, clays, etc. to check their shear strength.

In case of soils where pore pressure to acts effective parameters are in use i.e., effective stress, effective cohesion, effective frictional angle .

In simple words,

The strength of the soil resists the shear due is to applied load called shear strength. The soil has shear strength due to the interlocking of particles provide resistance, friction between the soil particles, and adhesion between the soil particles.

Before construction determination of shear strength is very important to know the bearing capacity and various mechanical properties of soil.

Following are the shear test performed on the soil to measure its shear strength:

1. Direct shear test (DST)
2. Unconfined compression test (UCT)
3. Triaxial compression test (TCT)
4. Vane shear test (VST)

Shear strength of soil is resistance against shear come from the footing of structure.

Shear strength gives soil bearing capacity of structure footing without excessive settlement.

Shear strength of soil is given by coulomb.

Shear strength = c +φ tan ∅

Where c= cohesion

φ= normal stress

∅ = angle of internal friction of soil

In clay angle of internal friction is very less and in sandy soil, cohesion is very less.

Some of the test is use for finding out shear strength of soil like

• Ven shear test
• Triaxial test
• Plate load bearing test
• Box test etc

The shear strength is the resistance offered by soil against the shear deformation it value is equal to the shear stress on critical plane.

The critical plane is that plane on which the resultant has a maximum angle of obliquity with the normal of that plane.

Angle of obliquity is the angle on which the resultant stress is most inclined with the normal of that plane. It is also called as internal angle of friction or angle of repose.

Shear strength of soil can be inferred as the internal resistance of soil against shear stresses developed at a point or on a plane due to the action of external forces. It depends on interparticle friction or cementation or both. The shear strength can be assessed with the help of various laboratory tests such as the triaxial test, Direct shear test, Vane shear test, etc. There are various models to interpret the shear strength of soil, such as the Coulomb failure criterion, Mohr Columb, Tresca, etc.