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 .