Shear strength of clays

Like the shear strength of sands, shear strength of clays are also very important. Shear strength of clays are greatly influenced by the following factors:

Drainage conditions during testing.
History of deposition of clays, i.e. over consolidated or normally consolidated clays.

For the detailed study of the shear strength of clays they are classified into following sub classes:

Normally consolidated clays ( NCC).
Over consolidated clays ( OCC).
Sensitive clays ( SC).

Normally consolidated clays

When we perform CD test on NCC, zero cohesion results along with a angle of friction, and thus behave as if it were a granular material.

When CU test is performed on it with pore pressure measurement it gives us a similar effective stress strength envelope.

Now when UU test is performed on the same unsaturated material with pore pressure measurement it will give us strength with is comparatively less than that in drained test.

Over consolidation clays

Talking about over consolidated clays as clear from their name that they have larger shear strength than normal consolidated clays of the same composition. An OCC will expand during shearing while on the other hand NCC will further undergo consolidation whilst. Thus they devel0op negative pore pressure when tested under undrained condition. And thus with the consequent increase in shear strength it increases the effective pressure.

Sensitive clays

Samples with high water contents such as marines, lake clays and organic slit usually have no measurable remolded strength. Deposit may be converted into viscous solid or there may be a considerable decrease in the shear strength of this soil, of there is a disturbance during the sampling. Such a type of clay is called as “sensitive clay” it is also known as “quick clay”.

From this we can drive the definition of sensitivity, as the ratio of undisturbed shear strength of a cohesion soil to remolded strength.