Unconfined compression test for determining shear strength

Unconfined compression test

The main difference between Unconfined compression test and triaxil compression test is that in this test the confining cell pressure is kept zero during the test, in fact it is a special case of triaxial test. Due to which without applying any lateral pressure like a concrete crushing test the cylindrical soil sample is crushed to failure. Although this test can also be done by using triaxil test in the laboratory but instead we move towards a more simple portable piece of equipment which is known as unconfined compression test apparatus. Due to its portable design the equipment can easily be shifted to the desired site and the test can be done within the field.

 In this test a cylindrical soil sample of height to diameter ratio 2 to 3 is compressed at a constant pressure of 1.25 mm/min in a loading frame until the 20% deformation is achieved.

Unconfined Compressive strength of Cohesive soil

It is defined as load per unit area at which an unconfined prismatic or cylindrical specimen of soil will fail in a simple Compressive test.

It is taken as the maximum load attained per unit area or the load per unit area at 20 % axial strain, whichever is secured first during test.

Unconfined compression test is the most simplest, easiest and least expensive test for cohesive soil. This test is particularly limited to cohesive soil. Because in this test no lateral supports are provided to soil sample. Soil sample must able to stand alone to perform this test. A non cohesive soil such as sand is not able to stand alone without lateral supports. The cohesion C is taken to be one half of the unconfined Compressive strength. 

A cohesive soil gets more of its shear strength from cohesion. For most cohesive soils, cohesion may be estimated from the results of unconfined compression test. However for soft and sensitive clay, the cohesion is commonly obtained from the results of field or laboratory vane tests.

This computation is based on the fact that minor principal stress is zero and the angle of friction of soil is assumed zero. It is often called an Undrained or U test.

Unconfined compression test does not generally provide a very reliable value of soil shear strength because of three reasons:

1. When the sample is removed from the ground, the effect of lateral restraint provided by the surrounding soil mass on the sample is lost. There is, however, some opinion that soil moisture provides a surface tension effect so that the sample is somewhat confined. This effect is more exaggerated if the sample is saturated.
2. The internal soil conditions such as degree of saturation, pore water pressure under stress transformation, and the effects of altering the degree of saturation cannot be controlled.
3. The friction on the ends of the sample from the loading platens provides a lateral restraint on the ends which alters the internal stresses by an unknown amount.

Error from the first two factors can be eliminated by using triaxial compression test. The third item has undergone considerable research, and the indication is that this factor is not as important as one might at first suppose.

Special end plates, or platens can be fabricated to reduce the friction effects if more refined test results are desired.