UCS test soil type and parameters

Soil type

The unconfined compression test is conducted on saturated, cohesive soils (clays) that can stand without lateral support, using undisturbed samples. It is not suitable for dry sands, gravels, or fissured clays, which cannot hold their shape without confining pressure.

Determination of strength parameters using Mohr circle

In the test, the lateral pressure is zero ($\sigma_3 = 0$) and the axial stress is increased to failure:

$$\sigma_1 = q_u = \frac{P}{A_c}, \qquad A_c = \frac{A_0}{1-\varepsilon}$$

The Mohr circle at failure passes through the origin with diameter $q_u$. For saturated clay, the undrained envelope is horizontal ($\phi_u = 0$), so the circle touches it at the top:

 tau
  |        ..--..
c_u ------*'     '*-------- envelope (phi_u = 0)
  |      /   C    \
  +-----+----+-----+----- sigma
  0 (s3=0)         q_u = s1

$$c_u = \frac{q_u}{2}$$

The failure plane would be at $45^\circ + \phi_u/2 = 45^\circ$ to the horizontal. If the sample is partly saturated and $\phi_u \neq 0$, a single unconfined test cannot give both parameters; triaxial tests at different confining pressures are needed to find $c_u$ and $\phi_u$. So the unconfined compression test is a quick test that gives only the undrained cohesion $c_u = q_u/2$.