GATE 2023 CE (CE2) – Question 32
An unconfined compression strength test was conducted on a cohesive soil. The test specimen failed at an axial stress of 76 kPa. The undrained cohesion (in kPa, in integer) of the soil is ______.
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Correct answer: 38
Explanation
**Unconfined compression test** (UCS): a cylinder of soil with no lateral pressure ($\sigma_3=0$) is loaded axially until it fails at $\sigma_1=q_u=76$ kPa.
For a saturated cohesive soil loaded undrained, the friction angle is $\phi_u=0$, so the Mohr circle has radius $q_u/2$ and the cohesion equals the shear strength:
$$c_u=\frac{q_u}{2}=\frac{76}{2}=\mathbf{38\ kPa}.$$