The correct answer is: C. unconsolidated drained test
In a triaxial compression test, the soil sample is subjected to three principal stresses:
- $\sigma_1$ is the major principal stress, which is applied in the vertical direction.
- $\sigma_2$ is the intermediate principal stress, which is applied in the horizontal direction.
- $\sigma_3$ is the minor principal stress, which is equal to the cell pressure.
The test is known as consolidated drained test if the soil sample is allowed to consolidate under the initial cell pressure $\sigma_3$ before the deviator stress $\sigma_1-\sigma_3$ is applied. This allows the soil to reach equilibrium under the initial stress state.
The test is known as consolidated undrained test if the soil sample is not allowed to consolidate under the initial cell pressure $\sigma_3$ before the deviator stress $\sigma_1-\sigma_3$ is applied. This means that the soil sample will be subjected to both the initial cell pressure and the deviator stress at the same time.
The test is known as unconsolidated drained test if the soil sample is not allowed to consolidate under the initial cell pressure $\sigma_3$, but is allowed to drain during the application of the deviator stress $\sigma_1-\sigma_3$. This means that the soil sample will be subjected to the initial cell pressure, but the deviator stress will be applied gradually, allowing the soil to drain and reach equilibrium under each increment of deviator stress.
In the question, drainage is allowed during the first stage (i.e. application of cell pressure) only and not during the second stage (i.e. application of deviator stress at constant cell pressure). This means that the soil sample is not allowed to consolidate under the initial cell pressure, but is allowed to drain during the application of the deviator stress. Therefore, the test is known as unconsolidated drained test.