Which test is most commonly used for measuring the shear strength of cohesive soils?

The vane shear test is the most commonly used method for measuring the shear strength of cohesive soils. This test is particularly advantageous because it can be performed in the field, allowing for direct measurement of in situ shear strength without requiring extensive sample handling or disturbance.

In the vane shear test, a device with a cylindrical vane is inserted into the soil and rotated. The torque required to shear the soil along the vane’s blades is measured, which can then be correlated to the undrained shear strength of the cohesive soil. This approach is useful for soils that exhibit a high plasticity and are sensitive to disturbance, making the results reliable for design and analysis.

In contrast, the other testing methods listed serve different purposes. The cylindrical consolidation test measures the compressibility characteristics of soil over time but does not directly assess shear strength. The California Bearing Ratio (CBR) test is primarily used for evaluating the bearing capacity of subgrades and is more relevant in pavement design than in assessing shear strength. The hydrometer test determines the particle size distribution of fine soils, which helps in classification but does not provide information about shear strength.

Understanding these differences underscores why the vane shear test is the preferred method for evaluating the shear strength of cohesive soils in practical applications.