Which in-situ test is specifically designed to determine the undrained shear strength of soft, saturated clays?

The in-situ test specifically designed to determine the undrained shear strength of soft, saturated clays is the Field Vane Shear Test, often referred to simply as the Vane Shear Test. Undrained shear strength, denoted as 'cu', represents the maximum shear stress a soil can withstand before failure without any change in its water content or pore water pressure, a critical condition for saturated clays under rapid loading. "In-situ" means the test is performed directly in the ground at the site, minimizing sample disturbance that can significantly alter the measured strength of soft clays. The test involves pushing a four-bladed vane, resembling a miniature propeller, into the undisturbed soft clay to the desired depth. Once the vane is embedded, it is rotated at a slow, constant angular velocity, typically 6 degrees per minute, by a torque-measuring device located at the ground surface. As the vane rotates, it shears the surrounding clay along a cylindrical surface defined by the height and diameter of the vane blades, causing the clay to fail in an undrained condition. The maximum torque required to cause this failure is measured. This measured torque is then directly related to the undrained shear strength of the clay using a specific formula that accounts for the geometry of the vane (its height and diameter). The Vane Shear Test is particularly effective for soft, saturated clays because it minimizes disturbance during installation compared to extracting disturbed samples, and it directly simulates an undrained failure condition which is representative of many engineering loading scenarios for these low-permeability soils.