The Vane Shear Test (VST) is a specialized in-situ method used in geotechnical engineering to measure the undrained shear strength (\(c_u\)) of soft, saturated fine-grained soils, most often clays. Determining this strength value is fundamental for engineers planning construction, as it directly influences foundation design and stability. The VST provides a quick and relatively straightforward way to obtain this measurement directly in the borehole. This test minimizes the disturbance of sensitive soil samples that might occur during traditional laboratory testing.
Anatomy of the Shear Vane
The device centers on a sturdy, slender rod that connects the surface mechanism to the measuring element submerged in the soil. Attached to the lower end of this rod is the vane, which consists of four thin, rectangular metal blades arranged perpendicularly to one another. This four-bladed configuration is designed to shear a cylindrical volume of soil when rotated.
The dimensions of the vane blades are standardized to ensure consistent and comparable results. A common configuration uses a height (H) to diameter (D) ratio of 2:1. Typical diameters may be around 65 millimeters or 90 millimeters, depending on the stiffness of the soil being tested. This specific geometry directly influences the calculation used to convert the measured torque into a strength value.
The above-ground component is the torque head, which applies a controlled rotational force to the rod. It simultaneously measures the resistance encountered in the soil. The measuring system is precisely calibrated to convert the angular rotation applied at the surface into a quantitative torque measurement, usually displayed on a dial or a digital readout.
Step-by-Step Field Operation
Before the test begins, the vane must be carefully advanced down a pre-drilled borehole until it reaches the specific depth required. Operators take great care during insertion to ensure the soil surrounding the vane is minimally disturbed, preserving its natural in-situ strength. Once positioned, the vane is ready to be rotated using the torque head at the surface.
The rotation must be applied very slowly and at a consistent speed to ensure the soil conditions remain undrained throughout the procedure. Maintaining undrained conditions means that the water within the soil pores does not have time to dissipate or move during the testing process. A typical, controlled rotation rate is between 0.1 and 0.2 degrees per second. This slow speed allows the test to accurately mimic how the soil would behave under rapid loading conditions.
As the rod turns, the operator continuously monitors the torque being applied and resisted as the blades cut through the soil. The torque increases steadily until it reaches a maximum value, known as the peak torque. This peak reading signifies the point at which the soil fails, or shears, along the cylindrical surface defined by the vane’s edges. This maximum torque value is the primary data point collected for determining the soil’s peak undrained shear strength.
After recording the peak torque, the operator continues to rotate the vane rapidly for several revolutions, typically five to ten rotations. This rapid spinning completely disrupts the soil structure around the vane, essentially “remolding” it. The torque required to maintain this rotation after the initial failure is significantly lower and is recorded as the residual or remolded torque. This remolded strength provides insight into how the soil might behave if its structure were completely destroyed.
Calculating Undrained Shear Strength
The final step in the Vane Shear Test is the conversion of the raw torque readings into the usable engineering value, the undrained shear strength (\(c_u\)). The peak torque (\(T_{max}\)) recorded during the test is directly proportional to the soil’s strength. This relationship is expressed conceptually as Torque equals the Shear Strength multiplied by a geometric constant.
This geometric constant, often denoted as \(K\), accounts for the specific shape and size of the vane blades used in the test. Since the vane creates a defined cylindrical shear surface in the soil, the geometric factor converts the rotational force (torque) into a force per unit area (strength). Engineers select the appropriate constant \(K\) based on the known dimensions (Height and Diameter) of the vane used in the field.
The calculation is performed twice using the two different torque values obtained during the field operation. The peak torque is used to calculate the soil’s maximum undrained shear strength. Separately, the residual torque value is used to calculate the remolded shear strength. Comparing these two calculated values helps engineers understand the soil’s sensitivity to disturbance.