Vane Shear Test
The shear vane test is a simple and effective way to measure the undrained shear strength of cohesive soils. This method uses a rod with vanes attached to it that is inserted into the soil and rotated. A gauge on top of the rod measures the torque required to cause failure of the soil and provides a conversion to shear strength. This equipment has been in use since at least 1948 and is governed by British Standard 1377 in the UK.
To perform the test, the shear vane consisting of a straight rod with four vanes arranged in a cruciform pattern and a handle/torque gauge is inserted into the soil to a depth of 500mm and rotated at a rate of 6 to 12 degrees per minute. Once the soil fails in shear, the gauge displays the maximum torque applied, which is marked with a scale that converts the torque into a shear strength measured in kiloNewtons per square metre.
The shear vane test is suitable for measuring the shear strength of clay soils classified as soft to firm. By remoulding the sample and retesting, an indication of the sensitivity of the clay can also be found. The test has also been used to measure the shear strength of mortar mixes, where it correlates with the slump value. Additionally, the equipment has been used since 1967 to assess the shear strength of packs of snow at risk of forming a slab avalanche.
However, there has been some dispute over the accuracy of the test since its inception. While pioneers of the test, L Carlson and AW Skempton, believed that it provided a higher value of shear strength than that indicated by unconfined compressive tests, some 1973 research claimed that the test actually provided an under-estimate of shear strength compared to compressive tests where a good quality of sample is obtainable. This discrepancy has been attributed to the disturbance of the sample caused by the insertion of the shear vane.
Despite this, the shear vane test remains a simple and portable method for assessing the undrained shear strength of cohesive soils, and it is widely used in the industry. With the proper application and interpretation of the results, the shear vane test can provide valuable information for the design of geotechnical structures.