A Shear Vane Technique to Measure The in Situ Strength of Weak Rock

An easy and inexpensive method is described for obtaining a quick indication of the in situ shear strength of weak rock. The technique is an adaptation of that used in soil mechanics and employs steel vanes of robust design which are driven into pre-drilled holes of suitable diameter in the rock. The peak shear resistance of the rotated vane is measured with a commercial torque wrench. The measured shear strength is affected by several factors such as the vane diameter and depth of embedment which are difficult to accurately quantify. It is therefore recommended that results obtained in this manner are used essentially as a strength index of the intact material. This test method has been found to be particularly well suited to fragile rocks such as fissile shales and slaking mudstones that are difficult to sample and from which it is not always possible to prepare regularly shaped specimens suitable for conventional laboratory strength tests.