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.
It is often desirable to rapidly obtain some quantitative indication of the intact strength of the rock instead of relying only on a general and, to some extent, subjective consistency description. This is particularly so during the early stages of an investigation before instituting detailed testing.
For soils there are many well established ways of easily obtaining a reasonable estimate of strength. At the other extreme end of the scale the actual strength of strong near-fresh rocks is seldom of immediate consequence though, if required, sampling and testing is usually not a problem. It is, however, over the intermediate range, from very stiff soils to weak and even moderately strong rock that strength values frequently become most relevant. Unfortunately, it is over this range that there appears to be a serious lack of inexpensive techniques that can provide a quick appraisal of strength, especially where the rock type is fragile and difficult to sample or from which it is not always possible to successfully prepare a regularly shaped laboratory test specimen.
Based on the standard shear vane test commonly employed in soil mechanics it has been found possible, with vanes of a suitable design and a somewhat different testing procedure, to extend this technique considerably in order to measure shear strengths of over 10 MPa. This would correspond to materials with an unconfined compressive strength of up to roughly 20 to 30 MPa, depending on the angle of internal friction.
The shear vanes for this kind of application must be of robust design. Typical shear vanes machined from tough alloyed steel are shown in Figure 1. The stubby nature of the individual blades is apparent from the illustration. Both four and six bladed shear vanes were used with the diameter ranging from 9 to 20 mm. Above about 20 mm diameter the torque at failure can become quite considerable and there is the danger of twisting or breaking the vanes. The small four bladed shear vane second from the left in Figure 1 clearly shows such excessive twisting.