Suggesting new protocol to determine point load strength index of rocks

Abstract

The point load test is widely used as an index test for determining the rock material strength and classification. The test provides an overall measure of rock strength known as the point load strength index (I_s). In the standard method, the minimum cross-sectional area (A) of a plane through the platen contact points is considered, and its equivalent core diameter is used to determine I_s. This paper suggests that the equivalent diameter of the specimen is calculated considering the actual area of the failure plane (A^*) recognized after the test. Accordingly, the relationships used for PLI calculation were updated. A number of blocks and irregular lumps were subjected to the point load test. The dimensions were measured once by a steel ruler and once using an image-processing technique. The results revealed that there is a strong positive correlation between the failure load (P) and A^*, while the correlation between P and A is weak and insignificant. Hence, A^* is a better parameter to normalize P and to yield a strength index (${I}_{s}^{*}$). Statistical analyses proved that the ${I}_{s}^{*}$ values have less dispersion around the mean and are more normally distributed compared to the I_s values. Therefore, ${I}_{s}^{*}$ regardless of the tool used to measure dimensions, seems a more reliable index.