Method of equivalent core diameter of actual fracture section for the determination of point load strength index of rocks

Point load strength test is one of the indirect methods to estimate the uniaxial compressive strength of rocks. Due to its ease of sample preparation, low cost, and simple testing, it has been widely used in rock engineering in the world. The common calculation method of point load strength index is the method of equivalent core diameter of minimum cross-section, followed by the method of actual fracture section and loading distance method. However, because there are different factors being considered in different methods, the point load strength indexes obtained from different formulas varies greatly, which will result in large errors when estimating the compressive strength of rocks. In this study, a large number of point load strength tests and uniaxial compression tests were carried out using four kinds of rock samples. Then, a series of statistical analyses were conducted based on the test data. The results indicated that the distance between two loading points and the width of actual fracture section played an important and non-negligible role for failure of rock specimens, but the effect of width of minimum cross-section of specimens could be ignored. Consequently, by combing the advantages of two traditional methods, a modified calculation method of PLSI was proposed. Finally, uniaxial compressive strength of four kinds of rocks estimated by different methods was compared. The results showed that the modified method had the least estimation error.