Block punch index (BPI) test—a new consideration on validity and correlations for basalt and rhyolite rock types

Abstract

The difficulties associated with performing direct compression strength tests on rock specimens have led to development of indirect test methods for the rock strength assessment. Over the years, indirect test methods have found wide acceptance as these methods are simple, more economical, less time consuming, and easy to adapt to any field situations. Among other indirect test methods, Block Punch Index (BPI) test is a relatively new compression index test in which a small segment of core with minimal sample preparation is subjected to an increasing load until the middle part of the specimen is punched out. As per the ISRM suggested method, for the BPI test to be valid, specimen should be broken into three parts: two end parts which are fixed in the apparatus and the middle-band which is punched out. The test should be rejected as invalid if the parallel fracture planes are either absent or not fully developed or cross joints develop. The primary aim of this study is to investigate the validity of the BPI test under secondary fracture development by laboratory testing and study of stress distribution in the BPI test specimen using 3D numerical models. Observations from laboratory experiments and numerical simulation studies indicate that the development of significant tensile stresses at the bottom-middle part of the specimen caused by bending, resulting in secondary failures in the middle part of the specimen after or at the time of the shear failure along the two parallel planes. However, the study suggests that, even with the secondary (cross) fracture development, the BPI test should be considered as a valid test as along as the middle portion of the specimen is punched out along two clean parallel shear planes. Finally, based on laboratory test data on basalt and rhyolite rock types, new correlations are developed between block punch index and other material properties of rock, such as, uniaxial compression strength, splitting tensile strength, and Young’s modulus.