Predicting the Uniaxial Compressive and Tensile Strengths of Gypsum Rock by Point Load Testing

Uniaxial compressive and tensile strengths are considered key properties for characterising rock material in engineering practice. They are determined, directly and indirectly, as described by the International Society for Rock Mechanics (ISRM) (1985) and the American Society for Testing Materials (ASTM) (1986). The point load strength index (PLSI), an indirect strength test, has been correlated empirically with both the compressive and tensile strengths of rock. The point load test (PLT) may be applied to cylindrical specimens, either along the axis, or the diameter, but as noted by Chau (1998), the diametral PLT is preferred (Bieniawski 1975). In the PLT, rock specimens (of cylindrical, prismatic, or irregular form) are loaded between two platen contact points, and they fail by the development of one or more extensional planes containing the line of loading. According to recommendations by ISRM (1985) and the standards of ASTM (1986), these types of failure mode are regarded as valid, whereas deviations from them are treated as invalid. Although relatively simple, the uniaxial compressive test is time-consuming and expensive, and it requires wellprepared cores. For these reasons, indirect tests such as the PLT, the Schmidt hammer rebound test, and sound velocity measurements (Sonmez et al. 2004, 2006) are often used to predict the uniaxial compressive strength (UCS). Tensile strength may be measured directly, but in practice it is difficult to do so. Instead, the indirect method of Brazilian testing can be used, as recommended by ISRM (1978). The Brazilian test measures tensile strength indirectly by developing tension across the diameter of a rock disc that is subjected to compression through a vertical load. In practice, however, the test has limitations, and more experimental data are needed to substantiate the correlation between the point load index and the UCS. The objective of this paper is to compare all of the PLT methods and their usefulness in practical applications. This study presents the PLSI (Is(50)) (air-dried and saturated states) obtained by three methods (axial, diametral, and irregular), and establishes the relationships between them. We then develop empirical equations relating UCS and Brazilian tensile strength (BTS) to Is(50) with the aim of obtaining simple, fast, practical and economical estimates of the UCS and BTS of gypsum rocks.