Utilizing the strength conversion factor in the estimation of uniaxial compressive strength from the point load index

The strength conversion factor (k) is the ratio between the uniaxial compressive strength (UCS) and the point load index (PLI). It has been used to estimate the UCS from the PLI since the 1960s. Many researchers have investigated the relationship between UCS and PLI for various rock types of different geological origins, such as igneous, sedimentary, and metamorphic rocks. In this study, the k values for subclasses of igneous (pyroclastic, volcanic, and plutonic), sedimentary (chemical and clastic), and metamorphic (foliated and nonfoliated) rocks were evaluated. For this purpose, UCS and PLI data for a total of 410 rock samples extracted from literature published around the world as well as UCS and PLI data obtained in this work for 80 rock samples taken from the Eastern Black Sea Region in Turkey were evaluated together to determine the k values of different rock classes. Strength conversion factors were obtained using zero-intercept regression analysis, formulation, and a graphical approach. This study confirmed that there is no single k value that is applicable to all rock classes. According to statistical analyses, k varied between 12.98 and 18.55 for the rocks studied. These findings demonstrate that the k values derived in this work can be reliably used to estimate the strengths of rock samples with specific lithologies.