Point load strength index of granitic irregular lumps: Size correction and correlation with uniaxial compressive strength

The Uniaxial Compressive Strength (UCS) of rock cores is a key parameter for design analysis of rock engineering projects such as rock tunnels and piles in rockets. Diametral and axial point load tests on cylinder cores have been used to obtain Point Load Strength Index (PLSI). This PLSI is correlated to UCS. Recently, point load tests on irregular lumps are also used to obtain PLSI which is then correlated to UCS. However, the data on this correlation are very limited, especially for Hong Kong rocks. This paper presents the methodology for specimen preparation, testing, and data analysis. Results from irregular lump point load tests, diametral and axial point load tests, and uniaxial compressive tests (a total of 754 specimens of granitic rocks were tested) are then presented, analyzed, and discussed. A correlation between PLSI from irregular lump tests and UCS is established with a suitable size correction function. It is found that the Is(50) of the slightly decomposed Granite is between 7.31MPa and 8.5MPa. The Is(50) of the altered rock is 2.35MPa, where the rock containing weakness plane is 5.59MPa. The moderately decomposed Granite varies from 2.28MPa to 4.92MPa. The weathering affected the PLIS a lot. It is found that the values of power index value m in the size correction function is 0.443–0.600 for slightly decomposed granite (fine to coarse grains) and 0.545–5.562 for moderately decomposed granite (fine to medium and medium grains). Based on our study, for slightly decomposed coarse grain and fine to medium grain Granite, the size correction factor F=(De/50)0.44 (lower bound) is recommend; while for all other weathering conditions and grain sizes, the size correction factor F=(De/50)0.55 (lower bound) is recommended. The correlation between the UCS and PLSI Is(50) of irregular lumps is found to be 22.27, which is very close to the values of 21.61 and 21.72 for diametral and axial PL tests using the size correction factor of F=(De/50)0.45. This indicates that the correlation between UCS and PLSI from point load tests on irregular lumps established in this paper is reliable.