Experimental investigation of rock strength using indentation test and point load test

Abstract Various methods have been proposed to indirectly assess the uniaxial compressive strength (UCS) of intact rock when drilled cores are not available. Indentation test (IT) and point load test (PLT) are the two widely used methods in the field to assess the rock strength by penetrating the irregular blocky fragments. The TBM muck produced by excavation contains a great number of blocky fragments which are suitable for an IT or a PLT. To use the IT or PLT on blocky fragments to estimate rock strength, it is necessary to find the relation between the indentation indices/modified point load strength ( I s ( 50 ) ) and the UCS. In the present study, the uniaxial compressive test (UCT), IT and PLT were conducted on the blocky fragments collected from a TBM-excavated tunnel to find the relationships between them. The effects of fragment sizes and indenter diameter (ID) on the indentation indices (critical transition force CTF and indentation modulus IM) were studied. The investigation shows that the indentation indices are influenced by the cross-sectional area and fracture surface area. Higher CTF and IM can be obtained as the ID increases. The corresponding normalizing methods were developed to reduce the dependency of test results to the fragment sizes and ID. Strong relations were found between the modified point load strength/estimated UCS and the indentation indices using linear and nonlinear regressions.