Case study: using the point load test to estimate rock strength of tunnels constructed by a tunnel boring machine

Rock strength is one of the most important parameters in geotechnical engineering. During excavation using a tunnel boring machine (TBM), a rapid and effective method to estimate rock strength is needed. The TBM muck produced during excavation contains blocky fragments which are suitable for a point load test (PLT). To use the PLT on blocky fragments to estimate rock strength, it is necessary to find the relation between the point load strength (PLS) of blocky fragments and uniaxial compressive strength (UCS) of surrounding rocks. In the present study, PLT was performed on both blocky fragments and core samples of limestone for the Yinsong Water Conveyance Project, China. The UCS of limestone was acquired. The PLS of blocky fragments and core samples was compared. The results show that the blocky fragments' PLS is lower than that of core samples; the declining ratio depends on the fractured degree of sampling sites; blocky fragments with width and thickness in a certain range are suitable for estimating rock strength; and when the size of the blocky fragment exceeds a certain range, it means the blocky fragment was not generated from the tunnel face. The limestone core samples simultaneously drilled from side wall have an average conversion factor from PLS to UCS of 24.61. The mean conversion factor from PLS of blocky fragments to UCS of limestone in the present study is 40.38.