Abstract: When using tunnel boring machine (TBM) to excavate a tunnel, engineers are hindered to observe the state of surrounding rock by the cutter-head and shield of TBM. Under this circumstance, the state of rock muck can be used to predict and evaluate rock mass conditions as well. The strength of blocky fragments in TBM muck can be obtained by the point load test. However, the relationship between blocky fragments and the intact surrounding rock has been unknown. In this study, point load tests are performed on the blocky fragments selected from the Yinsong Water Supply Project. Meanwhile, rock core samples are taken by drilling at corresponding locations of blocky fragments. Their strengths are also measured by point load tests, and then compared with uniaxial compressive strengths. In addition, geological conditions, the size of specimens and the breaking state are also recorded. The results show that the point load strength of blocky fragments reduces to 63.25% of core samples from side walls. It is found that the reducing ratio is greater when rock mass is more complete. The conversion factor from point load strength to uniaxial compressive strength for limestone is 25.3, whereas the conversion factor from the point load strength of blocky fragments to uniaxial compressive strength is 42.1. The breaking force depends on the breaking surface and oversize blocky fragments are not suitable for testing the strength. In conclusion, this study provides a useful method and basis for the rapid acquisition of rock strength in TBM tunnelling.

Key words: tunnel boring machine (TBM), blocky fragments, point load strength, damage, Yinsong Water Supply Project