Abstract: TBM (Tunnel Boring Machine) method and drilling and blasting method are two main methods in the excavation of underground projects. As for the two methods, disturbance range of surrounding rock, deformation and destruction characteristics, self-stability, and treatment measures are different. In order to accurately understand macroscopic mechanical characteristics of surrounding rock, we should analyze it at microscopic level. Diversion tunnel in JinpingⅡhydropower station ,with maximum embedded depth of 2 525 m, is taken as an example. We carry out tests of scanning electron microscope (SEM) and detect the depth of stress relaxation for marble tunnel under different excavation methods. The test results show that, 1) fracture surfaces of rock by scanning electron microscope in TBM method show pattern of wiping along crystal face and pattern of cutting crystal face, with shear failure as the main failure mechanism, whereas tensile failure for drilling and blasting method; 2) in the aspect of relaxation depth, stress-relaxation depths of surrounding rock excavated by drilling and blasting method are larger than those by TBM method, which vary from 40 cm to 100 cm. The research findings provide reference for failure mechanism of rock and reinforcement measures in projects with similar geological conditions.

Key words: TBM, drilling and blasting method, scanning electron microscope, depth of stress relaxation, failure mechanism, reinforcement measures