Influence of time-dependent of crack propagation of brittle rock on long-term stability of Jinping water diversion tunnel

The time-dependent propagation of cracks was observed from the failure of field surrounding rock and the monitored data of the water diversion tunnels in Jinping and became the dominating factor of the long-term stability of the tunnel. A long-term loading test was conducted on the marble from Jinping leading to the establishment of a fitted formula for the time-dependent behavior and a threshold ratio of driving stresses. A numerical specimen based on CPM model was set up considering the transitional characteristics from brittle-ductile-plastic and a numerical test of long-term loading was performed. A full scale PFC-based model was constructed for the water diversion tunnel and the long-term stability of the tunnel with various depths and rock types was assessed. It was found that in the test the failure time varied with the loading level exponentially. The subcritical crack began to expand if only a threshold of stress level, termed as the critical ratio of driving stresses, was reached. The ratio is 0.492 for Jinping marbles. The CPM-based numerical specimen reproduced nicely the triaxial behavior and the effect of confining pressure. The growth trend of the crack numbers was notable under the lower confining stress, but less noticeable under the higher confining stress. The number of the tensile cracks remained unchanged under the higher confining stress. During the numerical test of long-term loading, the axial strain and the growth of the number of cracks were in accordance with the three stage curve of creep. As the ratios of the driving stresses decreased, the strains due to creep increased. The development of the cracks due to the stress corrosion was also in accordance with the three stage curve of creep. A smaller ratio of driving stress led to a linear growth of the number of cracks of stress-corrosion but an exponentially decreased rate of growth of it. The results of the full scale PFC simulation suggested that after 100 years, the fracture zone of the tunnel would be with the range of 2.1-3.1 m for the class II marble and 3.3-4.5 m for the class III marble. It was thus confirmed that the long-term stability of the water diversion tunnel could be guaranteed.