MODEL TEST AND NUMERICAL SIMULATION FOR PROGRESSIVE FAILURE OF WEAK AND FRACTURED TUNNEL SURROUNDING ROCK UNDER DIFFERENT OVERBURDEN DEPTHS

By taking the weak and fractured surrounding rock(defined as class IV by the road tunnel code of China) for the three-lane highway tunnel with overburden depths of 25-60 m,similar materials and model test equipments are developed to reproduce the whole process of progressive failure of surrounding rock after excavation;and the characteristics of stress field in rock mass under different overburden depths are studied. Meanwhile,an elastoplastic damage constitutive model is introduced to simulate some cases of model test by FEM;and the related parameters of rock mass are determined by laboratory tests.The numerical simulation results show a good agreement with those of model test.By analyzing the results of model test and numerical simulation,the conclusions can be drawn as follows:(1) The failure zone of surrounding rock is the source of tunnel collapse load,and it is distributed mainly above the vault as well as partially at the bottom of both side walls and arch bottom.(2) The overburden depth has great influence on the dimensions of progressive failure zone around tunnelprofile.As the overburden depth increases,the corresponding failure zone shows a gradual expansion trend.(3) The circumferential stress within the surrounding rock rises firstly and then decreases gradually after tunnel excavation.The location of the maximum stress corresponds to the position of earth pressure arch.With the expansion of failure zone,it continues to develop towards the inner side of rock mass and form a dynamic pressure arch.(4) The location of earth pressure arch can be determined by measuring the maximum circumferential stress within rock mass,and thus to determine the tunnel collapse load,which is theoretically feasible.