# Study on the Damage Process and Numerical Simulation of Tunnel Excavation in Water-Rich Soft Rock
## 6 Conclusions  
This paper, based on the effect of excavation unloading and water-softening, studied the mechanism of rock damage after underground excavation in water-rich soft rock and derived equations for the evolution of damage variables and the deterioration of rock parameters considering time effect. Based on the above equations, a fluid-solid-damage multi-field coupling method considering the dual softening and time variation was proposed and applied in ABAQUS finite element software. Finally, taking a typical section of the Xujiadi tunnel as an example, the results of different models were compared and analyzed with field monitoring data to verify the accuracy and rationality of the new-prosed method for water-rich soft rock excavation. The main conclusions are as follows:  
When the confining pressure varies from large to small, the rock transitions from compression-shear damage to tensile failure. In the water-rock interaction, different rock fracture developments and their permeabilities cause progressive water-rock interaction. Therefore, the tensile damage and the time effect of water-softening during underground excavation must be taken into consideration. The fluid-solid coupling analysis ignores the softening damage process of surrounding rock under the action of excavation and water. So, the damage evolution equations of surrounding rock under dual softening actions and different failure modes are proposed. The equivalent plastic strain \(\overline{z}^{p}\) is used as the softening parameter for compression shear damage, and the compression shear damage variable \(D_{c}\) and \(\overline{z}^{p}\) are approximately linear; the equivalent strain \(\overline{z}\) is used as the softening parameter for tensile damage, and the tensile damage variable \(D_{l}\) and \(\overline{z}\) are nearly negative linear. From the engineering reality, taking into account the difference between excavation unloading and water-rock interaction, the time segmental function is used to reflect the softening effect of rock damage. Usually, the time of excavation unloading is short, and linear function is used; the time of water-softening is long, an exponential function is used according to the test results. Meanwhile, the calculation methods of softening damage variables corresponding to different working conditions are summarized. Following the derivation of damage evolution equations, an improved FEM analysis method based on ABAQUS is proposed for underground excavation in water-rich soft rock. The damage of surrounding rock is judged through equivalent strain and equivalent plastic strain based on the results of the previous step. Then the damage degree and deterred parameters of each damage element are obtained. The deteriorated parameters are used to replace previous parameters for the next calculation to reflect the progressive damage process of the surrounding rock under the dual softening. Finally, the simulated results of different models are compared with the field monitoring results, and it is found that the surrounding rock displacement and pressure calculated by fluid-solid coupling are much smaller than the monitoring data. When dual softening and time effect are considered, the calculated results of surrounding rock deformation and pressure are within 6% error with the field monitoring data, and the calculated time history curve is also close to the trend of field monitoring curve, which proves the reasonableness and accuracy of the method. This paper is also subject to some inadequacies. When the new simulation method is implemented, multiple models need to be built and 'inp' files need to be modified. Although satisfactory accuracy can be obtained by multiple iterations, the manual operation is slightly troublesome. Moreover, the calculation volume increases exponentially with the increase of observed points when the time effect is considered in the simulation.