Abstract: Safety of tunnel construction is a hot topic for the tunnel passing through complex loose deposits. Based on Monte Carlo stochastic principle and digital image processing technology, a mesostructure model for the excavation of Luodala tunnel is established with considering contact surface element and tensile strength. The deformation, failure process and instability mechanism of loose deposits surrounding rock during excavation are discussed and verified by application of the model in practical engineering. The results show that the model can reflect the failure process of surrounding rock during tunnel excavation, and the displacement contour of surrounding rock presents obvious fluctuation and asymmetry. The failure mode of surrounding rock is mainly shear failure, and tension-shear composite failure is also observed in some part. The failure zone extends from tip of the margin rocks to the deep layers, forming wedge-shaped shear zone and tension-shear loose zone, which are prone to cause local instability under construction disturbance. According to the failure characteristics of loose deposits, the grouting reinforcement measures of surrounding rock are proposed. After the reinforcement, the grouting reinforcement ring has been formed around the tunnel with good soil-rock bond and the deformation of surrounding rock can be controlled and the support structure is stable during the excavation and support process. The resutls can provide new ideas for the design and construction of tunnels in similar loose deposits.

Key words: loose deposits, tunnel, digital imaging technology, random simulation, mesostructure, grouting reinforcement