Experimental Study on Frost Heave Influence in Extreme State to Adjacent Tunnel in Freeze Reinforcement Construction of Shield Docking

The influence law of frost heave caused by shield connection freezing construction is the basis for evaluating the stability of adjacent tunnels and taking countermeasures. The constraint state of the stratum on the tunnel structure directly determines the distribution characteristics of the frost heave effect. Based on the large-diameter shield underwater connection project, the model tests of freezing reinforcement under two extreme boundary conditions of constant load free movement and fixation of the tunnel structure were designed and carried out. The evolution laws of stratum temperature, frost heave force and displacement during the freezing process were studied, and the mechanical response and displacement change process characteristics of the adjacent tunnel were obtained. Under the condition of scattered boreholes on the outside of the shield, all the space between the freezing pipe and the shield was frozen after 70 days of freezing, and the thickness of the frozen wall at the shield connection position could reach 6m, with an average temperature of about -15.2℃. The maximum frost heave displacement of the adjacent tunnel during the formation of frozen soil outside the shield was 9.6mm, and the maximum increase of the frost heave force acting on the tunnel surface was about 14.1% of the original stratum pressure. The compression deformation and dispersion effect of the unfrozen stratum between the frozen soil and the tunnel are the key factors affecting the tunnel structure's ability to withstand the frost heave effect. The average deformation modulus of the unfrozen stratum slightly increases with the expansion of the frozen soil, and the average deformation modulus of the silty clay stratum is between 20.8 and 49.3MPa. The research results show that the constraint state of the stratum affects the displacement and the change process of the frost heave force it bears, and the deformation of the compression of the unfrozen stratum absorbs the frost heave effect generated during the formation of the frozen soil, keeping the adjacent tunnel structure in a stable state.