Calculation of Passive Limit Support Pressure of Shallow Buried Tunnel Face in Sandy Soil Stratum

In order to better evaluate the passive instability failure mode of shallow shield tunnel face in sandy soil stratum, the existing three-dimensional calculation model is improved to make the failure area closer to the real soil failure contour. Considering the friction between the upper and lower blocks, the limit support pressure calculation formula is established by using the limit equilibrium method. The finite difference software FLAC^3D is used to construct the numerical model, and the variation law and failure mode of the passive failure support force of the tunnel face are analyzed. The limit support pressure solution obtained by the theoretical model is compared with the solution obtained by numerical simulation and the existing passive failure theoretical model to verify the reliability of the model. The results show that: The proposed theoretical calculation model can meet the limit calculation of passive instability of the tunnel face. The limit equilibrium method is used to calculate the friction between the upper and lower blocks of the model, and it is more reasonable to consider the static friction force. The optimal solution of the logarithmic spiral angle is affected by the internal friction angle. When the internal friction angle increases, the optimal solution of α will become smaller. The passive limit support pressure of the tunnel face of the shallow shield tunnel increases with the increase of the internal friction angle of the soil and decreases with the increase of the tunnel diameter.