Analytical Model for Prediction of Tunnel Deformations in Soft Rocks Considering the Softening and Expansion Effects

Accurate prediction of surrounding rock deformation can effectively prevent tunnel failure. To accurately predict the displacement of soft rock, an optimized prediction model is proposed, which simultaneously considered the phased release of geostress and the characteristics of soft rock. The existing stress release formula is modified based on the stress release law of three steps excavation. Considering the softening effect of soft rock, the softening factor is established and introduced into the Mohr-Columb strength criterion. At the same time, the influence of the expansion effect and rheological effect is considered in the derivation. Analytical model is used to verify actual soft rock tunnel engineering. The results show that the model can accurately predict the displacement. Combined with the actual engineering parameters, the calculated final displacement is 810.39 mm, which is less different from the actual monitoring vault displacements of 779.70 mm, 778.72 mm and 780.74 mm. The error of the final displacement is 3.787%, 3.907% and 3.658%, and the average error is 3.784%. Therefore, the analytical model has good applicability for similar soft rock tunnels, which can accurately calculate the displacement. The analytical model can be used to analyse the stability of soft rock tunnels.