Study on the stress characteristics of deep overburden large-span cut-and-cover tunnel under different unloading methods and prediction of lining structure deformation

In response to the widespread application of large-span high-fill cut-and-cover tunnels in Western China in recent years, model experiments were conducted to study the unloading effects of EPS (expanded polystyrene) boards and concrete columns used either individually or in combination. By analyzing earth pressure, internal forces, and displacements of the lining structure, the unloading mechanisms of these measures were elucidated. The safety of the lining structure under different unloading measures was evaluated using the damage stage method. Additionally, based on the principles of multiple linear regression, predictive equations for the displacements at the weak points of the lining structure (the crown and haunches) under a load of backfill were derived. The research results indicate that without any unloading measures, there is a significant concentration of earth pressure at the top of the cut-and-cover tunnel. The unloading method that combines EPS (expanded polystyrene) boards and concrete columns at the top and sides of the tunnel can maximally reduce the surrounding earth pressure. This method reduces the average vertical and horizontal earth pressure by 40.01% and 79.34%, respectively. However, this unloading method significantly decreases horizontal earth pressure, which weakens the soil's constraining effect on the lining structure, preventing the minimization of internal forces and displacements within the lining. On the other hand, EPS boards only at the top of the tunnel leverage the soil arching effect in the backfill. On the other hand, EPS boards only at the top of the tunnel leverage the soil arching effect in the backfill, this method not only reduces the increase in vertical earth pressure but also increases the horizontal earth pressure, thereby enhancing the soil's constraining effect on the lining structure and achieving optimal unloading results. After backfilling, with no unloading measures, the displacement at the top of the cut-and-cover tunnel is 29.15mm, and the bending moments at its weak points, the crown, and haunches, are 4111.88 kN•m and -3988.51 kN•m, respectively. After installing EPS boards at the top of the tunnel, the displacement at the top decreases by 50.15%, and the bending moments at the crown and haunches decrease by 77.48% and 75.71%, respectively. Furthermore, compared to other unloading measures, this method can effectively enhance structural safety. Using earth pressure as a variable, the displacement predictive equation for weak points of the lining structure, derived based on the principles of multiple linear regression, yields maximum and minimum errors between predicted and actual displacement values of 7.30% and 0.28%, respectively. This displacement prediction equation can be effectively applied to research on unloading measures and optimization of unloading parameters in large-span high-fill cut-and-cover tunnels.