Study on mechanical characteristics of deep soil-covered supper-span open cut tunnel under different unloading measures and deformation prediction of lining structure

In order to find out the unloading method that can effectively reduce the internal force and deformation of the super-span cut-and-cover tunnel in deep overburden, the unloading effect of EPS slab and concrete column used alone or in combination is studied through model test, the unloading mechanism is found out after analyzing the soil pressure, internal force and displacement of the lining structure, and the safety of the lining structure is evaluated by using the damage stage method. Based on the principle of multiple linear regression, the displacement prediction equation of the weak parts (vault and haunch) of the lining structure is obtained. The results show that the earth pressure will be concentrated on the top of the cut-and-cover tunnel without unloading measures, and the combination of cut- and-cover tunnel with concrete columns can reduce the earth pressure around the cut-and-cover tunnel to the maximum extent, and the average vertical and horizontal earth pressures can be reduced by 40. 01% and 79. 34%, respectively. However, this kind of unloading measure will greatly reduce the horizontal earth pressure and weaken the constraint effect of the soil on the lining structure, resulting in the internal force and displacement of the lining structure can not be reduced to the minimum. When the EPS is only laid on the top of the cut-and-cover tunnel, the horizontal earth pressure can be increased while the vertical earth pressure is reduced, and the constraint effect of the soil on the lining structure is strengthened to achieve the optimal unloading effect. After the completion of filling, without unloading measures, the displacement of the top of the cut-and-cover tunnel is 29. 15 mm, and the bending moments of the crown and haunch are 4111. 88 kN · m and 3988. 51 kN · m, respectively. After laying EPS board on the top of the cut-and-cover tunnel, the displacement of the top of the cut-and-cover tunnel is reduced by 50. 15%, and the bending moments of the crown and haunch are reduced by 77. 48% and 75. 71%, respectively. And compare with other unloading measure, that measure can effectively improve the structural safety. Taking the earth pressure as the variable, based on the principle of multiple linear regression, the maximum and minimum errors between the predicted value and the actual value of the displacement are 7. 30% and 0. 28%, respectively.The displacement prediction equation can be effectively applied to the study of unloading measures and the optimization of unloading parameters of super-span cut-and-cover tunnel in deep overburden soil.