Analysis of responses of existing shield tunnel to pressure-relief in confined aquifer

Sometimes, at the condition that a confined aquifer is not completely cut off, its pressure-relief can occur when dewatering via pumping is conducted to prevent the heave failure of pit formation above. Such pressure-relief can have negative effect on ground settlement considerable larger than that due to pit excavation. Therefore, such effect on the existing shield tunnel adjacent to the pit has to be considered. The effect is simulated with finite element method in this paper. The stress field of surrounding soil, the deformation and internal forces of the cross-sections of tunnel at different locations relative to the confined aquifer are analyzed. The results show that, when either most part of tunnel or the whole tunnel is located in the confined aquifer, its pressure-relief can cause not only overall upheaval or settlement, but also vertical convergence deformation of the tunnel. When the tunnel is located in the aquiclude above or under the confined aquifer, the pressure-relief only produces little influence on the tunnel deformation. However, when it is located in the upper aquiclude, the tunnel would exhibit obvious settlement due to the pressure-relief in the confined aquifer.