Analysis of Limit Support Pressure Due to Shield Tunnelling with a Shallow Overburden Under Seepage

Based on the semi-contained water model, the permeability of shield-crossing soil and overburden layers when the shield passed through the permeable soil was comprehensively analysed. The analytical solution of the head distribution along the tunnelling direction in the shield-crossing soil layer was derived, and the analytical solution of the two-dimensional seepage field was extended to the corresponding three-dimensional approximate solution. The active and passive failure modes of shallow-buried soil under steady-state seepage were determined using numerical modelling, and a corresponding cylinder-arc-corner-shaped model was established. Subsequently, the formulas of the two-limit support pressures at the tunnel face were obtained by introducing the above-mentioned three-dimensional seepage solution, and the newly calculated results were compared with those of the existing model. The results derived from the developed model were closer to the numerical solution. In addition, disturbance of the seepage field in front of the tunnel face was limited to three times the tunnel diameter and the values of the active and passive limit support pressure increased linearly with increasing head difference. The shield diameter and head difference were found to be the two main factors affecting the active limit pressure. The overburden thickness and shield diameter were the two major factors affecting the passive limit pressure. During tunnelling, the support pressure should be as close as possible to the in-situ transverse earth pressure using the approach that separately calculated the values for soil and water and should slightly fluctuate in the vicinity (preferably above it). The fluctuation range should be determined according to the deformation control standard.