Analysis of Limit Support Pressure due to Shield Tunnelling with Shallow Overburden under Seepage

Based on the semi-contained water model, comprehensively considering the permeability of the shield-crossing soil layer and the overburden layer when shield passed through the permeable soil, the analytical solution of the head distribution along the tunnelling direction within the shield-crossing soil layer was derived, combining this, the analytical solution of the two-dimensional seepage field was extended to the corresponding three-dimensional approximate solution. Meanwhile, the active and passive failure modes of shallow-buried soil under steady-state seepage were obtained using numerical method, and the corresponding cylinder-arc-corner shaped model was established, after that, the calculation formulas of the two limit support pressures at tunnel face were obtained by introducing the above-mentioned three-dimensional seepage solution, and the calculated results were compared with the existing one, the current results were closer to the numerical solution. Besides, the study found that the disturbance of the seepage field in front of the tunnel face is limited to three times the tunnel diameter, the values of active and passive limit support pressure increased linearly with the increase of the head difference. The shield diameter and head difference were two main factors affecting the active limit pressure, and the overburden thickness and the shield diameter were 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 approach calculating separately for soil and water, and should be slightly fluctuated in the vicinity (preferably above it). The fluctuation range should be determined according to the deformation control standard.