Influence of seepage and tunnel face opening on face support pressure of EPB shield

The hydraulic head on the tunnel face is smaller than that in the far-field ground when the earth pressure balance shield excavates in a saturated stratum. The seepage force could thus drag the soil to the tunnel face. However, the effect of the seepage on the tunnel face stability is not yet thoroughly understood. A series of centrifuge tests and numerical back-analyses by the Finite Difference Method were performed to investigate the influence of the ground anisotropic permeability and the tunnel face opening on the tunnel face stability under the seepage condition. The results show that the pore-water pressure in front of the tunnel face decreases with the increasing opening area on the tunnel face and the increasing kv/kh (the ratio of vertical hydraulic conductivity to horizontal hydraulic conductivity) as well. A higher support pressure is required for a higher kv/kh to keep the tunnel face stable. When the tunnel face is sealed, the minimum support pressure to keep stable is 1.92 times that in the condition with the fully permeable tunnel face. It indicates that seepage induced by the water ingress could benefit the tunnel face stability.

Automated summary

The study found that increasing the opening area of the tunnel face and the anisotropic permeability of the ground would reduce the pore-water pressure in front of the tunnel face, requiring higher support pressure to maintain the stability of the tunnel face.