Abstract: The external water pressure is the key factor for lining structural safety in construction, operation and maintenance condition. The traditional method for external water pressure estimation is with empirical formula. However, the effects of geology, boundary, excavation and lining construction aren’t adequately taken into account; and the solution results have risks and uncertainties. This paper reviews the general methods for external water pressure estimation, such as discount coefficient method, theoretically analytical method and numerical analysis method. The numerical results compared with the analytical results under different permeability environment and lining support forms illustrate the viability of the numerical analysis method. The numerical model scope is discussed to reduce the calculation error. The evolution process of seepage field is captured along with the tunnel excavation and lining supporting. The results show that the external water pressure on lining increases with the increase of permeability of surrounding rock and the lining thickness. The distance from the tunnel center to the boundary should not be less than 30 times tunnel diameter. Considering the transient effect, the tunnel seepage field tends to be stable within 10 days after the excavation; the water pressure distribution tends to be stable within 20 days after the lining supporting.

Key words: hydraulic tunnel, lining, external water pressure, discount coefficient, seepage flow