Abstract: To protect the environment and reduce the water pressure on lining as possible, the controlled drainage principle is proposed in high water-level mountain tunnels. But there is no specification for tunnel design with controlled drainage, so it is very urgent for tunnel design and construction. The mechanical character of the surrounding rock under coupling of seepage and stress is studied based on tunnel mechanics and seepage mechanics. The interaction between surrounding rock and lining with different drainages is studied by using character curve method; and the result is compared with numerical method. The result shows that the character curve and the support force are different and the effective tangential and radial stresses vary remarkably with different drainages; so the effect of drainage on the stress of surrounding rock and the support force can’t be ignored. Another conclusion is drawn that it is unsafe in traditional tunnel design that action of water is not considered during total drainage. All these conclusions can provide theoretical base for tunnel design with controlled drainage.

Key words: high water-level mountain tunnels, controlled drainage, coupling of seepage and stress, interaction between surrounding rock and lining, convergence-confinement method