Abstract：Subsea tunnels are located deeply under sea with high groundwater pressure. Therefore，external water pressure on lining should be considered when designing tunnel structure. With the idea of blocking groundwater and limiting discharge，external water pressure on tunnel lining can be reduced remarkably with low discharge，so tunnel structure design will be more economical. To find the appropriate parameters of grouting circle is the key issue to perfectly blocking groundwater and limiting discharge. Based on the case of Yuanliangshan tunnel located at high water pressure and water-enriched karst region，the basic regulations of the effects of grouting circle on tunnel gushing water and external water pressure on lining are studied by theoretical calculation and analysis. The concept of tunnel discharge ratio is put forward；and the relation between tunnel and external water pressure on lining is analyzed. Based on the above-mentioned research results，the method and program for choosing appropriate parameters of grouting circle are proposed；and the appropriate parameters of grouting circle for Yuanliangshan tunnel are given. The parameters of grouting circle are adopted；and satisfactory effects are achieved. The monitoring amount of gushing water after grouting is basically identical with theoretically calculated value. Research results indicate that the reduction coefficient of external water pressure on lining is decided by tunnel discharge ratio；it will be zero only when water seepage behind tunnel lining can be discharged absolutely，or there is external water pressure on lining. The effects of grouting circle will not share the external water pressure with tunnel lining，but block groundwater and reduce the external water pressure on lining remarkably with low discharge. The research results will provide references to tunnel waterproofing and drainage system design in high water pressure and water-enriched regions including subsea regions.
