The four hydropower tunnels of Jinping II Hydropower Station are mostly over burden of 2000 m

surrounded by more than 44.2MPa high initial stress and 10MPa pore water pressure. It is a great challenge for designers to determine the acceptable minimum interval under extremely complex geological conditions. So a integrated criterion for determining acceptable minimum interval is proposed according to (1) the plastic zone connecting or not; (2) the inflexion of displacement curve of key points in tunnel. (3) convergence displacement not exceeding the state specification. With above guidelines

the theoretical minimum interval of tunnel of 50 m is obtained

by hydro-mechanics coupling numerical simulation. At last

the modified result of 52 m is recommended for the acceptable minimum interval according to engineering analogy analysis of several typical deep tunnels. The conclusion shows that the design interval of 60 m at present is reasonable.