Study on acceptable minimum interval of long deeply burial hydropower tunnels in Jinping II hydropower station

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.