Assignment of the boundaries of a steel lining in hydraulic tunnels

1. The main problems when designing pressure tunnels are the maximum use of the bearing properties of the enclosing soil mass and prevention of leakage of water from the structure into the mass. The first condition is fulfilled completely in the case of using unlined tunnels, but here the second problem is not always solved. The optimal solution satisfying both requirements in high-head tunnels is the use of a combined lining consisting of an inner steel lining and outer concrete ring. 2. The length (boundary) of the steel lining in a pressure tunnel is determined on the basis of two criteria: the bearing capacity of the rock mass; the size and length of the seepage path from the tunnel into “dry” underground workings or to an open slope. 3. The criteria of the bearing capacity of the rock mass is characterized by two main parameters: deformation-strength characteristics and height of the stratum covering the tunnel. 4. The criterion of the size and length of the seepage path is determined by the seepage strength of the rock mass and stability of the underground workings an open, slopes located in the zone of the seepage flow issuing from the tunnel. 5. At present there is no unified method of determining the boundaries of a steel lining in pressure tunnels. This problem is solved in each particular case with consideration of the engineering-geologic, topographic, technological, operating, and economic conditions.