Unlined Tunnels of the Southern California Edison Company

The design, construction, and operating experience with eleven unlined power tunnels sited in granitic rock in the. Sierra Nevada Range in Central California, are summarized. The tunnels vary in age from less than 5 yr to more than 50 yr and vary in bore from 7ft to 24 ft. The advantages and disadvantages of unlined tunnels are compared with those for unlined tunnels, and the need for careful preproject geologic studies is emphasized. The results of tunnel friction loss tests are presented, and the effect of invert treatment on hydraulic and structural performance is reviewed. Case histories of leakage and rockfalls as well as the remedial work done to correct the problems are described. The need is emphasized for careful geologic examination of a tunnel interior and care in gunite treatment of soft seams and joints. It is concluded that under favorable geologic conditions, an unlined tunnel is a very economical type of tunnel to construct and maintain.