Electric
Mountain

Construction

Construction of the Main Caverns and Tunnels
The caverns, excavated deep from inside the hillside, house the main plant, associated control equipment, switchrooms, workshops and personnel amenities. Permanent cavern wall and roof rock support is similar throughout being a combination of rock anchors (minimum length 8m), rock bolts (3.7m long) and dowels together with mesh reinforced shotcrete to all exposed rock faces. Local reinforcing, where necessary, was also carried out during construction.

The caverns and their major dimensions.

Conventional reinforced concrete and steel structures were used for the various floors and rooms. However, computer studies needed to be carried out to evaluate stresses and to check natural frequencies of the structures to ensure vibration-free operation for the reinforced concrete structure supporting each pump-turbine/generator-motor.

All the tunnels are similar in construction being supported throughout with rock bolts to suit the particular rock structure and a layer of sprayed concrete, reinforced in places with steel mesh, applied to most of the roofs and walls. They are also similar in section having a flat invert (base) and semi-circular roof. The largest is the plant access tunnel (8.0m wide by 7m high and 711m long) designed for transportation of the largest plant item, the 400/18kV generator-motor transformer. This tunnel also serves as the main access tunnel for vehicles, personnel and plant to all areas of the cavern complex. The heating and ventilating tunnels provide a link between the main vertical ventilation shaft (chimney), which is 255m high by 5.0m in diameter, and the small ventilation shafts serving the main caverns. The tunnels alsoo provide access to part of the main cavern rock instrumentation installation. The cable tunnel carries the twin 400 kV main station cables between the main cavern area and the ground surface. Permanent and emergency lighting facilities are provided in most of the tunnels.