Mingtan Power Cavern is one of the largest power caverns in Taiwan. The optimization of orientation and shape of the cavern was determined from geology and stress analysis as well as construction considerations. Bedding parallel clay seams daylighting in the cavern roof were treated in advance of general excavation from small adits, by washing the weak seams with high pressure water jets and backfilling the washed seams with non-shrinkage mortar. At the same time, high strength tendons protected by corrugated sheathing against corrosion were installed from the adits to reinforce the cavern roof prior to excavation. The flexible support system of rockbolts, tendons and steel fibre reinforced shotcrete to form a self supporting rock mass throughout the cavern was adopted for the first time in such a large underground cavern excavation in Taiwan.

1.1 Description of project

The Mingtan Pumped Storage Project (the Project) is located near the geometrical center of the island of Taiwan, R.O.C. The primary function of the project is to convert surplus energy available from the power grid during off-peak time into stored energy to meet the ever-increasing peak load. The Project uses the existing Sun Moon Lake as the upper reservoir, and a dam across the Shuili River as the lower reservoir. The headrace pressure tunnel is about 3.8 km long, and the powerhouse is underground. The project has been designed by Sinotech Engineering Consultants, Inc. on behalf of Taiwan Power Company. The project layout is shown in Figure 1.

The Project calls for the excavation of a large power cavern 22.7 m wide, 46.95 m high and 158.7 m long, to house six units of reversible vertical draft pump-turbine power generating equipment, each unit rated 267 MW, having a total installed capacity of 1,600 MW. A transformer cavern, 13.5 m wide, 18.0 m high and 170.5 m long, is located downstream and parallel to the power cavern at a distance of 45 m, centerline to centerline. The cavern layout and the cross section of the power and transformer caverns are shown in Figure 2.

The main phase of construction of the project has been in full swing since September 1987. The first unit is scheduled for commission in April 1992. The total excavation of §he power cavern will amount to 170,000 m3 and is expected to be completed in August 1989. As of May 1988, the excavation of the pilot tunnel (top heading) was complete and slashing out of the haunches was in progress on both sides.

1.2 Site geology

The powerhouse and transformer caverns are located in the rock sequence of the Waichecheng Series which is mainly composed of bedded to massive, fine-grained to conglomeratic, and graywacke to quartzitic sandstone, occasionally with interlayers of siltstone or interbedded sandstone and siltstone. Figures 3 and 4 present the geology and rock mass quality of the powerhouse cavern.

The most prominent geologic feature in the cavern area is a series of bedding parallel faults (clay gouge seams parallel to bedding).

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