For some hydropower stations having a wide range of tailwater level, there will be many advantages if a small tailrace air cushion surge chamber is used to take the place of the conventional surge chamber. Generally, it can improve the stability of surrounding rock mass of openings, reduce construction period and construction cost, and improve the operational conditions of units. Air escaping from the chamber, supplement of air into it and inconvenience of operation as regards air cushion surge chamber are general problems of concern. But all these problems will be eliminated after some proper engineering measures are taken.
At present, about 100 large scale or medium-size power stations are under exploration, planning, design and construction in China, with a total installed capacity being 'approximately 1.1 ×105 MW. Amongthem, 5–6 × 104 MW will have been put into operation before 2000. A forty-- two percent of the 1. 1 × 105 MW will be installed in underground power stations mostly located at remote mountainous areas and deep river valleys in the southwest and northwest of China. The variation range of tail water levels is very wide, up to more than 20m, some even over 40m.
Including the range of oscillations caused by hydraulic transient, the total level variation range in the tailrace surge chamber of underground power station will be up to 50–70m. Probably, this will make the surge chamber larger than the main powerhouse cavern.
In some pumped storage power stations to be constructed, the conventional tailrace surge chamber will also become a very huge one since the installed elevation of the turbine is very low, and variation range of levels is relatively wide.
Table 1 is a list of dimensions of the conventional tailrace surge chambers adopted in some underground power stations during their feasibility or preliminary design stages.
(Table in full paper)
Tailrace surge chamber, main power house and main transformer chamber usually form three parallel underground openings, and between which some penstocks, tailrace tunnels, cable galleries and access tunnels are excavated. Therefore, the surrounding rock mass of these openings is likely unstable because it is repeatedly influenced by blasting and excavation.
In order to ensure the stability of the surrounding rock, measures should be taken, either increasing the supporting work, or enlarging the thickness of rock between caverns. As a rule of thumb, the net width of this rock wall should be 0.6–0.8 times of the height of the largest cavern.
However, the total distance from the upstream surface of powerhouse to the downstream surface of surge chamber will be rather long. In some large-scale stations it will be up to 150m.
If this range is too wide then it will bring about some unfavorable effects, such as:
The tailrace tunnel becomes too long, more than 15–20 times of the diameter of turbine runner.