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Fengtan HPP, located in Yuanling County of Hunan Province, has the first high hollow gravity arch dam (112.5m high) in China and an installed capacity of 800MW (including extension). It is mainly for power generation and concurrently for flood control, navigation, irrigation and fishery.
Investigation and design of the project was started in 1969, construction started in 1970, and the first unit was put into service in 1978. The project was basically completed in 1979.
A scheme of concrete hollow gravity arch dam plus powerhouse within the dam was adopted for the general project layout. The project consists of a concrete hollow arch dam (maximum height 112.5m), flood-discharging bottom outlets, headrace system (including intake and penstock), a powerhouse within the dam, a substation, a logway for ship and logs, and so on.
(1) Concrete hollow gravity arch dam and powerhouse within the dam
The scheme of concrete hollow gravity arch dam and powerhouse within the dam, compared to the scheme of common gravity dam and underground powerhouse and to the scheme of gravity dam and powerhouse within the dam, could save 250,000 m3 of rock tunnel excavation and 200,000 m3 of concrete, respectively. This practice of large hollow gravity arch dam and powerhouse within the hollow dam was the first time in China and has provided proven technologies in development of rivers with narrow valley and large discharge in the southern part of China.
(2) High-low bucket energy dissipation through strike in air
The 32,600m3/s large discharge from the dam would concentrate in radial direction and intensify scouring to the downstream areas. High-low bucket energy dissipation through strike in air was adopted. 13 stagger surface bays were arranged on the dam crest, 6 higher and 7 lower. In flood releasing, two layers (upper and lower) of flow would be formed and hit (in the air) transversely and longitudinally, be subject to strong disturbance and diffused aeration, and thus significantly increase the energy dissipation rate of water flow in the air and mitigate scouring. This type of energy dissipater has such advantages as good energy dissipation effect, shallow scouring pit, simple structure, and convenient construction, so it is suitable for arch dams with large discharge in narrow valley.
(3) Dam foundation consolidation by PU
To improve seepage control and consolidation of foundation defects of argillation intercalations and fault fracture zones of the dam foundation, compound impervious curtain of cement grouting and PU chemical grouting was employed, with cement grouting to fill big fractures in rock mass and create basic curtain, and PU grouting to fill tiny fractures of weak formations for reinforcement. The practice of Fengtan project has proven that PU chemical grouting can seal argillation intercalations of dam foundation and cement fracture zones, thus to play a role of dam foundation seepage control and consolidation.
(4) Improvement of quick arch closure method for hollow dam
Arch closing is critical in construction progress control of hollow dams. The method of arch closing with arch bracket was adopted for construction. Prefabricated arch rib and arch bracket were 3.85t and 9t, respectively, and have a span of 15.35m. They were designed as double hinged arch and positioned on the brackets extending from the dam with 10t gantry crane. The arch crown could be concreted after the arch rib and arch bracket are covered with prefabricated arch slabs. The practice has proven that this method is successful.
(5) Solving the problem of mutual interference between powerhouse and dam construction
For a project with its powerhouse located within the dam, construction interference is a focal problem. The construction interference between powerhouse and dam before arch closing mainly reflected on that the penstock and draft tubes must be installed while the fore and rear brackets were built. After arch closing, the concrete placement for the dam and construction and installation inside the powerhouse could be carried out at the same time. Since the height of the hollow was only one third of the dam height, the quantities of work over the arch were still large. Therefore, civil construction and electro-mechanical equipment installation inside the powerhouse would not impact the schedule for power generation.