Power Plant Engineering

11.9.1 ADVANTAGES AND DISADVANTAGES OF UNDERGROUND POWER-HOUSE
Advantages
1. Under suitable geological conditions, the underground conduit may prove the shortest and
sometimes even straight. The power conduit may be much shorter than the length of power
canal used for underground power house as the power canal usually built to follow the con-
tours of the terrain. By locating the power house underground, the number of restrictions as
safe topographical and geological conditions along the penstock and sufficient space at the
foot of the hill for constructing the power house are completely eliminated.
2. The construction of underground conduit instead of penstock results in considerable saving
in steel, the internal pressure is carried partly by the rock if it is of good quality. In sound high
quality rock, the penstock is replaced by an inclined or vertical pressure shaft excavated in
rock and provided with a steel lining of greatly reduced thickness in comparison with ex-
posed penstock ‘roe purpose’ of lining in such cases is protection against the seepage losses.
3. The reduced length of the pressure conduit reduced the pressures developed due to water-
hammer. Therefore, smaller surge tank is also sufficient.
4. For the economical arrangement, the ratio of the pressure conduit to the tail-race tunnel is
also significant. The overall cost of the system is lower if the tail-race tunnel length is rela-
tively large.
5. The construction work at underground power station can continue uninterrupted even under
severest winter conditions. The overall construction cost and period of construction is re-
duced due to continuity of work.
6. Much care is devoted today in many countries to preserve landscape features such as pictur-
esque rock walls, canyons, valleys and river banks in their original beauty against spoiling by
exposed penstocks, canal basins and machine halls. There is less danger of disturbance to
amenities with an underground power house and pipelines. The other advantages gained by
constructing underground power house are listed below. The six advantages mentioned above
reduce the constructional difficulties and overall cost of the plant and preserve the original

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POWER PLANT ENGINEERING
beauty of landscape. The overall cost is further reduced by the modern techniques in tunnel
work and better excavation process.
7. The shorter power conduit of underground power house reduces the head losses.
8. The regular maintenance and repair costs are lower for underground stations as the mainte-
nance required for rock tunnels is less.
9. The power plant is free from landslides, avalanches, heavy snow and rainfall.
10. The useful life of the structures excavated in rock is considerably longer than that of concrete
and reinforced concrete structures.
11. It is possible to improve the governing of the turbines with the construction of underground
power house.
12. The construction period is reduced mainly due to the possibility of full-scale construction
work in winter.
13. Underground power station is bomb-proof and may be preferred for military reasons: They
are perfectly protected against air-raids. The military considerations became more predomi-
nant with the increased shadow of the war and the building of underground power stations
underwent a rapid evolution after second world war.