HYDRO-ELECTRIC POWER PLANTS 359
The considerations supporting the construction of underground power stations are stated below :- Non-availability of a suitable site for a conventional surface station and good slope for penstock.
- Danger of falling rocks and snow avalanches particularly in narrow valleys.
- Availability of underground sound rock and avoidance of a long pressure tunnel and facility
for a convenient tail-race outlet. - Possibility of elimination of surge tank required for surface station due to long pressure tun-
nel - The rugged topographical features and the difficulties in finding a suitable short and steep
slope for pipe lines make it more economical to install the water conduit, the machine, trans-
former hall and tailrace system underground. - Foundation costs for overground power house become excessive in case of poor quality sur-
face layers. The construction of draft tube, spiral case and separating floors in loose weath-
ered rock is again more expensive than the excavation of corresponding parts underground.
The costs of underground machine hall are lower than those of the superstructure of a surface
powerhouse of similar dimensions.
11.9.1 Advantages and Disadvantages of Underground Power-House
Advantages- 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. - 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. - The reduced length of the pressure conduit reduced the pressures developed due to water-
hammer. Therefore, smaller surge tank is also sufficient. - 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. - 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. - 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