10 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
The stability of slopes will affect the design of all components
of a micro-hydro scheme and should therefore be analysed
thoroughly, particularly in the following key areas:
Above and below proposed canal routes.
Below the proposed location of a settling basin or a forebay
tank.
Along the proposed penstock alignment.
Above and below the proposed location of the power-house.
Threats in these areas will either take the form of weakening of
the support around the foundations through land slipping away
or collapsing, or damage to structures through falling debris, as
shown in Figures 2.3 and 2.4 and Photographs 2.1 and 2.2.
Indicators of slope instability are presented in Table 2.1.
The following features of the slope or rock face indicate
slope stability:
Complete vegetation cover, including trees standing
vertically
Straight, even, slope profile
Rock surfaces covered with moss, lichen or a weathere skin
Hard, impermeable rock
Rock with no or few joints
Closed rock joints
Well-packed debris, especially with fine material packed
into voids between coarse material
Well-established trees and shrubs
No active gullying (although a stable gully system may be
present)
The increased knowledge of the site gained from a thorough
investigation of the slopes will influence the design of the
whole scheme, particularly the location of principal structures.
The recommendations from the investigation of slope stability
should follow two basic rules:
Never construct on fill, that is, land which has been built-
up or filled using excavated material.
Avoid the location of structures close to landslide zones.
2.4.4 SOIL AND ROCK TYPES
The surveyor should investigate what local construction
materials such as soil and rock are available at site. Possible
uses of such materials are presented in Table 2.2.
The type of soil or rock also affects the foundation of structures
and the canal type. For example, if the soil type is sandy loam,
a larger foundation depth is required. On the other hand,
structures may be built directly on hard rock without any
excavation. Similarly, lining may not be required for headrace
canal if the soil type is clay. However, lining will be required
if the alignment is through sandy soil.
Subsidence is caused by the location of acid substances in
the local groundwater acting on soluble rocks such as
limestone, by the presence of rocks which are liable to splitting
and foliation, or by underground caverns which are prone to
collapse. The presence of thick layers of loose sandy soil may
also lead to subsidence.
These characteristics are identified by careful observation of
the site. Limestone outcrops, sinkholes (holes of 2-10 m in
diameter which form when the limestone beneath dissolves,
causing the soil above to collapse), the appearance of streams
or other seepage from depressions or cracks in the ground
surface are examples of characteristics to look for.
Undermining refers to the action of surface water on the
foundations of structures. The intake of the scheme and the
penstock are particularly prone to undermining where surface
water threatens the structures, but the headrace canal is also
vulnerable.
2.5 Hydrology and water availability
2.5.1 PREDICTION METHODOLOGIES
Hydrology dictates the size of various micro-hydro
components like the turbine, channel and the penstock. It
also has great influence on the scheme being designed over
or under capacity. The general practice in Nepal for micro-
hydro schemes is to visit the site during dry season and
measure the flow. The scheme is then designed based on
TABLE 2.2 Possible use of soil and rock in micro-hydropower
TYPE OF SOIL
OR ROCK
Sand
POSSIBLE USE IN
MICRO-HYDRO CIVIL WORKS
Concrete
Gravel
Rocks
Clay
Aggregate in concrete
Bed material in drains
Gabions and masonry
Canal lining
TREATMENT BEFORE USE
Must be selected or thoroughly washed to be free from
organic matter, fine particles, clay lumps and excessive
mica.
Must be carefully sorted and washed.
Must be washed.
Must be selected for correct type of rock, density and size.
Testing of clay content required to ensure that it will
serve as an impermeable layer.