Geological
explorations for reservoir sites
Though a
dam is constructed to build a reservoir, a reservoir has a large area
of spread and contained in a big chunk of the river valley upstream
of the dam. Hence, while identifying a suitable site for a proposed
dam, it is of paramount importance that the proposed reservoir site
is also thoroughly investigated and explored. The basis of planning
for such explorations is to have a rapid economical and dependable
pre-investment evaluation of subsurface conditions. It is also
necessary that a degree of uniformity be followed while carrying out
subsurface explorations so that the frame of reference of the
investigation covers all requisite aspects. In view of above, the
Bureau of Indian Standards has brought out a code IS: 13216 - 1991
“Code of practice for geological exploration for reservoir sites”,
that discusses the relevant aspects. According to the code since
reservoir projects in river valleys are meant to hold water;
therefore, the following aspects of the reservoirs have to be
properly investigated
(a) Water
tightness of the basins
(b)
Stability of the reservoir rim
(c)
Availability of construction material in the reservoir area
(d)
Silting
(e)
Direct and indirect submergence of economic mineral wealth
(f)
Seismo-techtonics
These
aspects are determined through investigations carried out by surface
and sub- surface exploration of proposed basin during the
reconnaissance, preliminary investigation, detailed investigation,
construction and post-construction stages of the project. The two
basic stages of investigation: reconnaissance and preliminary
investigations are explained below:
Reconnaissance
In the
reconnaissance stage, the objective of investigation is to bring out
the overall geological features of the reservoir and the adjacent
area to enable the designers, construction engineers and geologists
to pinpoint the geotechnical and ecological problems which have to be
tackled. The scale of geological mapping for this stage of work need
not be very large and the available geological maps on 1:50,000 or 1:
250,000 scales may be made use of. It is advantageous to carry out
photo geological interpretation of aerial photographs of the area, if
available. If a geological map of the area is not available, a
traverse geological map should be prepared at this stage preferably
using the aerial photos as base maps on which the engineering
evaluation of the various geotechnical features exposed in the area
should be depicted.
A
topographical index map on 1: 50 000 scales should be used at this
stage to delineate the areas which would require detailed study,
subsequently. To prevent an undesirable amount of leakage from the
reservoir, the likely zones of such leakage, such as major
dislocations and pervious or cavernous formations running across the
divide of the reservoir should be identified at this stage of
investigation for further detailed investigations. Major unstable
zones, particularly in the vicinity of the dam in tight gorges,
should be identified at this stage for carrying out detailed
investigations for the stability of the
reservoir
rim. The locations for suitable construction material available in
the reservoir area should be
pin
pointed at this stage so that after detailed surveys such materials
can be exploited for proper utilization during the construction stage
prior to impounding of reservoir. The rate of silting of the
reservoir is vital for planning the height of the dam and working out
the economic life of the project. Since the rate of silting, in
addition to other factors, is dependent on the type of terrain in the
catchment area of the reservoir, the major geological formations and
the ecological set up should be recognized at this stage to enable a
more accurate estimation of the rate of silting of the reservoir. For
example, it
should be
possible to estimate at this stage that forty percent of the
catchment of a storage dam project is covered by Quaternary sediment
and that this is a condition which is likely to a yield a high silt
rate or that ninety percent of the catchment of another storage dam
project is composed of igneous and metamorphic rocks and is likely to
yield a relatively low sediment rate. This information will also be
useful in examining whether or not tributaries flowing for long
distances through soft or unconsolidated
formations,
prior to forming the proposed reservoir, can be avoided and if not,
what remedial measures can be taken to control the silt load brought
by these tributaries. The impounding of a reservoir may submerge
economic/strategic mineral deposits occurring within the reservoir
area or the resultant rise in the water table around the reservoir
may cause flooding, increased seepage in quarries and mines located
in the area and water logging in other areas. It is, therefore,
necessary that the economic mineral deposits, which are likely to be
adversely affected by the reservoir area, are identified at this
stage of the investigation. For example, if an underground working is
located close to a proposed storage reservoir area, it should be
identified for regular systematic geo-hydrological studies
subsequently. These studies would establish whether the impoundment
of the water in the reservoir had adversely affected the underground
working or not. References should also be made to various agencies
dealing
with the economic minerals likely to be affected by the impoundment
in the reservoir for proper evaluation of the problem and suitable
necessary action. A dam and its reservoir are affected by the
environment in which they are located and in turn they also change
the environment. Impoundment of a reservoir sometimes results in an
increase of seismic activity at, or near the reservoir. The seismic
activity may lead to microtremors and in some cases lead to
earthquakes of high magnitude. It is, therefore, necessary to
undertake the regional seismotechtonic study of the project area. The
faults having active seismic status should be delineated at this
stage.
Simultaneous
action to plan and install a network of seismological observatories
encompassing the reservoir area should also be taken.
Preliminary
Investigation
The
object of preliminary investigation of the reservoir area is to
collect further details of the surface and subsurface geological
conditions, with reference to the likely problems identified during
the reconnaissance stage of investigation by means of surface mapping
supplemented by photo geological interpretation of aerial
photographs, hydro geological investigations, geophysical
investigations, preliminary subsurface exploration and by conducting
geo-seismological studies of the area.
On the
basis of studies carried out during the reconnaissance stage it
should be possible to estimate the extent of exploration that may be
required during the preliminary stage of investigation including the
total number of holes required to be drilled and the total number and
depth of pits, trenches and drifts as also the extent of geophysical
surveys which may be necessary. For exploration by pits, trenches,
drifts and shafts guidelines laid down in IS 4453: 1980 Name of IS
code should be followed.
The
potential zones of leakage from the reservoir and the lateral extent
of various features, such as extent of aeolian sand deposits, glacial
till, land slides, major dislocations or pervious and cavernous
formations running across the divide, should be delineated on a
scale of 1: 50000.
The
geo-hydrological conditions of the reservoir rim should be
established by surface and sub-surface investigation as well as
inventory, as a free ground water divide rising above the proposed
level of the reservoir is a favourable condition against leakage from
the reservoir. The level of water in a bore hole should be determined
as given in IS 6935: 1973.
The
extension of various features at depth, wherever necessary, is
investigated by geophysical exploration and by means of pits,
trenches, drifts and drill holes. For example, the resistivity survey
should be able to identify water saturated zones. The nature of the
material is investigated by means of laboratory and in situ tests, to
determine permeability and assess the quantum of leakage which may
take place through these zones on impoundment of the reservoir.
Moreover, permeability of
rocks/overburden
in the reservoir area is determined from water table fluctuations and
pumping tests in wells. For determining in situ permeability in
overburden and rock, reference should be made to IS 5529 (Part I):
1985 and IS: 5529 (Part II): 1985 respectively. The information about
permeability would enable the designers to estimate the treatment
cost for controlling leakage/seepage from the reservoir and to decide
whether it would be desirable to change the location of height of the
dam to avoid these
zones.
Major
unstable zones along the reservoir identified during the
reconnaissance stage and which are of consequence to the storage
scheme should be investigated in detail at his stage by means of
surface and sub-surface exploration.
The areas
should be geologically mapped in detail on a scale of 1: 2000. The
suspect planes/zones of failure should be identified and explored by
means of drifts, trenches and pits. Disturbed and undisturbed samples
of the plastic material should be tested for cohesion (c) and angle
of internal friction (φ) as well as for other relevant properties.
The stability of slopes should also be evaluated considering the
reservoirs operational conditions. These studies should provide the
designers with an idea of the magnitude of the problems that may be
encountered, so that they may be able to take remedial measures to
stabilize zones or to abandon the site altogether, if the situation
demands.
The areas
having potential economic mineral wealth and which are likely to be
adversely affected by the impoundment of the reservoir should be
explored by means of surface and sub-surface investigation to
establish their importance both in terms of their value as well as
strategic importance. This information would be necessary for
arriving at a decision regarding the submergence, or otherwise, of
the mineral deposit. The nature and amount of the existing seepage,
if any, in the existing mines and quarries in the adjacent areas of
the reservoir should be recorded and monitored regularly. This data
is necessary, to ascertain whether or not there has been any change
in the quantum of seepage in the mines and quarries due to the
impoundment of water in the reservoir, directly or indirectly.
Large
scale geological mapping and terrace matching across the faults with
seismically active status, delineated during the reconnaissance
stage, should be carried out on a scale of 1 : 2000 and the trend,
and behaviour of the fault plane should be investigated in detail by
means of surface studies and sub-surface exploration by pits,
trenches and drifts etc. A network of geodetic survey points should
be established on either side of the suspected faults to study
micro-movements along these suspected faults, if any, both prior to
and after impoundment of the reservoir. Micro earthquake studies
should be carried out using portable 3-station or 4-station networks
in areas with proven seismically active fault features.
On the
basis of the studies carried out during the preliminary stage it
should be possible to estimate the quantum of exploration which may
be required during the detailed stage of investigation including the
total number of holes required to be drilled and the total number and
depth of pits, trenches and drifts as also the extent of geophysical
survey which may be necessary.
Detailed
surface and sub-surface investigation of all features connected with
the reservoir should be carried out to provide information on leakage
of water through the periphery and/or basin of the reservoir area.
Based on these investigations and analysis of data it should be
possible to decide as to whether the reservoir area in question would
hold water without undue leakage. If, not, the dam site may have to
be abandoned in favour of suitable alternative site.
The
zones, which on preliminary investigation are found to be potential
zones of leakage/seepage from the reservoir, and which due to other
considerations cannot be avoided are geologically mapped on a scale
of 1 : 2 000 and investigated in detail at this stage by means of a
close spaced sub-surface exploration programme. The purpose of this
stage of investigation is to provide the designers sufficient data to
enable them to plan the programme of remedial treatment. The
sub-surface explorations are carried out by means of pits and
trenches, if the depth to be explored is shallow, say up to 5 meters,
and by drill holes and drifts, if the depth to be explored is greater
than 5 meters.
The
unstable zones around the reservoir rim, specially those close to the
dam sites in tight gorges, should be explored in detail by means of
drifts, pits and trenches so that the likely planes of failures are
located with precision. The physical properties including angle of
internal friction and cohesion of representative samples of the
material along which movement is anticipated should be determined.
The above information would enable the designers to work out details
for preventive measures, for example, it may be possible to unload
the top of the slide area or to load the toe of the slide with well
drained
material, within economic limits.
Sub-surface
explorations by drill holes, drifts, pits and trenches should be
carried out at possible locations of check dams and at the locations
of other preventive structures proposed to restrict the flow of silt
into the reservoir. These studies would enable the designers to
assess the feasibility of such proposals.
Detailed
plans, regarding the economic mineral deposits within the zones of
influence of the reservoir should be finalized during this stage by
the concerned agencies. The seepage investigations in the quarries
and mines within the zone of influence of the reservoir should be continued angle of internal friction (φ) as well as for other
relevant properties.
(source: CE, IIT Kharagpur)
