Geological Considerations - Dam

GEOLOGICAL CONSIDERATIONS - DAM
ENGR. MD. SHAMIM AHMED
LECTURER
DEPT. OF CIVIL ENGINEERING
SOUTHERN UNIVERSITY BANGLADESH

DAMS
A dam may be defined as a solid barrier constructed at suitable location
across river valley with a view of impounding water flowing through that
river.
Dams are created for the following objectives:
Generation of hydropower energy
Providing water for irrigation facilities
Fish farming
Fighting droughts
Controlling of floods
Water supply for domestic consumption
Providing navigational facilities

Terminologies
• Heel: contact with the ground on the upstream side.
• Toe: contact on the downstream side.
• Abutment: Sides of the valley on which the structure of the dam rest.
• Galleries: small rooms like structure left within the dam for checking
operations.
• Diversion tunnel: Tunnels are constructed for diverting water before
the construction of dam. This helps in keeping the river bed dry.
• Spillways: It is the arrangement near the top to release the excess
water of the reservoir to downstream side.
• Sluiceway: An opening in the dam near the ground level, which is
used to clear the silt accumulation in the reservoir side.

Types of Dams
On the basis of design:
o Gravity dams
o Buttress dams
o Arch dams
o Earth dams

1. Gravity dam
• A solid concrete or masonry structures, that withstands the water
pressure, by virtue of its weight
• All forces acting on the dam are assumed to be directly transmitted
to the foundation rocks
• They are generally of triangular profile and are among the safest

2. Buttress dam
• They have a continuous upstream face, supported at regular
intervals, by buttress walls on the downstream side.
• They are lighter than solid dams.
• Likely to induce greater stresses at the foundation, since most of the
load passes through the buttress walls and is not spread uniformly over
the foundation.

3. Arch dam
• Arch-shaped, convex at the upstream side.
• Major portion of the thrust forces acting on the dam are transmitted
by arch action, onto the abutment rocks.
• Structural efficiency is higher than that of gravity dams, the presence
of sound abutments is a prime necessity.
• Uses less amount of concrete

4. Earth dam
• Non-rigid structures, built with naturally available materials such as
earth and rock.
• Ideal, where the dam site is weak to support concrete dams, or
where competent rocks are found at great depths.
• They are trapezoidal in shape, relatively smaller in height and broad
at the base.

4. Composite dam
• Composite dams are combinations of one
or more dam types.
• Most often a large section of a dam will
be either an embankment or gravity dam,
with the section responsible for power
generation being a buttress or arch.

Geotechnical Considerations for Dam Site Selection
• Topography- a place is selected ideally in narrow gorge or small valley
with enough catchment area behind dam is so that calculated amount
of water can be easily stored in the reservoir created upstream..
• Technically- a site should have strong, impermeable and stable rock
body.
• Constructionally- site should not be much away from the deposits of
material which required for construction.
• Economical
• Environmental

Geological investigation
•General geology of area-
–In detailed mapping of the area reveals the facts like
 Topographic features
 Natural drainage patterns
 General characters and structures of rock formation like
stratification, folding and faulting
 Trend and rate of erosion in the area.

Geological investigation …
• Lithology-
–It is the most important factor in dam construction investigation.
–Lithology in and around area with surface and subsurface lithology is
investigated.
–These studies reveals the composition and textures of rock at site.
–It gives the idea about the rock type as igneous, sedimentary or
metamorphic.
–It also shows that the area is made up of single thick layer of same
rock type or with multiple layers.

• Structures-
–These involves detail study of and mapping of planes of weakness like faults,
fold, joints, bedding plane etc.
–Dip and strike:
 Strength of unfractured stratified rock is greater when stresses are acting
normal to the bedding planes .
 Horizontal beds are stable base
 Dipping upstream are stable
 Dipping down stream unstable

– Fault
 The faulted rocks are generally shattered and weak along the rupture
surface
 Faulted land surfaces are the favorable sites for shocks during
earthquakes.
 Small faults and shear zones are
treated with some methods
 Deep and major faults are avoided
at construction site.

– Folds:
In syncline bends dams placed on the upstream limbs would cause leakage
from beneath the dam.
– Joints:
According to the nature of joints the appropriate engineering technique is
used to feel up the joints.

RESERVOIR
A reservoir is a large, artificial lake created by constructing a dam across a
river.

SELECTION OF SITE FOR A RESERVOIR:
•At the site river valley should be narrow so the length of the dam to be constructed is less.
•Basin should be watertight and free from fissures, cracks, so that there is no loss of water
due to percolation.
•The site should be such that as far as possible minimum land and property is submerged in
the reservoir.
•It should be such that it avoids water from the tributaries which carry usually high content of
sediment.
•The site must be such that adequate reservoir capacity is available for the desired purpose.
•The site should be such that a deep reservoir may be formed so that the land costs per unit
of capacity are low, evaporation loss is less and there is less likelihood of weed growth.
•The soil and mass at the reservoir site should not contain any objectionable soluble
minerals and salts which may get dissolved in water and deteriorate water quality.

SELECTION OF SITE FOR A RESERVOIR:
•The quality of water stored in the reservoir must be satisfactory available for its intended
use.
•The site should be easily accessible by road or railway.
•The construction materials for the dam should be available in the vicinity of the site.
•The site should be such that the costs of associated works such as roads, rails, housing
colonies for workers and staff, etc. should be low.

Geological Considerations - Dam

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