Lecture 5

Reservoir
 Definition of Reservoir
 Investigations of reservoir site
 Geological problems in
reservoirs
 Types of Reservoirs
 Storage Capacity of Reservoirs
2
Dam
 Dam types and Loads on
Dams
 Site Investigation of Dam
Sites
 Selection of Type of Dam
 Geological consideration of
dam site
 Causes of dam failure
Chapter 5 Outline

Reservoir
1. Definition of Reservoir
2. Investigations of reservoir site
3. Geological problems in reservoirs
4. Types of reservoir

At the end of this chapter
Students will be able to understand about different
dams and reservoirs
Students will be able to understand problems in dam
foundation, abutments and reservoir
Students will be acquire knowledge to fix problems in
dam
Students will be familiar with dam and reservoir site
investigation
4

5
Introduction: definitions and concepts
Reservoir: a water body or lake which could be created when a barrier is
constructed across a river or a stream.
Advantages/uses of reservoirs:
 Water supply.
 Irrigation.
 Hydroelectric power generation.
 Recreation.
 Flood control.
 Navigation, and others.
Disadvantages of reservoirs:
 Detract from natural settings, ruin nature's work.
 Inundate the spawning grounds of fish, and the potential for
archaeological findings.
 Inhibit the seasonal migration of fish, and even endanger some
species of fish.
 Foster diseases if not properly maintained.
 Water can evaporate significantly.
 Induce earthquakes.

Factors that affect Reservoir
The most important factor are:
a) Location of the dam
b) Run-off characteristics of the catchment area.
c) Water tightness of the proposed reservoir basin.
d) Reservoir rim stability.
e) Rate of sedimentation in the reservoir.
f) Water quality and
g) Seismic activity induced by the reservoir.
6

Reservoir
 Factors which influence the feasibility and economics of a
proposed reservoir site are:
Location of the dam

Reservoir
Run-off characteristics of the catchment area

Reservoir
Water tightness of the proposed reservoir basin

Reservoir
Reservoir rim stability

Reservoir
Rate of sedimentation in the reservoir

Reservoir
Water quality(the effect of water and its contents) on
building materials, especially concrete)

Reservoir
Seismic activity induced by the reservoir
Present land use and social factors

Investigation of Reservoir Sites
The investigation of reservoir sites can be
concentrated on
Topographic surveys
Geological investigations
Hydrological investigations
14

Topographic Surveys
Conducted for dam, reservoir and other associated
work.
Topographic survey of the area is carried out to
evaluate the landscapes of the area
evaluate accessibility of the area
get most economical reservoir area
15

Hydrological Investigations
The hydrological investigations are conducted for the
following purposes :
(i) To study the runoff pattern and storage
capacity.
(ii) To determine the maximum discharge
at the site.
16

Hydrological Investigations Cont.
In an investigation of a potential reservoir site, consideration must be
given to :
Rainfall
 Runoff
Infiltration
Evapotranspiration
Topography
Geological condition
 Vegetation cover/land use and land cover
 The two basic type of data needed for reservoir design are:
Topographical maps
Hydraulic records

Topographical Maps
 Storage capacity
 Catchment area
and drainage
density
 Reservoir volume
Scale 1 Km
Example
Estimate the storage capacity of reservoir for Tefenya dam at MRL 2200m. Contour
interval is 20m.

Solution
 Storage capacity of the
reservoir is equal to
(A+B)CI/2 + (B+C)CI/2
+ (C+D)CI/2.
 Where A, B, C and D
are the area between
consecutive contour
and CI is the contour
interval.

Area No. Area b/n consecutive contour in m2
A-1 5899.18
A-2 5519.34
A-3 5240.67
A-4 5410.52
A-5 2319.08
A-6 317.77

Hydrological Records
 Hydrological records: amount of water available for
storage purpose.
 Flood peaks and Volumes
Rainfall
Stream flow records
 The hydrological elaborations which are reported hereafter
are aimed at the study of the following aspects:
mean annual runoff and its monthly variability,
 flood
sediment transport

Geological Investigation
Geological investigations of the dam and reservoir site
are done for the following purposes.
(i) Suitability of foundation for the dam.
(ii) Watertightness of the reservoir basin
(iii) Location of the quarry sites for the
construction materials.
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Engineering Geological Investigation
1. Watertightness of the reservoir..
 Hence the objective of the reservoir is to store water, so it is important to
examine carefully, the lithological and geomorphological condition in the
reservoir and surrounding areas because
 Water can escape…
 Through permeable rock slope…
 Through Continuous Fissures…
 Through Terrace Deposits
 Through ancient Buried Valleys
Reservoir impounded water may escape through permeable beds into the adjoining valley

2. Stability of the reservoir slopes
The slope stability condition of the area can be
 affected the function of the dam.
 reduced the storage capacity of the reservoir.
 caused over topping of the dam.
Wave generation in
reservoir due to
landslide
Example: Vajoint dam, Italy, in 1964 experienced such seiches,
generated due to a catastrophic failure in the reservoir area, killing
2000 people in the low lying areas.

3. Siltation of the reservoir
 The siltation rate of the reservoir is depend on
 the amount and rate of inflow and the amount of
solid material supplied.
 the size of a drainage basin
The rock type, drainage density and gradient of
slope.
The time required to fill the critical storage
volume.
The land use and land cover condition of the
areas

Considerations for successful reservoirs
A reservoir can be claimed to be successful if:
♠ It is water tight or it does not suffer from any serious leakage of water.
♠ It has a long life due to a very low rate of silting in the reservoir basin.
♠ It should have adequate capacity to hold a large and desirable quantity
of water.
♠ It should have less chances of occurrence of seismicity and landslides.
♠ It has a foundation rock with a good bearing capacity.
Problems associated with reservoirs
• The main geological problems associated with the reservoirs are
♠ Ground water conditions
♠ Silting
♠ Permeable rocks
26

Ground water conditions
Rivers which loses water are known as influent rivers.
Hence there is leakage under reservoirs.
Rivers which gain water from the ground water are
known as Effluent Rivers. Hence there is no leakage.
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Fig.8.(a) Influent and (b) Effluent river
condition at reservoir site

Silting of Reservoirs
The amount of silt produced and supplied to the rivers depends mainly on:
♠ lithological character and
♠ topography of the catchment area.
The rivers flowing over the soft rocks and high gradient areas carry greater
amounts of silt. The measures that help to reduced silting of reservoirs are:
♠ Vegetation
♠ Covering with slabs on weak zones
♠ Terracing of the slope and construction of retaining walls
♠ Check dams
♠ By diversion of sediment-loaded waters.
Permeable rocks
The rocks, which are highly porous, are likely to cause series leakage from the
reservoir.
The following methods used to seal permeable zones:
♠ Natural silting
♠ Grouting
♠ Covering weak zones with concrete slabs. 28

Site Selection
The following points are important to select
suitable sites for reservoir
Large storage capacity
River valley should be narrow, length of dam to
constructed is less.
Water tightness of reservoir.
Good hydrological conditions
Deep reservoir
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Small submerged area
Low silt inflow
No objectionable minerals
Low cost of real estate
Site easily accessible
30
Site Selection Cont.

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5.4 Types of reservoirs
Depending upon the purposes they serve, reservoirs may be classified into
the following:
i. Storage or Conservation Reservoirs:
 Reservoirs constructed across a stream or river to store the
seasonal flows (low flows and peak flows).
 Commonly used for water supply, irrigation, hydropower generation,
recreation, etc.
ii. Flood Control Reservoirs:
 Stores a portion of the flood flow in such a way as to minimize the
flood peaks at the areas to be protected downstream
iii. Distribution Reservoirs:
 Temporary storage facilities provided to store flows during low flows
to be used for peak flows. E.g. Water supplies ponds, Night storage
pond for irrigation.
iv. Multipurpose Reservoirs:
 A reservoir planned and constructed to serve not only one purpose
but various purposes together. E.g. Irrigation, water supply,
recreation, hydropower, flood control, navigation etc.

Reservoir:
 Important physical characteristics of reservoir:
Storage capacity
Yield/Safe yield
32

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Storage Capacity of Reservoirs
 The volume of water stored in the reservoir can be refereed as
reservoir capacity.
 It is determined after identifying the surface area covered
by water and elevation differences between consecutive
areas.
 The surface areas are determined from topographic maps of the
reservoir area.
 There are a number of options to determine the storage capacity
of a reservoir (E.g. End area method, prismoidal).

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Storage Capacity
calculation formulae:
1. Trapezoidal formula.
2. Cone formula.
3. Prismoidal formula.
4. Storage Volume from
cross-sectional areas.

Gross storage = Live storage + Dead storage
Live storage
Dead storage
Dam
36

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Basic Terms and Definitions of Reservoir Zone
(1) Full reservoir level (FRL): is the highest water level to which the
water surface will rise during normal operating conditions. Also called
the full tank level (FTL) or the normal pool level (NPL).
(2) Maximum water level (MWL): is the maximum level to which the
water surface will rise when the design flood passes over the spillway.
Also called the maximum pool level (MPL) or maximum flood level
(MFL).
(3) Minimum pool level: is the lowest level up to which the water is
withdrawn from the reservoir under ordinary conditions. It corresponds
to the elevation of the lowest outlet (or sluiceway) of the dam.
 However, in the case of a reservoir for hydroelectric power; the
minimum pool level is fixed after considering the minimum working
head required for the efficient working of turbines.

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(4) Useful storage: volume of water stored between the full reservoir level and the
minimum pool level. Also known as the live storage.
(5) Surcharge storage: is the volume of water stored above the full reservoir level
upto the maximum water level. The surcharge storage is an uncontrolled storage
which exists only when the river is in flood and the flood water is passing over the
spillway. This storage is available only for the absorption of flood and it cannot be
used for other purposes.
(6) Dead storage: volume of water held below the minimum pool level. The dead
storage is not useful, as it cannot be used for any purpose under ordinary operating
conditions.

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(7) Bank storage: If the banks of the reservoir are porous, some water is
temporarily stored by them when the reservoir is full.
(8) Valley storage: The volume of water held by the natural river channel in
its valley upto the top of its banks before the construction of a reservoir is
called the valley storage. May be important in flood control reservoirs.
(9) Yield from a reservoir: Yield is the volume of water which can be
withdrawn from a reservoir in a specified period of time. The yield is
determined from the storage capacity of the reservoir and the mass inflow
curve.
(10) Safe yield (Firm yield): is the maximum quantity of water which can
be supplied from a reservoir in a specified period of time during a critical dry
year. Lowest recorded natural flow of the river for a number of years is
taken as the critical dry period for determining the safe yield.
(11) Secondary yield: is the quantity of water which is available during the
period of high flow in the rivers when the yield is more than the safe yield. It
is supplied on at the lower rates. The hydropower developed from
secondary yield is sold to industries at cheaper rates.

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(12) Average yield: is the arithmetic average of the firm yield
and the secondary yield over a long period of time.
(13) Design yield: is the yield adopted in the design of a
reservoir. Fixed after considering the urgency of the water
needs and the amount of risk involved. The design yield should
be such that the demands of the consumers are reasonably
met with, and at the same time, the storage required is not
unduly large.

Reservoir problems
The main reservoir problems which are linked with
geologic characteristics of the area are:
Seepage and leakage:
- no reservoir is free of seepage,
- leakage is the abnormally large escape of water
from the reservoir
Leakage may occur along
- buried channels,
- solution cavities in soluble rocks,
- joints, faults, and other weakness planes
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Leakage buried channels beneath drift
50 km
Modern river/valley
Ancient river/valley Sautet
dam and
reservoir
Bypass of reservoir in drift
Reservoirs: leakage
42

river
reservoir
before
after
water table divide
Leakage to next valley
Bedrock with a water
table and finite
permeability
new
water
table
Reservoir problem: water table leakage-1
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river
before
Bedrock with low
permeability: aquiclude
High
permeability
layer
Water table in aquifer
reservoir
after
High
permeability
layer
Modified water table in aquifer
Leakage to next valley
Reservoirs: water table leakage-2
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Before
Water table
river
After - 1
reservoir
Raised water table
After - 2
reservoir
Failure and
slumping
due to
weakened
rock mass
Reservoirs: raised water table
45

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Reservoir problem: Sedimentation

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Consequences of Reservoir Sedimentation
Loss of Storage (yield; reliability)
Upstream: loss of navigable depths
Downstream: degradation of channel; loss of land and
habitats
Hydropower: downstream deposits can increase and
decrease efficiency HP
Abrasion of turbines

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How do we control sedimentation??
1. Reduce sediment inflow
erosion control and upstream sediment trapping.
2. Route sediments
Some or all of the inflowing sediment load may be hydraulically
routed beyond the storage pool by techniques such as off-stream
reservoirs, sediment bypass, and venting of turbid density currents.
3. Sediment removal
Deposited sediments may be periodically removed by hydraulic
flushing, hydraulic dredging, or dry excavation.
4. Provide large storage volume
Reservoir benefits may be considered sustainable if a storage
volume is provided that exceeds the volume of the sediment
supply.

DamDam
Dam
 Dam types and Loads on Dams
 Site Investigation of Dam Sites
 Selection of Type of Dam
 Geological consideration of dam site
 Causes of dam failure

Brain Storm Questions
What is dam?
Why we build dam
Discuss forces that exert on dams
What are different types of dams
Explain site conditions for dams
 What are parameters we have to consider
during geological and engineering geological
study of dam sites?
 Discuss the remedial measurements if there
engineering problems encountered at proposed dam
site??

Dam and Dam Site
A Dam is a solid barrier, which is constructed across a river to store
water.
A dam is built mainly:
♠ To store water for irrigation
♠ To generate hydro electric power
♠ To supply water to industries
♠ To supply water for domestic use
♠ To Control flood
♠ To control Siltation
A dam that serves more than one purpose is called a multi purpose
Dam
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A Dam and Its Parts
Heel: is that part of a dam
which comes in contact with
the ground on the upstream
side
Toe: It is that part of the
dam which comes in contact
with the ground on the
downstream side
Spillways: are the openings
made in a dam near the top
to let off excess water of the
reservoir to the downstream
side
They are commonly placed
on a sound foundation
within or out side the body
of the dam and the openings
are controlled by suitably
designed gates

Sluices: are openings in the
dam near the ground level
They are useful in clearing
the slit of the reservoir
Cut of wall: is an under
ground wall like structure
of concrete in the heel
position. It is useful to
prevent leakage and uplift
pressure (or under thrust)
under the foundation and
thereby to avoid
undercutting of the heel of
the dam
A Dam and Its Parts

Galleries: are small rooms
left within a dam for
checking operations
Free board: It is the part
of the dam structure
between top of the dam
and highest storage level
Abutments: These are the
sides of the valley
supporting the dam
structure
Diversion tunnels: are the
tunnels constructed
beforehand for diverting
the river water. These help
in keeping the river bed
dry at the dam site and
facilitate dam construction
A Dam and Its Parts

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Classification of Dams
(a) Based on function:
 Storage dams
 Detention dams
 Diversion dams
 Debris dams
 Coffer dams - a temporary dam constructed for facilitating
construction.
 It is an enclosure constructed around a site to exclude
water so that the construction can be done in dry.

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(b) Based on Hydraulic
Design:
 Overflow dams.
 Non-overflow dams.

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(c) Based on Materials of
Construction:
 Masonry dam
 Concrete dam
 Earth dam
 Rock fill dam
 Timber dam
 Steel dam
 Combined concrete-earth dam
 Composite dam.

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(d) Based on Rigidity
Rigid dams: A rigid dam is quite stiff. It is constructed of
stiff materials such as concrete, masonry, steel and timber.
• These dams deflect and deform very little when
subjected to water pressure and other forces
Non-rigid dams: A non-rigid dam is relatively less stiff
compared to a rigid dam.
• The dams constructed of earth and rock fill are non-
rigid dams. There are relatively large settlements and
deformations in a non-rigid dam.
Rock fill dams are actually neither fully rigid nor fully
non-rigid. These are sometimes classified as semi-rigid
dams.

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(e) Based on structural
action:
 Gravity dams
 Embankment dams
 Earth dams
 Rock fill dams
 Arch dams
 Buttress dams
 Others
 Steel dams
 Timber dams

Dam Types, Advantages and Disadvantages
60

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Gravity Dams
 A gravity dam resists the
water pressure and other
forces due to its weight (or
gravitational forces).
 Rigid, heavy, massive,
monolithic structure
–Made of large amounts of
concrete
–Resistance is due to their own
weight
•Favorable site:
–Constricted area of a valley
–Close sound bed rock, both
in the floor and abutments

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Masonary Gravity Dam
(Non-overflow )

63
Concrete Gravity Dam with Overflow Section

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Advantages of Gravity Dams:
Gravity dams are quite strong, stable and durable.
 are quite suitable across moderately wide valleys/gorges having steep slopes
where earth dams, if constructed, might slip.
 can be constructed to very great heights, provided good rock foundations are
available.
 are well adapted for use as an overflow spillway section.
 Earth dams cannot be used as an overflow section.
 Even in earth dams, the overflow section is usually a gravity dam.
 are specially suited to such areas where there is very heavy downpour. The
slopes of the earth dams might be washed away in such an area.
 maintenance cost of a gravity dam is very low.
 does not fail suddenly.
 There is enough warning of the imminent failure and the valuable property and
human life can be saved to some extent.
 can be constructed during all types of climatic conditions.
 sedimentation in the reservoir on the upstream of a gravity dam can be
somewhat reduced by operation of deep-set sluices.

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Disadvantages of Gravity Dams:
 Gravity dams of great height can be constructed only on sound rock
foundations. These cannot be constructed on weak or permeable foundations on
which earth dams can be constructed.
 initial cost of a gravity dam is usually more than that of an earth dam. At the
sites where good earth is available for construction and funds are limited, earth
dams are better.
 usually take a longer time in construction than earth dams, especially when
mechanized plants for batching, mixing and transporting concrete are not
available.
 require more skilled labour than that in earth dams.
 subsequent raising is not possible in a gravity dam.

66
Earth Dams
 An earth dam is made of earth (or soil) and resists the forces exerted
upon it mainly due to shear strength of the soil.
 Are usually built in wide valleys having flat slopes at flanks
(abutments).
 Can be homogeneous when the height of the dam is not great.
 Are of zoned sections, with an impervious zone (called core) in the
middle and relatively pervious zones (called shells or shoulders)
enclosing the impervious zone on both sides. Nowadays majority of
dams constructed are of this type.
The highest dams of the world are earth dams (Rongunsky dam Russia, 325
m and Nurek dam, Russia, 317 m).

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Advantages of Earth Dams:
 are usually cheaper than gravity dams if suitable earth for
construction is available near the site.
 can be constructed on almost all types of foundations, provided
suitable measures of foundation treatment and seepage control are
taken.
 can be constructed in a relatively short period.
 skilled labour is not required in construction of an earth dam.
 can be raised subsequently.
 are aesthetically more pleasing than gravity dams.
 are more earthquake-resistant than gravity dams.

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Disadvantages of Earth Dams:
 are not suitable for narrow gorges with steep slopes.
 cannot be designed as an overflow section.
 a spillway has to be located away from the dam.
 cannot be constructed in regions with heavy downpour, as the
slopes might be washed away.
 maintenance cost of an earth dam is quite high.
 it requires constant supervision.
 sluices cannot be provided in a high earth dam to remove slit.
 fails suddenly without any sign of imminent failure.
 a sudden failure causes havoc and untold miseries.

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Rock fill Dams
 A rock fill dam is built of rock fragments and boulders of large size.
 An impervious membrane (cement concrete or asphaltic concrete or
earth core) is placed on the rock fill on the upstream side to reduce
the seepage through the dam.
 A dry rubble cushion is placed between the rock fill and the
membrane for the distribution of water load and for providing a
support to the membrane.
 Side slopes of rock fill are usually kept equal to the angle of repose of
rock (1.4:1 or 1.3:1).
 Rock fill dams are quite economical when a large quantity of rock is
easily available near the site.

72
Advantages of Rock fill Dams:
Rockfill dams have almost the same advantages and disadvantages over
gravity dams as discussed for earth dams.
 are quite inexpensive if rock fragments are easily available.
 can be constructed quite rapidly.
 can better withstand the shocks due to earthquake than earth dams.
 can be constructed even in adverse climates
Disadvantages of Rock fill Dams:
 Rock fill dams require more strong foundations than earth dams.
 Rock fill dams require heavy machines for transporting, dumping and
compacting rocks.

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Arch Dams
• Composed of a single concrete wall of high strength,
curved in plan with its convex face pointing upstream.
• Transmits most of the horizontal thrust of the water load
to the adjacent abutments by arch action.
• Impose high stresses upon narrow zones
• The rock mass at the abutments and immediately down
valley of the dam must be strong.
Favorable site:
• Narrow gorges
• Walls should be capable of withstanding the thrust
produced by the arch action
• Well-keyed into the abutments

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Advantages of Arch Dams:
 requires less concrete as compared to a gravity dam as the
section is thinner.
 are more suited to narrow, V-shaped valley, having very
steep slopes.
 uplift pressure is not an important factor in the design of an
arch dam because the arch dam has less width and the
reduction in weight due to uplift does not affect the stability.
 can be constructed on a relatively less strong foundation
because a small part of load is transferred to base, whereas in
a gravity dam full load is transferred to base.

76
Disadvantages of Arch Dams:
 Requires good rock in the flanks (abutments) to resist the
thrust. If the abutments yield, extra stresses develop which
may cause failure.
 Requires sophisticated formwork, more skilled labor and
richer concrete.
 Cannot be constructed in very cold climates because spalling
of concrete occurs due to alternate freezing and thawing.
 Are more prone to sabotage.
 The speed of construction is relatively slow.

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Buttress Dams
Consists a slab/deck or reinforced concrete which slopes
upstream and supported by a number of buttresses from the
downstream side whose axis are normal to the slab.
The buttresses support the deck and transmit the water
load to the foundation
Favorable site:
–Competent foundation rock

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Advantages of Buttress Dams:
 Buttress dams require less concrete than gravity dams.
 Uplift/ice pressure is generally not a major factor.
 can be constructed on relatively weaker foundations.
 Power house and water treatment plants, etc. can be housed between
buttresses.
 Vertical component of the water pressure on deck prevents the dam
against overturning and sliding failures.
 Can be designed to accommodate moderate movements of foundations
without serious damages.
 Heat dissipation is better in buttress dams.
 Back of the deck and the foundation between buttresses are accessible
for inspection.
 Can be easily raised subsequently by extending buttresses and deck
slabs.

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Disadvantages of Buttress Dams:
 Buttress dams require costlier formwork, reinforcement
and more skilled labor.
 Consequently, the overall cost of construction may
be more than that of a gravity dam.
 Buttress dams are more susceptible to damage and
sabotage.
 Buttress dams cannot be constructed in very cold
climates because of spalling of concrete.
 Because the upstream deck slab is thin, its deterioration
may have very serious effect on the stability.

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Composite Dams
• 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.
The Bloemhof Dam on the Orange River of South Africa is an excellent example of a gravity/buttress dam.

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Loads on Dams
(a) Primary loads:
 Water loads.
 Seepage forces.
 Self-weight loads.
(b) Secondary loads:
 Sediment load: generates horizontal thrust.
 Hydrodynamic wave action: wave action.
 Ice load: extreme climate.
 Thermal load: concrete dams (change in Temperature,
cement hydration and cooling).
 Abutment hydrostatic load: internal seepage load in
abutment rock mass (arch dams).
 Interactive effects: from relative stiffness and
differential deformation of dam and foundation.
(c) Exceptional loads: seismic loads and tectonic effects.

85
Schematic of principal loads: gravity dam profile.

86
Internal uplift and pressure envelopes

87
Questions:
 What is the purpose
of drain holes/
gallery in gravity
dams?
 Where is the
position of the
drain holes:
upstream or
downstream of
curtain grout?
 Is uplift pressure a
concern for
embankment dams?
Why?

Engineering Geological Dam Site
investigation
I. Reconnaissance Study
1. Evaluation of the data having at archives
2. Field investigation for limited time (Reconnaissance
Study)
3. Some maps in small scale, for example 1:25.000 or
1:50.000
4. Some hydraulic data about
a. Basin
b. Precipitation area
c. Runoff, maximum discharge
5. Some approach to the reservoir area, dam site and type of
dam and height of dam...etc
6. Photogeological studies 88

Cont’d
II. Prelımınary Studıes at the reservoır area and dam
sıte
1. Dam site investigations
 Location of dam axis
 Location of diversion tunnel
 Location of spillway
 Location of powerhouse...etc
2. Geological studies
3. Geophysical surveying
4. Underground investigations
 Boreholes
 Drilling tests
5. Surveying for materials
 Field surveying
 Laboratory tests 89

Preliminary studies… cont’d
6. Slope stability investigations
7. Earthquake hazard & risk analysis
8. Environmental studies
9. Leakage possibilities from reservoir area
10. Leakage possibilities from dam site
11. Erosion, sedimentation & siltation
90

Detaıled Investıgatıon at Dam Sıte
1. Topographic surveyings
2. Geological mappings
1:5000 – 1:1000 or 1:500
3. Underground explorations
Boreholes, adits....etc
4. Hydrogeological studies
5. Slope stability analysis
91

Factors Affectıng Dam Type Selectıon
• Topography
• Geology
• Bearing capacity of the underlying soil
• Foundation settlements
• Permeability of the foundation soil
 Material availability
 Spillway position
 Earthquakes
 Safety
 Height
 Aesthetic view
 Qualified labour
 Cost 92

Factors Affecting Dam Axıs
 Topography
 Geology
 Materials
 Spillway location availability
 Sediments in the flowing water
 Water quality
 Earthquake possibility
 Downstream water rights 93

Geological considerations in the selection of a
dam site
The important geological requirements, which should be
considered in the selection of a dam site, are as follows:
•Narrow river valley
•Occurrence of the bedrock at a shallow depth
•Competent rocks to offer a stable foundation
•Proper geological structures
94

Narrow River Valley
At the proposed dam site,
if the river valley is
narrow, only a small dam
is required, which means
the cost of dam
construction will be less.
On the other hand, if the
valley is wide, a bigger
dam is necessary which
means the construction
cost will be very high. 95

Occurrence of the Bedrock at a shallow depth
To ensure its safety and stability a dam has to necessarily rest
on (Physically) very strong and (Structurally) very stable (i.e.
bedrocks).
If such competent bedrocks occur near the surface or at
shallow depths, the foundation cost of the dam will naturally
be less.
On the other hand, if competent bedrocks occur at great
depths, the cost of the foundation will be very high because it
involves extensive work of excavation of loose overburden
and concrete refilling.
96

Problems Related to Incompetence of Rocks.
♠ Dams on shale
♠ Dams on soluble rocks
♠ Dams on Volcanic rocks
Dam on shale
♠ Shale is soft rock and when saturated with water under
pressure likely produces lubricating material making a
slippery base.
♠ Shale’s bearing capacity is low and it becomes plastic
when wetted.
97

Dam on soluble rocks
♠ The soluble rocks include limestone, dolomite and marble.
♠ These rocks are generally strong to support the weight of
the dam.
♠ But they may contain underground openings due to
dissolution.
98
Dams on Volcanic rocks
 all plutonic rocks like Granites, Syenites, diorites and gabbros are
very competent and desirable rocks.
 However, volcanic rocks which are vesicular or amygdaloidal,
are not equally desirable, obviously because these character
contributes to porosity, permeability and hollowness which, in
turn, reflect the strength of the rocks.

Problems Related to Improper Geological
Structures
♠ Dam on horizontal strata
♠ Dam built across the strike of the rocks
♠ Dam on strata dipping up stream.
♠ Dam on strata dipping down stream.
♠ Dam on jointed and permeable rocks.
♠ Dam on faults
99

Dam on horizontal strata
Here the load of the dam acts
perpendicular to the bedding
planes and the beds as a whole
can withstand the pressure with
full competence
Also, the compressing weight
prevents seepage
Thus, leakage is checked and
uplift pressure is avoided
This is a safe situation for a dam
site

Dam built across the strike of the rocks
If a dam is aligned across the strike of the strata, then
its foundation will be on different rock types of
varying properties.
This situation leads to unequal settlements of the dam
foundation.
101
Water
Dam

Where the strike direction is parallel to the axis of
the dam: i. Strata with gentle (100 to 450)
upstream dip
The resultant force acts more or less
perpendicular to the bedding planes
The formations are best positioned to
withstand the loads effectively
The infiltrated water is directed upstream
side by the bedding planes. So the scope
for leakage downstream side is restricted
No scope for uplift pressure
Gentle upstream dip is ideal for dam
location better than the earlier situation

the dam:
ii. Strata with steep (more than 450) upstream dip
The bedding planes are not
perpendicular to the
resultant force and hence
this is not as ideal as the
previous situation
Percolated water returns to
upstream side and no scope
for seepage
There will not be uplift
pressure
Situation is good but not as
good as the previous one

the dam: iii. Strata with 100 to 450 downstream dip
The resultant force and bedding planes
are in the same direction
This situation is harmful to the dam for
the following reasons:
1. The resultant force and the dip of
bedding planes are in the same direction
so it is vulnerable for slip
2. The scope for percolation of water
along the bedding planes is enhanced
3. It enhances uplift pressure
4. Significant loss of water due to
seepage
5. If a clay formation is there, it acts as
a slippery plane.
Situation of this kind is very undesirable
and dangerous

the dam: vi. Strata with steep (more than 450)
downstream dip
Here the resultant force
and the bedding planes are
almost parallel and they
are vulnerable for slip
Situation here is worse
than the previous one
Such geological structure
is bad and disadvantageous

the dam: v) Vertical strata
 Perfectly vertical beds are uncommon in
nature and they normally have some
inclination. If such situation is present:
 i. It will not pose the problem of uplift
pressure
 ii. It does not allow percolation and seepage
 This situation is better than the above two
situations
 Here the load of the dam acts parallel to the
beds as a whole are resistant enough to with
stand pressure, but not much as horizontal
beds

Folded Strata
Folds are generally less dangerous
than faults
They consist of two limbs and each
limb can be considered as a set of
dipping strata
When viewed from this approach
the influence of a fold at the dam
site can be interpreted either as
advantageous or disadvantageous
just as in the case of occurrence of
inclined beds

Folded Strata
Case 1: This case is similar to the situation
where the strata dip upstream side and it is
advantageous
Case 2: Here the dam is on the crest of the
fold and the strata dip in the upstream side.
This situation is similar to the case where
the strata dip upstream side and it is also as
advantageous
Case 3: This case is similar to the situation
were the strata dip downstream side and it
is disadvantageous.
However, it should be borne in the mind
that unlike simple tilted strata, the folded
rocks also are highly fractured along the
crests because of the strain. Hence grouting
and other precautions are to be considered
to improve the stability and competence of
the rocks at the dam site

Faulted Strata
Occurrence of a fault irrespective of the attitude of the strata (strike
and dip) at the dam site is most undesirable
If the fault is active, under no circumstance, dam construction can
be taken up
It causes not only displacement of the site but also possible
occurrence of earth quakes
If crushed or intensively fractured, it becomes physically
incompetent to bear the forces of a dam
Due to the associated porosity and permeability, the water is
percolated and seepage causes uplift
However if need arises, and if the tectonic history of the faulted
region indicates that it has become stable and has no threat of
possible recurrence then such site can be considered after necessary
treatment

Faulted StrataCase 1: i. If a fault occurs upstream
side and dips upstream side, the fault
face needs to be sealed to avoid
possible leakage
Case 1: ii. If a fault occurs upstream
side and dips downstream side, it is
not desirable because it has all the
disadvantages like uplift pressure,
heavy leakage of water, etc.
Case 2: If the dam has to rest on
inactive faults, it is harmful unless
strong precautions are taken
Case 3: If faults occur downstream
side, they are not at all harmful
irrespective of their attitude

113
Geological and geotechnical problems of dam sites in
Ethiopia
Problems related to dams and reservoirs:
 Stability, settlement/consolidation of dam foundation,
 Leakage/seepage,
 Geohazards: landslides and earthquakes,
 Sedimentation of reservoirs.
Main aspects to be covered during engineering geological
investigation include:
 Determination of geotechnical parameters: (a) deformability
(settlement/consolidation), (b) shear strength, and (c) permeability.
 Evaluation of the hydrological aspects of the foundation rock mass.
 State of natural stress in the foundation rock.
 Natural hazards for the dam site and reservoir area.
 Construction materials for the project: (a) quality, (b) quantity and
(c) proximity.

Geological factors to be given due attention in
the design and construction of Dams
Reading assignments
On the effects of rock units for design and construction
of Dams
114

Main causes of Dam Failure
(1) Failure of concrete dams
Lack of shear strength and discontinuity in
foundation
Excessive uplift in the foundation (inadequate or
non-existent drainage)
Lack of dam stability
Excessive or differential deformation of the
foundation
Piping and erosion in the foundation caused by high
permeability
Flaw/error in design
Lack of supervision during construction
No monitoring or warning system (systems were out
of order) 115

Main causes of Dam Failure… cont
 Human error during site investigation, design,
construction and operation of concrete dams:
 Inadequate foundation investigation
 Incomplete data on available material
 Poor design
 Negligible construction supervision
 Incomplete first impoundment
 Incorrect operation of flood gates
 Insufficient monitoring and data analysis
 Lack of preventive measures or repair work
116

Main Causes of Dam Failure
(II) Failure of Embankment Dam
 Overtopping during flood discharge because of inadequate
spillway capacity or non-functioning flood gate
 Internal erosion along the dam-foundation interface or
along embankment with adjoining or embedded
appurtenant structures or concentrated piping in the
embankment itself because of inadequate or non-existent
filter zones
 Non-homogeneity in the foundation or dam (leading to
foundation failure or erosion)
 Large settlement in the foundation
 Crack following the settlement, with resulting piping effect
 liquefaction
117

118
Generally, Earth dam failures are mainly caused by improper design, lack of
investigations, inadequate care in construction and poor maintenance. Various causes of
failures can be grouped into three categories.
 Hydraulic failures
 Seepage failures
 Structural failures
(a) Hydraulic failures:-The hydraulic failures may occur due to one or more of the
following causes:
 Over topping
 Erosion of U/S face
 Erosion of upstream face
 Erosion of D/S toe
(b) Seepage failures:- seepage failures may occur due to the following causes:
 Piping through the dam
 Piping through the foundation
 Conduit leakage
 Sloughing of downstream toe
Piping is the progressive backward erosion starting from the exit point and subsequent
removal of the soil from with in the body of the dam and the formation of pipe-like
conduit inside the dam.

119
(c) Structural failure – structural failures in earth dams are generally
shear failures leading to sliding of the embankment or the foundation.
Structural failures in the earth dams are of the following types:
 Slides in embankments: upstream sudden drawdown,
downstream steady seepage.
 Foundation slides.
 Liquefaction slides – flow of fine sand and silt in loose
condition.
 Failures by spreading – occurs when the earth dam is located
above a stratified deposit that contains layers of site clay.
 Failure due to earthquakes etc.

Envıronmental Impacts of Constructıon
Phase Of Dams
 River pollution
 Erosion
 Loss of aesthetic view
 Air pollution
 Noise pollution
 Dust
120

GEOLOGICAL FACTORS
CONSIDERED
IN
THE SELECTION
OF
A DAM SITE

Lecture 5

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Lecture 5