3. Dams
• Dam is a solid barrier constructed at a suitable
location across a river valley to store flowing water.
• Storage of water is utilized for following objectives:
• Hydropower
• Irrigation
• Water for domestic consumption
• Drought and flood control
• For navigational facilities
• Other additional utilization is to develop fisheries
5. • 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
• Sluice way: An opening in the dam near the ground level, which is used to clear the silt
accumulation in the reservoir side.
6. • Crest: The top of the Dam. These may in some cases be used for providing
a roadway or walkway over the dam.
• Parapet walls: Low Protective walls on either side of the roadway or
walkway on the crest.
• Free board: The space between the highest level of water in the reservoir
and the top of the dam.
• Dead Storage level: Level of permanent storage below which the water will
not be withdrawn.
• Diversion Tunnel: Tunnel constructed to divert or change the direction of
water to bypass the dam construction site. The dam is built while the river
flows through the diversion tunnel.
7. Selection of site
• The type and size of dam constructed depends on the need for and
the amount of water available, the topography and geology of the
site, and the construction materials that are readily obtainable.
• Dams can be divided into two major categories according to the type of material
with which they are constructed, namely, concrete dams and earth dams.
• The former category can be subdivided into gravity, arch and buttress
dams, whereas rolled fill and rockfill embankments comprise the
other.
• As far as dam construction is concerned, safety must be the primary concern,
this coming before cost. Safety requires that the foundations and abutments be
adequate for the type of dam selected.
8. Based on the functions of dam, it can be
classified as follows:
• Storage dams: They are constructed to store water during
the rainy season when there is a large flow in the river.
Many small dams impound the spring runoff for later use in
dry summers. Storage dams may also provide a water
supply, or improved habitat for fish and wildlife. They may
store water for hydroelectric power generation, irrigation or
for a flood control project. Storage dams are the most
common type of dams and in general the dam means a
storage dam unless qualified otherwise.
9. Based on the functions of dam, it can be
classified as follows:
• Diversion dams: A diversion dam is
constructed for the purpose of diverting water of
the river into an off-taking canal (or a conduit).
They provide sufficient pressure for pushing
water into ditches, canals, or other conveyance
systems. Such shorter dams are used for
irrigation, and for diversion from a stream to a
distant storage reservoir. A diversion dam is
usually of low height and has a small storage
reservoir on its upstream.
10. Based on the functions of dam, it can be
classified as follows:
• Diversion dams: Roza Diversion Dam,
located 10 miles north of Yakima, diverts water
from the Yakima River (tributary of Columbia
river, Washington). The dam is a concrete one,
486 feet long at the crest, 67 feet high, and
contains 21,700 cubic yards of concrete. In 2010,
Reclamation modified the 110-foot long west
roller gate to reduce the amount of water leakage
under the gate.
11. Based on the functions of dam, it can be
classified as follows:
• Detention dams: Detention dams are constructed for
flood control. A detention dam retards the flow in the river
on its downstream during floods by storing some flood
water. Thus the effect of sudden floods is reduced to some
extent. The water retained in the reservoir is later released
gradually at a controlled rate according to the carrying
capacity of the channel downstream of the detention dam.
Thus the area downstream of the dam is protected against
flood.
12. Based on the functions of dam, it can be
classified as follows:
• Debris dams: A debris
dam is constructed to retain
debris such as sand, gravel,
and drift wood flowing in
the river with water. The
water after passing over a
debris dam is relatively
clear.
13. Based on the functions of dam, it can be
classified as follows:
14. Based on the functions of dam, it can be
classified as follows:
• Coffer dams: It is an enclosure
constructed around the construction site to
exclude water so that the construction can be
done in dry. A cofferdam is thus a temporary
dam constructed for facilitating construction. A
coffer dam is usually constructed on the
upstream of the main dam to divert water into a
diversion tunnel (or channel) during the
construction of the dam. When the flow in the
river during construction of the dam is not much,
the site is usually enclosed by the coffer dam and
pumped dry. Sometimes a coffer dam on the
downstream of the dam is also required.
15. Based on the functions of dam, it can be
classified as follows:
16. TYPES OF DAMS
• Gravity Dams:
• These dams are heavy
and massive wall-like
structures of concrete
in which the whole
weight acts vertically
downwards
Reservoir
Force
• As the entire load is transmitted on the small area of foundation, such
dams are constructed where rocks are competent and stable.
• They are designed to hold back large volumes of water. By using
concrete, the weight of the dam is actually able to resist the horizontal
thrust of water pushing against it.
• This is why it is called a gravity dam. Gravity essentially holds the dam
down to the ground, stopping water from toppling it over.
17. GRAVITY DAMS
Gravity dams are dams which resist the horizontal thrust of the
water entirely by their own weight.
They use their weight to hold back the water in the reservoir.
18. Buttress dam
•If the valley is wide and the foundation
rocks have varying strengths
•Consists of an upstream sloping deck that
bears the load of impounded water and
vertical buttresses aligned normal to the
plane of this slab.
•Vertical buttresses transmit the load of the
deck, impounded water and accumulated
sediments to the foundation.
19. BUTTRESS DAMS
Buttress dams are dams in which the face is held up by a series of supports.
Buttress dams can take many forms -- the face may be flat or curved.
Usually, buttress dams are made of concrete and may be reinforced with steel bars.
20. Buttress dam
•Buttress dams are of three types : (i) Deck type, (ii)
Multiple-arch type, and (iii) Massive-head type.
i.A deck type buttress dam consists of a sloping deck
supported by buttresses. Buttresses are triangular
concrete walls which transmit the water pressure from
the deck slab to the foundation. Buttresses are
compression members. Buttresses are typically spaced
across the dam site every 6 to 30 metre, depending
upon the size and design of the dam. Buttress dams are
sometimes called hollow dams because the buttresses
do not form a solid wall stretching across a river
valley. The deck is usually a reinforced concrete slab
supported between the buttresses, which are usually
equally spaced.
21. Buttress dam
ii. In a multiple-arch type buttress
dam the deck slab is replaced by
horizontal arches supported by
buttresses. The arches are
usually of small span and made
of concrete.
22. Buttress dam
iii. In a massive-head type buttress dam, there is
no deck slab. Instead of the deck, the
upstream edges of the buttresses are flared
to form massive heads which span the
distance between the buttresses. The buttress
dams require less concrete than gravity
dams. But they are not necessarily cheaper
than the gravity dams because of extra cost
of form work, reinforcement and more
skilled labor. The foundation requirements
of a buttress dam are usually less stringent
than those in a gravity dam.
25. ARCH DAMS
Curved dam which is dependent upon arch action for its strength.
Transmits most of horizontal water thrust behind them to the
abutments by the arch action.
Thinner and requires less material than any other type of dam.
Used only in narrow canyons.
26. • These type of dams are
concrete or masonry dams
which are curved or convex
upstream in plan
• This shape helps to
transmit the major part of
the water load to the
abutments
• Arch dams are built across
narrow, deep river gorges,
but now in recent years they
have been considered even
for little wider valleys.
Arch Dams:
27. • Arch dams can only be built where the walls of a
canyon are of unquestionable stability. They must also
be impervious to seepage around the dam, as this could
be a source of dam falure in the future.
• Because of these factors, Arch dams can only be built in
very limited locations.
• Arch dams use less materials than gravity dams, but are
more expensive to construct due to the extensive
amount of expertise required to build one.
30. Arch dam
•The section of an arch dam is
approximately triangular like a
gravity dam but the section is
comparatively thinner. The arch
dam may have a single curvature
or double curvature in the
vertical plane.
31. Earth Dams: • They are trapezoidal in shape
• Earth dams are constructed
where the foundation or the
underlying material or rocks
are weak to support the
masonry dam or where the
suitable competent rocks are at
greater depth.
• Earthen dams are relatively
smaller in height and broad at
the base
• They are mainly built with
clay, sand and gravel, hence
they are also known as Earth
fill dam or Rock fill dam
32. Embankment dams are massive dams made of earth or rock.
They rely on their weight to resist the flow of water, just like
concrete gravity dams.
33. Earth/Embankment dams
•An earth dam is made of earth (or soil) built up by compacting
successive layers of earth, using the most impervious materials to form
a core and placing more permeable substances on the upstream and
downstream sides.
•A facing of crushed stone prevents erosion by wind or rain, and an
ample spillway, usually of concrete, protects against catastrophic
washout should the water overtop the dam.
•Earth dam resists the forces exerted upon it mainly due to shear
strength of the soil.
•Although the weight of the earth dam also helps in resisting the
forces, the structural behaviour of an earth dam is entirely different
from that of a gravity dam.
34. Earth/Embankment dams
•The earth dams are usually built in wide valleys having flat slopes at
flanks (abutments).
•The foundation requirements are less stringent than those of gravity
dams, and hence they can be built at the sites where the foundations
are less strong.
•They can be built on all types of foundations. However, the height of
the dam will depend upon the strength of the foundation material.
•Examples of earthfill dam: Rongunsky dam (Russia) and New
Cornelia Dam (USA).
35.
36.
37. Harddap Dam, near Mariental, Namibia, and example of an embankment
dam.
38. Earth/Embankment dams
•Rockfill Dams: A rockfill dam is built of rock fragments and boulders of large
size. An impervious membrane is placed on the rockfill on the upstream side to reduce
the seepage through the dam. The membrane is usually made of cement concrete or
asphaltic concrete. In early rockfill dams, steel and timber membrane were also used,
but now they are obsolete.
39. Earth/Embankment dams
•A dry rubble cushion is placed between the rockfill and the membrane for the
distribution of water load and for providing a support to the membrane.
•Sometimes, the rockfill dams have an impervious earth core in the middle to check the
seepage instead of an impervious upstream membrane.
•The earth core is placed against a dumped rockfill.
•It is necessary to provide adequate filters between the earth core and the rockfill on
the upstream and downstream sides of the core so that the soil particles are not carried
by water and piping does not occur.
•Examples of rockfill dam: Mica Dam (Canada) and Chicoasen Dam (Mexico
41. Composite dams
• Some sites that are geologically unsuitable for a specific type of dam
design may support one of composite design.
• For example, a broad valley that has strong rocks on one side and
weaker ones on the other possibly can be spanned by a combined
gravity and embankment dam, that is, a 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.
42. The Bloemhof Dam on the Orange River of South Africa is an excellent
example of a gravity/buttress dam.
Gravity Dam
Buttress Dam
44. • Bhakra Dam is the highest
Concrete Gravity dam in Asia
and Second Highest in the
world.
• Bhakra Dam is across river
Sutlej in Himachal Pradesh
• The construction of this project
was started in the year 1948 and
was completed in 1963 .
• It is 740 ft. high above the deepest foundation as straight concrete dam being more
than three times the height of Qutab Minar.
• Length at top 518.16 m (1700 feet); Width at base 190.5 m (625 feet), and at the
top is 9.14 m (30 feet)
• Bhakra Dam is the highest Concrete Gravity dam in Asia and Second Highest in
the world.
45.
46. Implications
• The construction of a dam and the filling of a reservoir behind it
impose a load on the sides and floor of a valley, creating new stress
conditions.
• These stresses must be analyzed so that there is ample assurance that
there will be no possibility of failure.
• A concrete dam behaves as a rigid monolithic structure, the stress
acting on the foundation being a function of the weight of the dam as
distributed over the total area of the foundation.
• In contrast, an earthfill dam exhibits semi-plastic behaviour, and the
pressure on the foundation at any point depends on the thickness of
the dam above that point.
47. Gravity
-depends on its own weight for stability
-usually straight in plan although slightly curved
Forces on Gravity Dam
1. Gravity (weight of dam)
W = V γ = (volume)(specific weight of material)
(lb) = (ft3
)(lb/ft3
)
2. Hydrostatic pressure
Hh = γ h2
/ 2 (horizontal component)
(lb/ft) = (lb/ft3
) (ft)2
/2
where,
h = depth of water at that section
γ = specific weight of water
Hv = γ V / h (vertical component)
(lb/ft) = (lb/ft3
) (ft3
) / ft
where,
48.
49.
50.
51. Gravity
Forces on Gravity Dam
3. Uplift
the water under pressure that comes b/t dam and foundation and
results
in upward (uplift) forces against the dam
h1 = depth of water @ upstream face, “heel” (higher)
h2 = depth of water@ downstream face, “toe” (lower)
γ = specific weight of water
t = base thickness of dam
4. Ice pressure
pressure created by thermal expansion exerts thrust against
upstream face of the dam
5. Earthquake forces
results in inertial forces that include vertical motion, oscillatory
increase, or decrease in hydrostatic pressure (all put force against
dam)
52. Implications
• Vertical static forces act downward and include both the weight of
the structure and the water, although a large part of the dam is
submerged and, therefore, the buoyancy effect reduces the influence
of the load.
• The most important dynamic forces acting on a dam are wave
action, overflow of water and seismic shocks.
• Horizontal forces are exerted on a dam by the lateral pressure of
water behind it.
• These, if excessive, may cause concrete dams to slide.
• The tendency towards sliding at the base of such dams is of
particular significance in fissile rocks such as shales, slates and
phyllites.
53. Implications-Lithology
• Weak zones, such as interbedded ashes in a sequence of basalt lava
flows, can prove troublesome.
• The presence of flat-lying joints may destroy much of the inherent
shear strength of a rock mass and reduce the problem of resistance of
a foundation to horizontal forces to one of sliding friction, so that the
roughness of joint surfaces becomes a critical factor.
• The rock surface should be roughened to prevent sliding, and
keying the dam some distance into the foundation is advisable.
• Another method of reducing sliding is to give a downward
slope to the base of the dam in the upstream direction of the
valley.
54. Sliding failures of masonry dams
• Horizontal forces tend to push the dam downstream and, if
excessive, may cause the dam to slide.
• St. Francis Dam (California), Austin Dam (Texas), Teton Dam
(Idaho) are some examples.
58. St. Francis Dam California
• This was a solid gravity dam, curved on a radius of 500 ft, the crest being 16 ft
thick, the base 175 ft thick, and height of the dam 205 ft.
• Storage began on Mar. 1, 1926 and the failure occurred on Mar. 12, 1928.
• Leakage through the dam, especially through the foundation preceeded the
failure.
• The bottom and one slope of the canyon were underlain by laminated mica
schist whereas the opposite slope was underlain by reddish conglomerate.
• The contact between the two rock types was along a fault.
• The dam was placed astride the fault, partly on schist and partl on
conglomerate.
• Failure occurred near the fault and was basically due to the softening and
disintegration of the conglomerate by percolating waters.
• 236 people died.
61. Teton Dam Idaho
• The dam was 305 ft high above the Teton River bed and
about 3000 ft long.
• It was topped out in November 1975.
• The foundation was in rhyolite, a closely and heavily
jointed volcanic rock, there being even human sized
openings found beneath the dam location.
68. Implications-Pore water pressure
• Variations in pore water pressure cause changes in the state of stress
in rock masses.
• They reduce the compressive strength of rocks and
cause an increase in the amount of deformation they
undergo.
• Pore water also may be responsible for swelling in certain rocks
and for acceleration in their rate of alteration.
• Pore water in the stratified rocks of a dam foundation reduces the
coefficient of friction between the individual beds, and between the
foundation and the dam.
69. Implications-Pore water pressure
• Percolation of water through the foundations of concrete dams, even
when the rock masses concerned are of good quality and low
permeability, is a decisive factor in the safety and performance of
such dams.
• Such percolation can remove filler material that may be occupying
joints that, in turn, can lead to differential settlement of the
foundations.
• It also may open joints, which decreases the strength of the rock
mass.
70. Implications-Uplift pressure
• Uplift pressure acts against the base of a dam and is caused by water
seeping beneath it that is under hydrostatic head from the reservoir.
• The uplift pressure on the heel of a dam is equal to the depth of
the foundation below water level multiplied by the unit weight of
the water.
• In the simplest case, it is assumed that the difference in hydraulic
heads between the heel and the toe of the dam is dissipated
uniformly between them.
• The uplift pressure can be reduced by allowing water to be
conducted downstream by drains incorporated into the foundation
and base of the dam.
71. Implications-Settlement and rebound problems
• Under the action of its weight and other vertical forces imposed
upon it, the dam settles.
• Filling the reservoir causes additional settlement of the dam.
• In the case of extremely large reservoir, the weight of the reservoir
water and silt can cause the entire region surrounding the reservoir
to subside eg. In Hoover Dam in Nevada.
• It should be noted that any settlement consists of an
elastic (reversible) part and so called plastic
(irreversible) part.
• The latter constitutes a small, progressive sinking of the dam into the
underlying material and is absent in relatively old dams in which this
small amount of sinking is already achieved.
72. Implications-Settlement and rebound problems
• The settlement problem is simple if the foundation rock is sound and
strong, such as only slightly fractured and jointed granite.
• In dealing with weaker rock types, especially clayey in character,
such as soft shales, claystones or siltstones, the question of
deformation under heavy load becomes critical.
• The settlement of a dam may be increased by the presence or
formation of cavities under the existing structure, for instance by
removal of salt and gypsum beds by solution.
73. Implications-Settlement and rebound problems
• When load is removed from a rock mass on excavation, it is subject to rebound.
• The amount of rebound depends on the modulus of elasticity of the rocks concerned,
the larger the modulus of elasticity, the smaller the rebound.
• In the case of dams, the rebound problem may be serious if, during construction, a thick
layer of unreliable rock layer is removed to prevent excessive settlement of, or leakage
under, a dam.
• The rebound process in rocks generally takes considerable time to achieve completion
and will continue after a dam has been constructed if the rebound pressure or heave
developed by the foundation material exceeds the effective weight of the dam.
• Hence, if heave is to be counteracted, a dam should impose a load on the foundation
equal to or slightly in excess of the load removed.
• If the excavated rock has been exposed for a certain time, it may swell, and a structure
built on that foundation will settle more than expected.
• Nonuniform rebound may be responsible for differential settlement of the whole
structure.
74. Implications-Seepage of water
• When water is held back by a membrane, such as a masonry dam,
there is a permanent movement of water from the reservoir under
and around the membrane and at the rims of the reservoir.
• This normal phenomenon is seepage which is to be
distinguished from leakage.
• The latter is abnormally large escape of water from the reservoir
through the fissures and openings in the rock.
• Attention should be not only given to the dam itself and its reservoir,
but also to the pattern of joints, vertical and oblique channels,
together with general topography and geology of the concerned area.
75. Implications-Seepage of water
• It is not uncommon to find “buried” channels in the
reservoir rim.
• Leakage often may be due to solubility of the dam foundation or
reservoir material, such as limestone, gypsum or salt rock.
• The non-soluble rocks such as granite or quartzite do not present
serious leakage problems unless severely fissured.
• If problems are present, fortunately such rocks are amenable to
grouting without great difficulties.
76. Implications-Seepage of water-Prevention
• In highly permeable rock masses, excessive seepage beneath a dam
may damage the foundation.
• Seepage rates can be lowered by reducing the hydraulic gradient
beneath the dam by incorporating a cut-off into the design.
• A cut-off lengthens the flow path, reducing the
hydraulic gradient.
• It extends to an impermeable horizon or some specified depth and
usually is located below the upstream face of the dam.
• The rate of seepage also can be effectively reduced by placing an
impervious earthfill against the lower part of the upstream face of a
dam.
79. Geology and Dam sites
• Of the various natural factors that directly influence the design of
dams, none is more important than the geological, not only do they
control the character of the foundation but they also govern the
materials available for construction.
• The character of the foundations upon which dams are built and their
reaction to the new conditions of stress and strain, of hydrostatic
pressure and of exposure to weathering must be ascertained so that
the proper factors of safety may be adopted to ensure against
subsequent failure.
83. Geology and Dam sites-Igneous rocks
• Excluding the weaker types of compaction shales,
mudstones, marls, pyroclasts and certain very friable types
of sandstone, there are few foundation materials deserving
the name rock that are incapable of resisting the bearing
loads even of high dams.
• In their unaltered state, plutonic igneous rocks essentially
are sound and durable, with adequate strength for any
engineering requirement.
• In some instances, however, intrusives may be highly
altered by weathering or hydrothermal attack.
84. Geology and Dam sites-Igneous Rocks
• Thick massive basalts make satisfactory dam sites but many basalts
of comparatively young geological age are highly permeable,
transmitting water via their open joints, pipes, cavities, tunnels, and
contact zones.
• Foundation problems in young volcanic sequences are twofold.
1. Firstly, weak beds of ash and tuff may occur between the basalt flows that give
rise to problems of differential settlement or sliding.
2. Secondly, weathering during periods of volcanic inactivity may have produced
fossil soils, these being of much lower strength.
• Rhyolites, and frequently andesites, do not present the same severe
leakage problems as young basalt sequences. They frequently offer
good foundations for concrete dams, although at some sites chemical
weathering may mean that embankment designs have to be adopted.
85. Geology and Dam sites-Igneous rocks
• Pyroclastics usually give rise to extremely variable
foundation conditions due to wide variations in strength,
durability and permeability.
• Their behaviour very much depends on their degree of
induration, for example, many agglomerates have high
enough strengths to support concrete dams and also have
low permeabilities.
• By contrast, ashes are weak and often highly permeable.
86. Geology and Dam sites-Igneous Rocks
• One particular hazard concerns ash not previously
wetted, that is, it may be metastable and so undergoes a
significant reduction in its void ratio on saturation.
• Clay/cement grouting at high pressures may turn ash into a
satisfactory foundation.
• Ashes frequently are prone to sliding.
• Montmorillonite is not an uncommon constituent in these
rocks when they are weathered, so that they may swell on
wetting.
87. Geology and Dam sites-Metamorphic rocks
• Fresh metamorphosed rocks such as quartzite and hornfels are very
strong and afford excellent dam sites.
• Marble has the same advantages and disadvantages as other
carbonate rocks.
• Generally, gneiss has proved a good foundation rock for dams.
• Cleavage, schistosity and, to a lesser extent, foliation in regional
metamorphic rocks may adversely affect their strength and make
them more susceptible to decay.
88. Geology and Dam sites-Metamorphic rocks
• Moreover areas of regional metamorphism usually have suffered
extensive folding so that rocks may be fractured and deformed.
• Some schists, slates and phyllites are variable in quality, some being
excellent for dam site purposes, others, regardless of the degree of
their deformation or weathering, are so poor as to be wholly
undesirable in foundations and abutments.
• For instance, talc, chlorite and sericite schists are weak rocks
containing closely spaced planes of schistosity.
89. Geology and Dam sites-Metamorphic rocks
• Some schists become slippery upon weathering and, therefore, fail under
moderately light loads.
• On the other hand, slates and phyllites tend to be durable.
• Although slates and phyllites are suitable for concrete dams where good load-
bearing strata occur at a relatively shallow depth, problems may arise in
excavating broad foundations.
• Particular care is required in blasting slates, phyllites and schists, otherwise
considerable overbreak or shattering may result.
• Consequently, rock fill embankments are being increasingly adopted at such
sites.
90. Geology and Dam sites-Weakness zones
• Joints and shear zones are responsible for the unsound rock
encountered at dam sites on plutonic and metamorphic rocks.
• Unless they are sealed, they may permit leakage through foundations
and abutments.
• Sheet or flat-lying joints tend to be approximately parallel to the
topographic surface and introduce a dangerous element of weakness
into valley slopes.
• Their width varies and, if they remain untreated, large quantities of
water may escape through them from the reservoir.
91. Geology and Dam sites-Weakness zones
• Indeed, Terzaghi (1962) observed that the most objectionable feature
in terms of the foundation at Mammoth Pool Dam, CaliforniaMammoth Pool Dam, California,
which is in granodiorite, was the sheet joints orientated parallel to
the rock surface.
• Moreover, joints may transmit hydrostatic pressures into the rock
masses downstream from the abutments that are high enough to
dislodge sheets of rock.
• If a joint is very wide and located close to the rock surface, it may
close up under the weight or lateral pressure exerted by the dam and
cause differential settlement.
92. Geology and Dam sites-Sedimentary rocks
• Sandstones have a wide range of strength, depending largely on the amount and
type of cement matrix material occupying the voids.
• With the exception of shaley sandstone, sandstone is no subject to rapid surface
deterioration on exposure.
• As a foundation rock, even poorly cemented sandstone is not susceptible to
plastic deformation.
• However, friable sandstones introduce problems of scour at the foundation.
• Moreover, sandstones are highly vulnerable to the scouring and plucking action
of the overflow from dams and have to be adequately protected by suitable
hydraulic structures.
93. Geology and Dam sites-Sedimentary rocks
• A major problem of dam sites located in sandstones results from the
fact that they normally are transected by joints, which reduce
resistance to sliding.
• Generally, however, sandstones have high coefficients of internal
friction that give them high shearing strengths, when restrained
under load.
• Sandstones frequently are interbedded with shale.
• These layers of shale may constitute potential sliding surfaces.
Sometimes, such interbedding accentuates the undesirable properties
of the shale by permitting access of water to the shale–sandstone
contacts.
94. Geology and Dam sites-Sedimentary rocks
• Contact seepage may weaken shale surfaces and cause
sliding in formations that dip away from abutments and
spillway cuts.
• Severe uplift pressures also may develop beneath beds of
shale in a dam foundation and appreciably reduce its
resistance to sliding.
• Foundations and abutments composed of interbedded
sandstones and shales also present problems of settlement
and rebound, the magnitude of these factors depending on
the character of the shales.
95. Geology and Dam sites-Sedimentary rocks
• Limestone dam sites vary widely in their suitability.
• Thick-bedded, horizontally lying limestones, relatively free from solution
cavities, afford excellent dam sites.
• Also, limestone requires no special treatment to ensure a good bond with
concrete.
• On the other hand, thin-bedded, highly folded or cavernous limestones are
likely to present serious foundation or abutment problems involving bearing
capacity or watertightness or both (Soderburg, 1979).
• Resistance to sliding involves the shearing strength of limestone. If the rock
mass is thin bedded, a possibility of sliding may exist. This should be guarded
against by suitably keying the dam structure into the foundation rock.
• Beds separated by layers of clay or shale, especially those inclined downstream,
may serve as sliding planes under certain conditions.
96. Geology and Dam sites-Solution caves
• Some solution features are always be present in limestone.
• The size, form, abundance and downward extent of these features depend on
geological structure and presence of interbedded impervious layers.
• Individual cavities may be open, they may be partially or completely filled with
clay, silt, sand or gravel mixtures or they may be water-filled conduits.
• Solution cavities present numerous problems in the construction of large dams,
among which bearing capacity and watertightness are paramount.
• Few dam sites are so bad that it is impossible to construct safe and successful
structures upon them but the cost of the necessary remedial treatment may be
prohibitive.
• In fact, dam sites should be abandoned where the cavities are large and
numerous, extending to considerable depths.
• Sufficient bearing strength generally may be obtained in cavernous rock by
deeper excavation than otherwise would be necessary
97. Geology and Dam sites-Solution caves
• Watertightness may be attained by removing the material from cavities, and
refilling with concrete.
• The establishment of a watertight cut-off through cavernous limestone presents
difficulties in proportion to the size and extent of the solution openings.
• Grouting has not always proved successful in preventing water loss from
reservoirs on karstic terrains.
• For example, Bozovic et al. (1981) referred to large caverns in
limestone at the Keban Dam site in Turkey that exceeded 100,000 m3
in
volume.
• In fact, despite 36,000 m of exploratory drilling and 11 km of
exploratory adits, a huge cavern over 600,000 m3
went undiscovered.
• This illustrates the fact that risk in karstic areas cannot be completely
eliminated even by intensive site investigation.
98. Geology and Dam sites-Solution caves
• Even though these caverns were filled with large blocks of rock
(0.5 ¥ 0.5 ¥ 0.5m) and aggregate, and an extensive grouting
programme carried out, leakage on reservoir impoundment
amounted to some 26 m3 s-1.
• A classic case of leakage was associated with the Hales Bar Dam,
Tennessee, which was founded on the Bangor Limestone.
• After completion of the dam in 1917, it underwent several episodes
of extensive grout treatment.
• None were successful, and leakage had increased to more than 54
m3 s-1 by the late 1950s. Consequently, the dam was demolished in
1968.
99. Geology and Dam sites-Solution caves
• Another difficult project has been described by TurkmenAnother difficult project has been described by Turkmen
et al. (2002), namely, the Kalecik Dam in Turkey. Thereet al. (2002), namely, the Kalecik Dam in Turkey. There
seepage through the karstic limestone beneath led to a 200seepage through the karstic limestone beneath led to a 200
m long and 60 m deep grout curtain being constructedm long and 60 m deep grout curtain being constructed
beneath the axis of this rockfill dam. Unfortunately, thisbeneath the axis of this rockfill dam. Unfortunately, this
did not solve the seepage problem. A further investigationdid not solve the seepage problem. A further investigation
showed that seepage paths existed between the dam andshowed that seepage paths existed between the dam and
the spillway. Therefore, it was recommended that a newthe spillway. Therefore, it was recommended that a new
grout curtain be constructed beneath the spillway.grout curtain be constructed beneath the spillway.
100. Geology and Dam sites-Solution caves
• The removal of evaporites by solution can result in subsidence and collapse of
overlying strata.
• Indeed, cavities have been known to form in the United States within a matter of
a few years where thick beds of gypsum occurred beneath dams.
• Brune (1965) reported extensive surface cracking and subsidence in reservoir
areas in Oklahoma and New Mexico due to the collapse of cavernous gypsum.
• He also noted that a sinkhole appeared in the sediment pool shortly after the
completion of the Cavalry Creek Dam, Oklahoma, which caused much water to
be lost.
• Investigations, however, have shown that when anhydrite and gypsum are
interbedded with marl (mudstone), they generally are sound.
101. Geology and Dam sites-Solution caves
• Well cemented shales, under structurally sound conditions, present
few problems at dam sites, though their strength limitations and
elastic properties may be factors of importance in the design of
concrete dams of appreciable height.
• They, however, have lower moduli of elasticity and lower shear
strength values than concrete and, therefore, are unsatisfactory
foundation materials for arch dams.
• Moreover, if the lamination is horizontal and well developed, then
the foundations may offer little shear resistance to the horizontal
forces exerted by a dam.
102. Geology and Dam sites-Solution caves
• Keying the dam into such a foundation is then required.
• Severe settlements may take place in low grade compaction shales.
• As a consequence, such sites are generally developed with earth
dams, but associated concrete structures such as spillways will
involve these problems.
• The stability of slopes in cuts is one of the major problems in shale
both during and after construction.
• Cuttings in shale above structures must be made stable.
• This problem becomes particularly acute in dipping formations and
in formations containing montmorillonite.
103. Geology and Dam sites
• Earth dams are usually constructed on clay soils as they have insufficient load-
bearing properties required to support concrete dams.
• Beneath valley floors, clays may be contorted, fractured and softened due to
valley bulging so that the load of an earth dam may have to spread over wider
areas than is the case with shales and mudstones.
• Settlement beneath an embankment dam constructed on soft clay soils can
present problems and may lead to the development of excess pore water
pressures in the foundation soils (Olson, 1998).
• Deep cuts involve problems of rebound if the weight of removed material
exceeds that of the structure.
• Slope stability problems also arise, with rotational slides being a hazard.
104. Geology and Dam sites-Glaciation problems
• Among the many manifestations of glaciation are the presence of buried
channels; disrupted drainage systems; deeply filled valleys; sand–gravel
terraces; narrow overflow channels connecting open valleys; and extensive
deposits of lacustrine silts and clays, till, and outwash sands and gravels.
• Deposits of peat and head (solifluction debris) may be interbedded with these
glacial deposits.
• Consequently, some glacial deposits may be notoriously variable in
composition, both laterally and vertically.
• As a result, some dam sites in glaciated areas are among the most difficult to
appraise on the basis of surface evidence.
• Knowledge of the preglacial, glacial and postglacial history of a locality is of
importance in the search for the most practical sites.
105. Geology and Dam sites-Glaciation problems
• A primary consideration in glacial terrains is the discovery of sites
where rock foundations are available for spillway, outlet and
powerhouse structures.
• Generally, earth dams are constructed in areas of glacial deposits.
Concrete dams, however, are feasible in postglacial, rock-cut
valleys, and composite dams are practical in valleys containing rock
benches.
106. Geology and Dam sites-Glaciation problems
• The major problems associated with foundations on alluvial deposits generally
result from the fact that the deposits are poorly consolidated.
• Silts and clays are subject to plastic deformation or shear failure under
relatively light loads and undergo consolidation for long periods of time when
subjected to appreciable loads.
• Embankment dams are normally constructed on such soils as they lack the load-
bearing capacity necessary to support concrete dams.
• The slopes of an embankment dam may be flattened in order to mobilize greater
foundation shear strength, or berms may be introduced into the slope.
• Nonetheless, many large embankment dams have been built on such materials,
but this demands a thorough exploration and testing programme in order to
design safe structures.
107. Geology and Dam sites-Glaciation problems
• Soft alluvial clays at ground level generally have been removed if economically
feasible.
• Where soft alluvial clays are not more than 2.3 m thick, they may consolidate
during construction if covered with a drainage blanket, especially if they are
resting on sand and gravel.
• On the other hand, coarser sands and gravels undergo comparatively little
consolidation under load and therefore often afford good foundations for earth
dams. Their primary problems result from their permeability.
• Problems relating to underseepage through pervious strata may be tackled by a
cut-off trench, if the depth to bedrock is not too great or by a grout curtain.
• Otherwise, underseepage may be checked by the construction of an impervious
upstream blanket to lengthen the path of percolation and the installation on the
downstream side of suitable drainage facilities to collect the seepage.
108. Geology and Dam sites-Glaciation problems
• Landslips are a common feature of valleys in mountainous areas, and large slips
often cause narrowing of a valley that therefore looks topographically suitable
for a dam.
• Unless they are shallow seated and can be removed or effectively drained, it is
prudent to avoid landslipped areas in dam location, because their unstable
nature may result in movement during construction or, subsequently, on filling
or drawdown of the reservoir.
• Fault zones may be occupied by shattered or crushed material and so represent
zones of weakness that may give rise to landslip upon excavation for a dam.
• The occurrence of faults in a river is not unusual, and this generally means that
the material along the fault zone is highly altered.
• A deep cut-off is necessary in such a situation.
109. Construction materials
• Large amounts of soil, sand, stone and aggregate and concrete are
need for dam construction.
• If available, these materials will be collected as near to the site of the
dam as possible.
• The extraction of these materials requires large amounts of fossil
fuels to operate the machinery.
• Air and water pollution result from the dust and mud that is created
from this process
