Dams and spillways

DAMS AND SPILLWAYS
Prof.M.B Chougule DKTE’S YCP Ichalkaranji

DAMS
• A solid barrier constructed at a suitable location across a river
valley to store flowing water.
• The selection criteria for site of a dam is-
1. Good foundation should be available.
2. It should be located in a narrow valley.
3. It should have sufficient space for spillway.
4. It should have impervious bed and site so as to reduce
erosion.
5. It should fulfill the purpose i.e. irrigation, drinking etc.
6. Materials should be easily available near by site.
7. It should have less submergence area i.e. not more than 10%.
8. The site should be such that the length of dam should be.
minimum as it directly affect cost.
9. It should be easily accessible throughout the year.

Classification dams on the basis of
methods of construction .
• 1)Earthen Dam
• 2) Rock fill Dam
• 3) Gravity Dam
• 4)Steel Dam
• 5)Timber Dam
• 6) Arch Dam

selection criteria for suitable type of DAM
• 1. Topography: It decides the choice of a particular type of
dam.
• U- narrow Shaped valley - concrete overflow dam
• Rolling Plane country – Earth fill dam with spillway separate
site
• Narrow V-Shaped valley – An Arch Dam.
• 2. Geology and foundation Condition: Foundation of dam
requires carrying weight of dam therefore it is important.
• On solid rock foundation – All types of dam can be
constructed
• Gravel foundation – Only earth dam
• Silt and fine sand foundation – Earthen dam or low gravity
dam.
• Clay foundation – Earthen dam

3. Availability of material-
• Construction material must be locally available as
their quantity required is very large
• If good soil available easily - Earthen dam are
suitable
• If sand, cement, And stone easily available -
concrete gravity dam are constructed
• If Above material are to be transported from large
distances – Hollow concrete dam available
• 4. Spillway site and location : In masonry dam
construction spillway can be constructed in
continuation with the embankment but in earthen
dam separate site for spillway is required

.
• 5. Earthquake zone: Concrete or masonry dam cannot
sustained earthquake whereas earthen dam are suitable in
earthquake zone areas which can absorb small shocks
• 6. Height of dam: Earthen dam are suitable for heights up to
30 m. for greater height gravity dams are generally preferred
• 7. Man power/ skilled labour: If skilled labour are available
then masonry dam can be constructed otherwise earthen
dam can be constructed.
• 8. Life of dam: Concrete or masonry dam has more life as
compare to earthen dam. Depending upon the requirement
or planning type of dam can be considered.
• 9. Roadway width: width of Road way on masonry dam is less
as compare to earthen dam. If access is to be provided than
earthen dams are constructed.
• 10. Funds available: Earthen dam can be constructed with
less cost as compare to masonry or concrete dam

OTHER FACTORS
• MATERIAL AVAILABLE
• FUNDS AVAILABLE
• NATURE OF FOUNDATION
• SPACE FOR SURPLUS WORK( i.e. spillways)
• NATURE OF VALLEY.
• SEEPAGE AND UPLIFT UNDER DAM.

* GRAVITY DAMS *
• Gravity dam is a solid structure made up of
masonry or concrete and are constructed across
river to create reservoir on its upstream side.
• These are strong and durable. The dead weight of
dam is distributed such that whole section
withstand pressure of water on upstream side.
These are at right angles to flow of river.
• Masonry Dams: This type of dam is constructed
with masonry. It could be either of rubble or
coursed masonry.
• Concrete Dams: They are constructed block by block
by pouring good quality concrete.

FORCES ACTING ON GRAVITY DAM.
• Following are the forces acting on gravity Dam:
• 1. Water pressure on upstream side
• 2. Water pressure on downstream side
• 3. Weight of the dam
• 4. Upstream silt pressure
• 5. Seismic forces
• 6. Uplift forces
• 7. Ice pressure
• 8. Wind pressure

.
• 1. Water pressure on upstream side: Water pressure is
the major external force acting on the dam. This is
overturning force.
• P1 = Wh2 acting at h/3 from the base.
• 2
• Where, W = specific weight of water
• H = height of water
• P2 = weight of wedge water on upstream slpoe acting
downwards through C.G. This is retaining force.
• 2. Water pressure on downstream side: Weight of
water on downstream wedge acting at C. G.
downwards. This is retaining force.
• P3 = Whd 2
• 2
• Where, Hd = depth of water on downstream side.

.
• 3. Weight of dam: It is stabilizing force.
• W = Area of cross-section x Unit weight of dam material
acting at
• C. G. of the dam section downward.
• 4. Upstream silt pressure:
• P silt = Wb hs 2 X 1-sinϕ acting horizontally at hs/3 from the
base. 2 1+sinϕ
• Where, Wb = weight of submerged silt.
• Φ = Angle of internal friction of the silt.
• Hs = Depth of silt
• 5. Seismic forces :
• These forces are considered only in such area of country
which comes under seismic zones where possibility of
earthquake is more and these are taken as,
• Pe = 500h2 Approximately acting at 0.42h from the base,
horizontally downstream direction. It is overturning force.

.
• 6. Uplift force:
• It is the pressure due to the seepage of water through the
foundation.
• It acts upwards on the foundation of the dam and reduces the
effective weight.
• U = Kwb ( h + hd)
• 2
• Where, K = Permeability of the foundation
• = 0 for hard impervious rock.
• = 0.2 to 0.6 for other rocks.
• 7. Ice Pressure: In extreme cold climate, the top surface of the
reservoir freezes into ice. Due to variation in temperature, such ice
expands during the day time and exerts pressure on the dam. This
force acts along the length of the dam at the reservoir level. The
magnitude of this force Varies from 25 to 150 t/m2.
• 8. Wind pressure: The wind acting on all exposed faces of the dam
exerts pressure in the wind direction. This pressure depends on
the speed of wind.

THEORETICAL /ELEMENTARY PROFILE
• It means cross section of dam which is designed for
the maximum height of dam above rock foundation.
• It is imaginary section having right angle shape
• Base on rock and apex on HFL.
• Water face vertical.
• Water thrust
• Weight of DAM
• Uplift pressure

.• To find base width-
• Assume that profile is triangle ABC
• AB=H G= special gravity of dam masonry
• BC=b (2.3 to 2.8)
• K= permeability of rock (0.4 to 0.6)
• Consider 1m length of DAM normal to the plane of
paper.
• Water thrust P=WH² KG act H/3 from base
• 2
• Weight of dam W=GwbH
• 2 acting at Centre of
gravity

.
• Uplift pressure U=KwbH
• 2 act upward
through m
• Net vertical force= W-U=WbH -(G-K)
• 2
CONDITION TO BE FULFILL ARE:
NO TENSION
• For no tension resultant must pass through middle
third that is at point N on base. Since point N is in
direction of resultant sum of moment of all forces
about N must be zero.
NO SLIDING: For no sliding and minimum value of
safety factor is 1 neglecting shear.

.• Elementary profile found safe if it is modified in a
practical profile by adding:-
• Free board to prevent water splashing to provide
additional storage.
• Width at top- to accommodate road
• Additional upstream width- to balance load due to
top width
• Recommendation:
• To keep resultant within middle third of the base in
the reservoir the above that is top width and free
board are added arbitrarily.

.• FREE BOARD FB=0.9
• FB ‗̃1.54Hw Hw= height of
wave
• TOP WIDTH a= 6 to 10m
• And for economy a=014(U/S depth of water)
• ADDITIONAL UPSTREAM WIDTH= a/16
• STABILITY REQUIREMENTS OF GRAVITY DAM
• Resistance to sliding (neglecting shear).
• Resistance to sliding ( considering shear).
• Resistance to compressive stress.
• Résistance to internal tension.
• Resistance to overturning.

GALLERIES IN GRAVITY DAM
• Gallery is a passage provided in the body of dam.
These may run parallel or transverse to the axis of
dam and located at different levels.
• These all are inter connected by shaft, lift, stair etc.
• The usual sizes of galleries are 2 to 2.5m wide and
2.5 to 4m deep and corners are rounded.

.• Importance of drainage gallery:
1) For inspection of dam from inside.
2) To drain off seepage water through the body of
dam.
3) It provides access to spillway gate.
4) It helps in locating pumps, observation devices.
5) It provides access for grouting.
TYPES:-
i) Foundation gallery or Drainage gallery.
ii) Inspection gallery.

.
• Functions:
• i) Foundation gallery: It is provided near the rock foundations
serve to drawn off the water which percolates through the
foundations. It is also helpful for drilling and grouting of the
foundations.
• Size 1.5x2.2m to 1.8x2.4m after completing foundation grouting
drainage holes are drilled to collect seepage water and then it is
drained off by cross galleries.
• ii) Inspection Gallery: These galleries are provided at various
elevations and meter connected by vertical shafts.
• a) These galleries besides draining of seepage water serve
inspection purpose.
• b) They provide access to dam interior for observing and
controlling the dam.
• c) They provide access for carrying pipes, etc.
• d) They provide access for grouting the contraction joints.
• e) They provide access to all outlets, spillway gates, valves.
• They provide space for drilling and grouting of the foundations

JOINTS IN GRAVITY DAM
• Importance of joints:
• 1) Construction joints are used for opposing
contraction stresses.
• 2) Construction joints are provided for ease in
construction of dam.
• 3) Contraction joints helps to reduce tensile stresses
formed in concrete and temperature variations.
• 4) Transverse joints allow contraction and prevent
cracks in the dam.

There are two types of joints –
• 1) Construction joint
• 2) Contraction joint
• (1) Construction Joint: In solid gravity dams, the
height between horizontal joints is usually limited
to 1.5 m. This height between two successive
construction joints or horizontal joints is known as
lift. The surface should be properly treated to make
the horizontal joint water tight
• (2) Contraction Joint : They are provided to avoid
cracks formed due to shrinkage of concrete due to
temperature changes. These joints are of two types
–

.
• a) Transverse Joint b) Longitudinal Joint
• (2.a) Transverse Joint: These are provided normal to
axis of dam.
• They prevent the transverse cracks due to contraction
of concrete. The joint is filled with asphaltic filler.
• (2.b) Longitudinal Joints : These are provided parallel
to the axis of dam to prevent longitudinal cracks. Water
stops are provided to prevent leakage of water. The
spacing of these joints is limited to
15m.
• Key ways :- Key ways are invariably provided in
vertical longitudinal joints. Function of keyway is to
permit transfer of shearing stress from one block to
other.

VARIOUS STAGES IN THE
CONSTRUCTION OF GRAVITY DAM
• 1) DIVERSION PROBLEM IN DAMS CONSTRUCTION:-
Before construction of dam, initially the water of the river
should be temporarily diverted.
Diversion can be done by two ways:
1)provision of a diversion tunnel
2)by constructing the dam in two stages.
2)CONSTRUCTION OF GALLERIES IN GRAVITY DAM:-
Two types of galleries are to be constructed
1)foundation galleries
2)inspection galleries

.
• Measures to control cracking in gravity dam:
1. Using minimum amount of cement in a given mix of specified
strength. The quantity of cement can be decreased by better grading
the aggregate.
2. When concrete is poured, it is poured up to a certain height in the
first attempt. This height is called ‘ Lift ’. Generally 1.5m lift is used in
modern dams. If lift is reduced, more horizontal joints will get
developed and also sufficient cooling time between two successive
pours shall be obtained thus reducing cracking.
3. By providing suitably spaced contraction joints, in addition to the
normal construction joints.
4. Special low heat cements may be used.
5. The materials which go into the concrete ,may be cooled before
mixing.
6. Further cooling is accomplished by circulating cold water through
pipes embedded in concrete. This is quite an expensive measure and
is adopted only for large gravity dams.
7.Installation of water stops. Grouting foundation surface.
8. Providing construction and contraction joints , key ways.

EQUIPMENTS AND MACHINERY REQUIRED
FOR CONSTRUCTION OF GRAVITY DAM:-
• 1)adjustable steel props or telescopic props.
• 2)steel props.
• 3)H- frames.
• 4)telescopic girder.
• 5)steel plates or centering plates.
• 6)wooden planks.
• 7)steel pan.
• 8)spade pick-axe.
• 9)excavation equipments like tractor, bulldozer,
grader, scraper, power shovel, truck mounted drag
line.

.
• 10) Dragers if the site is situated along the bank of
river, lake or sea.
• 11) trencher or ditcher.
• 12) rippers.
• 13) motor grader.
• 14) compaction equipment's.
• 15) dumpers.
• 16) pumped concrete unit.
• 17) elevators or conveyers used for transporting
various construction materials
• 18) wheel mounted mobile crane.
• 19)telescope type jib crane.

.
• 20) mast tower crane.
• 21) static tower crane.
• 22) travelling tower crane.
• 23) climbing crane.
• 24) water pumping machine i.e. centrifugal pump.
• 25) drilling machine.

DRAG LINES
It is most flexible excavating machine tool.
It is favourable when other equipments are not useful.
It has more reach than shovel either for excavation or for disposal.
It can dig below its base level.
In drag line the boom of shovel is replaced by crane boom and shovel
dipper by drag line bucket.
Depending upon the type of material to be excavated the buckets are
used.
The bucket size varies from 1.15cum to 27 cum
Weight of bucket 1550kg to 32000kg
capacity also varies from 1.15cum to 3cum
They are truck mounted, crawler mounted, wheel mounted.
The length of drag line is 15m to 74m, boom angle 20 to 45 degree,
Dumping radius 12 to 26m, digging depth 6 to 12m, speed 2 to
50km/hr

WORKING OF DRAGLINE
• It has both operation of excavation and
disposal.
• At the time of working , the bucket swings and
kept on material to be excavated and then
hauling its front towards the base of machine,
excavation and filling is done simultaneously.

BACK TRENCH HOE EXCAVAOR
used for dragging foundation trenches below
operating level

SKIMMER EXCAVATOR
• Used for surface excavation and leveling

GIRDERS OR TRESTLES
• It is horizontal member temporarily give the
support to the formwork of R.C.C slab. It is generally
used for longer span and shorter span of R.C.C. slab
to be casted,.
• Adjustable telescopic girder is recent innovation in
scaffolding industry. Required span can be adjusted
with the help of telescopic girder and non
adjustable girders are available in various length of
span, generally for shorter span.
• Simple girder (I-section) is also used to support the
centering work of the R.C.C slab steel plates or
wooden planks.

.
• Fabricated robust and durable range of adjustable
telescopic steel props which are manufactured out of
continuous welded steel tubes, these props are made
according to IS1161 grade YST22. the outer tube is 50
I.D. And inner tube is 40 I.D.
• The adjustable telescopic props are braced in both
directions by tubes and double couplers at
approximate 250mm above prop nut when used for
height beyond 3.6m.
• Telescopic steel props are widely used in all types of
construction of various structure like multistoried
building, bridges, dams etc. for supporting the
centering work or formwork made for various member
such as slabs, beams and any other.

STEEL FORMWORK
• It consist of H- frames, steel plates, steel props,
telescopic prop, girder or trestles, etc. which is
commonly used in the various construction work
like multistoried building, bridges, dams, culverts
etc. for giving the temporary support for concreting
work. After getting the required strength to the
concrete after casting, steel formwork is dismantled
and can reused for any other construction work.

Conveying RMC:
• After preparation of concrete in plant or at
transportation or on site, it is very important that
prepared concrete should be transported at
required place without disturbing its characterizes
like strength, workability, consistency likewise. So
transportation of RMC becomes prime important
factor. Concrete can be transported by a variety of
methods and equipment's.
• These equipment's maintains homogeneity of
concrete through travel from source to destination
point if proper precaution are taken. The methods
adopted for conveying concrete are.

.
• 1) mortar pan.
• 2) belt conveyors.
• 3) pump and pipeline.
• 4) truck mixer and dumper.
• 5) transit mixer.
• 6) skip and hoist.
• 7) wheel borrow, hand cart.
• 8) chute.
• 9) helicopters.

Transit mixer:-
• It is a very effective and popular equipment used
for transportation of concrete over long distance
used in RMC plants. Transit mixer is simply a truck
mounted mixer having capacity 4 to 7m³.
• Functionally there are two types of transit mixers.
In one type, mixed concrete is transported to the
site by keeping it agitated all along at a speed of 2
to 6 RPM.
• In other type concrete is batched at plant and
mixing is done in truck mixer during travel or at
destination.

.
• For longer transportation transit mixer gives better
result. This mixer contains a rotating drum rotating
about central axis which is mounted on drum. Speed
on rotating is between 4 to 6 RPM. Revolutions are
limited to 300 for mixing and agitation.
• Now-a-days, for effective transportation of concret,
concrete pumps are mounted on truck carring transit
mixer placer pumps are also provided with concrete
pump so that concrete discharged from mixer is
pumped and placed directly in formwork of structure.
• The safe time of transportation of RMC is lower value
of –
• 90minutes
• time taken for 300 revolutions of drum.

Stationary Equipment's
• These machines remains stationary.
• They used to perform specific job like
excavation, loading of muck in truck dredging
of soil.
• e.g.- face shovel, dragline, JCB
• The machines are truck mounted and it can be
functioned effectively on slippery soils too

BULLDOZERS:
• These are the most cheapest for excavating and moving
earth up to 100m
• These are either crawler or wheel type
• Cutting blade is perpendicular to the direction of travel
• It can also be set at some angle to the direction
• Used in hill roads to push material in slopes
• V shape blade may also attached to the front use to cut
trees
• The size of bulldozer is indicated by size of blade and
its capacity
• Size of blade may be 1700mm X 700mm to 3380mm X
1140mm & having 0.7m3 to 3.7m3 capacity

.
• Power shovel is an equipment used for excavating
earth with good control over digging its out put
depends on type of material, depth of cutting
• The dipper is lowered to the floor with teeth
pointing in the face of crawling force through shaft
at that time tension is applied on hoisting line and
some force is applied on dipper. Dipper will fill as it
comes up and out. The size of power shovel is
indicated by the size of dipper

JCB (1945)
• manufactured by JCB excavators ltd (Joseph Cyril
Bamford ) england, that’s why it is called JCB.
• It is multipurpose excavator based on tractor power
unit.
• Available in small and medium size.
• Tractor is normally diesel powered wheel based.
• And hydraulically operated.
• Shovel is fitted at front.
• And bucket or hue at rear of vehicle.
• Weight of machine is removed form axles by jacks
at corners at the time of operating.

It has 4 cylinder diesel engine giving maximum speed
of 27km/hr
• It has attachment of loader bucket having width of
2000mm and capacity of 0.67cum
• It also have attachment of excavator bucket at back
side with 430mm width and capacity of 0.16cum
• Maximum discharge height of front loader is
3000mm and for rear excavator it is 49000mm
• Excavator arm can be positioned at 530mm on
either side, can rotate at 180 degree and excavation
up to 3710mm depth
• Chair of operator can revolve in any direction for
doing work on either side

DRAG LINES
It is most flexible excavating machine tool.
It is favourable when other equipments are not useful.
It has more reach than shovel either for excavation or for disposal.
It can dig below its base level.
In drag line the boom of shovel is replaced by crane boom and
shovel dipper by drag line bucket.
Depending upon the type of material to be excavated the buckets
are used.
The bucket size varies from 1.15cum to 27 cum
Weight of bucket 1550kg to 32000kg
capacity also varies from 1.15cum to 3cum
They are truck mounted, crawler mounted, wheel mounted.
The length of drag line is 15m to 74m, boom angle 20 to 45 degree,
Dumping radius 12 to 26m, digging depth 6 to 12m, speed 2 to
50km/hr

COMPACTION
• To increase the bearing capacity, strength of earth
structure the soil is compacted and stabilised.
• It is done by applying proper method and useing
many types of rollers depending upon the
requirements and degree of compaction.
• TYPES OF ROLLERS:
• Plain rollers
• Sheep footed rollers
• Pneumatic rollers
• Vibratory rollers
• rammer

PLAIN ROLLER
• Length 5 to 15 m
Used for ordinary rolling where deep compaction is
not required
It consist of two axle and three wheels from which
front one is used for steering and rear used for
driving.
Its weight ranges from 10 to 12 MT
It may be self propelled or tractor mounted.
As the roller moves over the soil the feet penetrate
the soil to produce a compaction from bottom to
top

SHEEP FOOTED ROLLERS
• When the compaction is to done at grate depth the
place like embankment or canal sheep footed rollers
are used.
• It consist of hollow steel drum around its periphery of
which welded projections or sheep's fetes are used,
these varies from 15 to 20cm.
• In the working of these roller soil is supposed to be
compacted and consolidate.
• when compaction by projecting teeth is not more than
12mm depth the top layer is finished with smooth
wheel roller.
• These rollers are called temping rollers.
• Pressure variation below teeth is 4 to 7 kg/cm and 25
to 70 kg/cm for light and heavy rollers respectively.

PNEUMATIC ROLLERS
• In pneumatic tyre roller its weight plays an important
role while compacting soil with the help of kneading
and vibrating.
• This type of rolling compaction is due to its tyre wheel
• The arrangement is done as track of forward wheel is in
between rear wheel
• Thus uniform compaction is achieved while moving
forward
• 4 to 8 tyres are used
• Weight up to 200MT
• These rollers can compact up to 60cm depth and
suitable for any type of soil

VIBRATORY ROLLER
• It is recant development of compacting dry
lean soil and concrete.
• Heavy rollers which vibrates while rolling for
the compaction of dry lean concrete and soil,
bitumen carpet. Mainly used for construction
of dam and pavements.

RAMMING
• Ramming is one of the method used in hand
compaction, it should be done with care.
• The light ramming is permitted in case of
unreinforced foundation concrete or ground
floor construction.
• It is not in case of RCC or where concrete is to
be placed in the form work supported on
struts as it may adversely affect reinforcement
and may disturb it

Graders
• It is a road grader. It is a long machine with long blade
used to maintain or create flat surface.
• Similar to bulldozer. The only difference is they are
fitted with the adjustable blade at centre of machine.
• For finishing work this grader are used.
• Not used for excavating work.
• OPERATION:-its main function is to finish the grade
where rough grading is performed by the heavy
equipment.
• It also can produce inclined surface to produce
camber/cant
• It is mainly used in construction of dirt or gravel roads
• In paved roads they are used to prepare the base
course.

Earthendam
• The dam which are constructed of earthen material
such as gravel sand, silt and clay are called earthen
dam.
• It is trapezoidal section with top width of 3 to 10m
and upstream slope of 1V: (2.5 TO 4) H.
downstream slope of 1V TO 3H.
• TYPES OF EARTHEN DAM
• 1) zoned type earthen dam.
• 2) Homogeneous type earthen dam
• 3) diaphragm type earthen dam

Type of earthen dams based on methods of construction:
• 1) rolled fill earth dam.
• 2) hydraulic fill earth dam.
• 3) semi- hydraulic earth dam.
• MERITS OF GRAVITY DAM OVER EARTHEN DAM :-
• 1) Comparatively there is less seepage through
gravity dam.
• 2) Diversion of flow during construction of dam is
not that much costly as that of earthen dam.
• 3) Life is more.
• 4) Maintenance cost is less.

1) Hydraulic fill method:
• a) In this method, the dam body is constructed by
excavating and transporting soils by using water pipes
called flumes, which are laid along the outer edge of
the embankment. The soil materials are mixed with the
water and pumped into these flumes. The slush is
discharged through the outlets in the flumes at suitable
intervals along their length.
• b) The slush flowing towards the center of the bank,
tens to settle down. The coarse particle gets deposited
soon after discharge near the outer edge. While the
fines gets carried and settle at the center, forming a
zoned embankment having impervious central core.
Because of slow drainage from core this method is
susceptible to settlement over long period. Therefore
this method is seldom adopted these days.

2) Rolled fill method:
• a) The embankment is constructed by placing
suitable soil materials in thin layer and compacting
them with rollers. The soil is brought to the site
from burrow pits and spread by bulldozers in
layers.
• b) These rollers are compacted by rollers of
designed weights. Ordinary road rollers can be used
for low embankments. The moisture content of the
soil fill must be properly controlled. Compaction of
gravel can be done by vibrating equipment. This
method is used for construction of dam.

CONSTRUCTION OF EARTHEN DAMS:
• Usual order of construction includes:
• 1) construction of monuments i.e. reference points.
• 2) stripping or benching.
• 3) excavation of cut off trench.
• 4) excavation and filling of seepage drains.
• 5) construction of rock toe.
• 6) rising of upstream casing and hearting zone.
• 7) keying to flanks.
• 8) foundation of masonry work like head regulator and
spillway.
• 9) earthwork in hearting and casing.
• 10) pitching on U/S as well as D/S if required.
• 11) gorge filling, finishing downstream slope.
• 12) construction of drains and berms.

TYPICAL SECTION OF EARTHEN DAM

.
• 1. Hearting: It forms the central impervious section
constructed with clayee soil, silty clay, loam etc. It is
compacted at OMC. It provides water tightness to the dam
and adequate shear resistance against slipping. It controls the
seepage flow through the body of the dam.
• 2. Casing: It forms outer portion of the dam. It is constructed
with murum soft rock or sand and gravel etc. It is compacted
at its OMC. Casing provides a cover to the hearting protecting
it from cracking. It develops shear resistance against slip and
provides stability to the dam. It also helps in drainage.
• 3. Cut-off trench: It is excavated below ground level under
the hearting zone and filled with clayee soil and well
compacted.
• The function of cut-off is to prevent or reduce seepage flow
through the pervious foundation. It prevents piping of dam
through foundation.

.
• 4. Rock toe: It is constructed from rock pieces or
boulders larger than 20cm size. It helps to prevent
sloughing of the toe due to the seepage flow and
increases the stability of dam.
• 5. Pitching: Pitching 30cm to 45cm thickness is
provided by laying stones of 30cm size and 40kg to
50kg weight on a dressed upstream slope. It
prevents the erosion of material on the upstream
face caused due to wave action and protects the
slope from sudden drawdown.
• 6. Turfing: it is planting of special type of grass
called harali on the downstream face of the dam. It
protects the downstream slope from eroding action
of rain water.

.
• 7. Berms: these are offsets provided on
downstream at 8 to 10 m vertical intervals for 3 to
5m width. These are provided to collect the rain
water and dispose it off safely, to provide roadway
for vehicles, to reduce the velocity of rain water
falling on slope, to provide minimum cover of 2m
above the Seepage line.
• 8. Drains: A network of drains is provided with
longitudinal drains, cross drains and toe drains on
downstream side of the embankment.

.
•
• L- drain: It is filter laid along the downstream toe of
hearting to collect the seepage through the
embankment and divert it into cross drains.
• Cross drains: Cross drains are laid at 450, 600, or 900
angle to the axis of dam. It is filled with sand, gravel
and rock. The object of cross drain is to collect the
seepage from the L-drain and downstream casing and
lead it to the toe drain.
• Toe drain: It is an open continuous drain. It collects the
discharge of seepage from cross-drains and discharges
it into the river or nalla.
• 9. Transition filter: It is graded filter placed in between
clayee core and sandy shells. It helps in draining of
hearting and helps reduce the pore pressure.

CRITERIA FOR SAFE DESIGN OF EARTHEN DAM
• The following criteria should be followed for the
safe design of earthen dam are,
• 1) dam should not be overturn and hence according
to that necessary measures should be taken.
• 2) both slope i.e. U/S Slope and D/S slope should be
stable under worst condition.
• 3) seepage should be within D/S face.

FAILURE OF EARTHEN DAM
• Following are the points due to which earthen dam
may fail.
• 1) improper construction method and techniques.
• 2) improper maintenance.
• 3) improper design due to insufficient in
investigation.
• The failure of earthen dam may be grouped into
following categories.
• 1) HYDRAULIC FAILURE.
• 2) SEEPAGE FAILURE
• 3) STRUCTURAL FAILURE.

.
• Causes of failure:
• 1. Hydraulic failures:
• By overtopping
• Erosion of upstream slope
• Cracking due to frost action
• Erosion of downstream slope
• Erosion of the downstream toe
• 2. Seepage failure:
• Piping through the body of the dam
• Piping through foundation
• 3. Structural failure:
• Upstream and downstream slopes slide
• Faulty construction and improper maintenance

.
• (1) Hydraulic Failure : It may be caused by –
• a) Overtopping :- If the actual flood discharge is
much more than the estimated flood discharge or
the free board is kept insufficient or there is
settlement of the dam or capacity of spill way is
insufficient, then it results in the overtopping of the
dam. During overtopping the crest of the dam may
be washed out & the dam may collapse.
• b) Erosion : If the stone protection on u/s side is
insufficient, then the u/s face may be damaged by
erosion due to wave action. The d/s side also may
be damaged by tail water, rain water etc. The toe of
the dam may also get damaged by water flowing
through spillways.

.
• Seepage:
• The flowing of water through the pores of the soil is
called seepage.
• Various methods used to reduce seepage in
earthen dam:
• A] Control of seepage through Embankment
• 1] Provision of impervious core
• 2] Provision of rock toe
• 3] Provision of drainage blanket
• B] Control of seepage through Foundation
• 1] Cut off trench
• 2] Concrete cut off walls

Seepage line:
• The line within the dam section below which
there is positive hydrostatic pressure in dam is
called as seepage line or saturation line.
• It gives following information
• It gives soil which is dry and submerged which
helps in finding shear strength of soil.
• It gives an idea that dam will not fail due to
softening of the downstream face, if it does
not cut the downstream face
• It also helps in drawing flow net.

.
• a) Methods to control seepage through body of earthen dam:
• 1. Provision of Impervious core: The effective method of seepage
• control is zoned type section with impervious core. The seepage
• line is changed due to provision of core.
• 2. Provision of rock toe: rock toe is provided to change the path of
• seepage line and prevent sloughing of downstream toe. It reduces
• the submerged area of the dam section.
• 3. Horizontal drainage blanket: the seepage can be controlled by
• providing a horizontal drainage blanket along with the rock toe.
• b. Methods to control seepage through foundation of earthen
• dam:
• 1. Cut-off trench: a trench is excavated below hearting zone,
• filled with impervious soil a properly compacted.
• 2. Concrete cut-off walls: vertical impervious cut-off made of
• concrete or sheet piles may be provided at upstream side of
• earthen dam. Such cut-off should generally be extended
• through the entire depth of pervious foundation so as to
• achieve effective seepage control.
• OTHER METHOD:
• Slurry trench cut off, grout curtains, upstream impervious blanket and relief wall

.
• (2) Seepage Failure : It may be caused by –
• a) Piping or Undermining : Due to continuous
seepage, flow through the body of the dam &
through the sub-soil below the dam. The d/s side
gets eroded or washed out & a hollow pipe like
groove is formed which extends gradually towards
the u/s through the base of the dam. This
phenomenon is known as piping or undermining. It
weakens the dam & ultimately causes the failure of
the dam.

.
• b) Sloughing : The crumbling of the toe of the dam is known
as sloughing. When the reservoir runs full, for the longer
time, the d/s base of the dam remains saturated. Due to the
force of seepage water the toe of the dam goes on crumbling
gradually. Ultimately the base of dam collapses.
• (3) Structural Failure :
• a) Sliding of side slopes : Sometimes it is found that the side
slope of the dam slides down to form some steeper slope.
Then the dam goes on depressing gradually & then
overtopping occurs which leads to the failure of the dam.
• b) Damage by Earthquake : The earthquake cracks may
develop on the body of the dam . It may eventually collapse.
• c) Damage by burrowing animals : Some burrowing animals
like craw, fish, snake, squirrel etc. causes damage to the dam
by digging holes through the foundation & body of the dam.

.
• 1. Upstream and downstream slope failures due to construction
• pore pressure: When dam is built of relatively impervious
• compressible soil, the drainage is extremely slow and excess pore
• pressure develops during and immediately after construction.
When
• the permeability is low, there may be no substantial drop in pore
• pressure in central zone of the dam by the end of construction if
this
• lies within usual range of 2-4 years. An initial pore pressure up to
• almost 140% of total weight of soil, above the point considered
has
• been more critical from the point of stability.
• 2. Upstream slope failure due to sudden drawdown: When
• upstream slide occurs due to sudden drawdown, the pore pressure
• along the surface of slide is dissipated to large extent.

.
• 3. Downstream slope failure during full reservoir
condition:
• Critical condition for downstream slope occurs
when the reservoir is
• full and percolation is at its maximum rate. The
direction of seepage
• tends to decrease stability.

.
• 4. Foundation slide: Spontaneous liquefaction- If the
foundation is
• laid on soft soil then dam will slide. It may also slides if
strata
• consist of weathered rock, shales etc. i.e. if a soft and
weak strata
• exits in foundation. The dam may slide over some
expansion of
• clayee soil on saturation may cause lifting dam.
• 5. Failure by spreading: Failures by spreading have
been observed
• only on connection with fills located above stratified
deposits that
• contain layers of soft clay

.
• 6. Failure by earthquake: it depends upon the intensity of
• earthquake. The most serious damages and failures may be
due to the following effects due to earthquake:
• a. Cracks in the core of dam leading to leakage and piping
failure
• b. Settlement of the crest due to compression of
foundation and / or embankment
• c. Shaking of reservoir bottom causing slow waves
• d. Liquefaction of sand below foundation.
• 7. Slope protection failure: Slopes are generally protected
by riprap over a layer of gravel or filter blanket. During a
heavy storm the waves on the surface of reservoir beat
repeatedly against the slope just above the reservoir level
which causes erosion.

.
• 8. Damage caused by water soluble materials: The
leaching of natural deposits of water soluble
materials such as gypsum may cause excessive
settlement.

.
• Remedial measures to avoid failure of earthen dam :
• 1) Control of seepage through embankment
• a) Provide Hearting in the central portion of dam.
• b) Provide casing over the hearting.
• c) Provision of horizontal drainage blanket
• 2) Control of seepage through foundation
• a) Provide cutoff trench under hearting zone.
• b) Provide concrete cut-off wall
• 3) Control of seepage in general
• a) Provide rock toe on d/s face at toe.
• b) Provide pitching on u/s slope.
• c) Provide turfing on d/s slope.
• d) Provide berms at 8 to 10 m vertical interval on d/s.

Types of repair and maintenance for an
earthen dam:
• the eight types of repairs and maintenance works for
an earthen dam.
• Following are the repairs and maintenance work for an
earthen dam-
• 1. Maintenance of pitching
• 2. Checking of upstream slope
• 3. Checking of downstream slope
• 4. Checking of Berms
• 5. Repairing of Turfings
• 6. Maintenance of rock toe
• 7. Maintenance of drain
• 8. Checking of top of dam including parapet wall

significance of phreatic line in earthen
dam
• Significance of phreatic line in earthen dam-
• 1. It gives us a divide line between dry and submerged
soil. The soil above the seepage line will be taken as dry
and the soil below the seepage line shall be taken as
submerged for computations of shear strength.
• 2. It represents the top streamline and hence helps us
in drawing the flow net.
• 3. The seepage line determination helps us to ensure
that it does not cut the downstream face of the dam.
This is extremely necessary for preventing softening or
sloughing of the dam.

Spillway-
• It is an arrangement provided at the crest of dam to expel the
excess water rises above the full reservoir level.
• Functions of spillway-
• To effectively dispose off the surplus quantity of water from
upstream to downstream side of the reservoir.
•
• Spillway is safety valve for dams:-
• 1) Spillway is an arrangement provided at the crest of dam to
expel the excess water rises above the full reservoir level.
• 2) This is necessary otherwise water will go on rising even
above HFL and will start flowing from top of dam which may
affect stability of dam.
• 3) Therefore it is very essential to provide spillway to dispose
surplus water on downstream side.

.
• Necessity and location of emergency spillway:
• Emergency spillway is provided to dispose-off the excess flood
water more than the designed flood. The top of emergency
spillway is kept below the top of main dam, but slightly above
the H.F.L. When abnormal high intensity flood occurs the
weaker portion gets washed and flood water flows through that
portion which acts as additional spillway and thus avoids
possibility of failure of the dam. It can be reconstructed
afterwards. Thus emergency spillway helps main spillway in
emergency.
• LOCATION:
• 1) in the body of dam.
• 2) at one end of the dam
• 3) away from the body of dam independently.
• Generally in gravity dam spillways are provided in body of dam
• Separate spillway is provided in earthen dam

.
• the advantages of spillway gates in dam :-
• 1) Saving in height of dam
• 2) Saving in land acquisition
• 3) More useful storage for the same height of dam
• 4) Reduction in the length of spillway & saving in cost
• Different types of spillway :
• 1) Main / Service spillway
• 2) Ogee spillway
• 3)Bar spillway
• 4)Side channel spillway
• 5)Trough / Chute spillway
• 6)Shaft spillway
• 7) Emergency Spillway
•

Essential requirements of spilway
• 1) it must have adequate discharge capacity.
• 2) it should be provided with some device for
energy dessipitation.
• 3) its discharge should be such that it should not
cause flooding on downstream side.
• 4) it must be safe.
• 5) its surface must be erosion resistant.
• 6) it should be so located that it should not errode
downstream toe of dam.

FACTORS AFFECTING SPILLWAY CAPACITY:
• Inflow.
• capacity of out late.
• Possible damage.
• Available storage capacity.
• Gates of spillway.
• Site condition.
• Type of dam and its purpose.
• Height of the crest of spillway.
• Solid material brought by river its nature and
amount.
• Geological condition.

COMPONENT PARTS OF SPILLWAY
• BODY, WEIR OR SPILLWAY:
• The weir or body may be ogee shaped, U shaped,
semicircular or circular.
• These are sometimes provided with gates to control or
to regulate flow of water.
• APPROCH CHANNEL:
• It draws excess water from reservoir and directed it to
downstream side.
• It admits water when its level goes above FRL into the
spillway thus helps in controlling discharge.
• ENERGY DISSIPATORS:
• When water flows over from crest it falls rapidly
depending upon height of crest, due to its high velocity
it may scour the downstream end and may damage the
dam.

.
• Therefore energy dissipaters are provided at
downstream end of the discharge channel.
Generally sky jump bucket, roller bucket or some
other suitable devices are provided to dissipate the
energy.
• TAIL CHANNEL:
• It conveys spillway discharge to the downstream
side.
• GUIDE WALL:
• It guide the water to flow directly into the river
downstream.

emergency spillway
• Emergency spillway is also called as breaching section and is
• provided in earthen dams and rock-fill dams. In such spillway
part of the length of earthen dam is kept weak. It is
constructed to dispose-off the excess floodwater more than
the designed flood. The top of emergency spillway is kept
below the top of main dam, but slightly above the H.F.L.
• When abnormal high intensity flood occurs the weaker
portion gets washed and flood water flows through that
portion which acts as additional spillway and thus avoids
possibility of failure of the dam. It can be reconstructed
afterwards. Thus emergency spillway helps main spillway in
emergency.
• When there is mal functioning of spillway gate.
• When actual flood exceeds the design flood etc.

SERVICE SPILLWAYS:
• The service spillway are masonry or concrete
structure provided with necessary
component. It is necessary for all dams and in
most it is the only spillway designed to pass
design flood.

OGEE spillway
• The shape of spillway is ogee or S shaped. The main
difference between free over fall spillway and ogee
spillway is that in case of free over fall spillway
water flowing over the crest of spillway drops
vertically as free set where in ogee shaped spillway
water is guided smoothly over the crest and is made
to guide over the downstream face of the spillway.
• It is ideal spillway as water flowing over the crest of
spillway always remains in contact with the surface
spillway.
•

BAR SPILLWAY
• It consist of rectangular masonry concrete bar with
crest width 1 to 1.5m.
• It is low height spillway, founded on concrete block,
rest on hard rock foundation sometime precast
concrete blocks are used for coping of the crest.
• The top of crest is at FRL.

SPILLWAY GATES:
• Advantages of spillway gates:
• 1) economical construction is possible due to
reduction in height.
• 2) useful storage up to some extent is possible
• 3) saving in land acquisition.
• 4) reduction in spillway length is possible
• These are also called as crest . The gates which are
provided at the crest of spillway are called crest
spillway gates.
• There are different type of gates and these are non
automatic and automatic.
• These are fitted in steel frames fitted in piers

NON AUTOMATIC GATES:
• The non automatic gates are lifted by an electronic
distant control system.
• They are classified as
• 1) vertical lift gate
• 2) taintor gate/ radial gate
• 3) flash board
• 4) drum gate

Vertical lift gate:
• These are made of steel sheets fitted on frames.
• These gates are suspended by iron chain or wire
rope within frame and guides with rollers to reduce
and lifting force.

radial gate
• 1) A radial gate has a curved water supporting face made of
steel.
• 2) It is properly braced by a steel framework which is pivoted
on horizontal shafts.
• 3) The gate can rotate about fixed horizontal axis.
• 4) Hoisting cables are attached to the gate and lead to
winches on hoisting platform.
• 5) The gate is pulled up by using cables and water is released
through the gate.
• 6) It is used for big spans varying from 4 m to 15 m, height 3
m to 10 m
• 7) The gate in cross – section is seen as a sector of a circle.
• It is used for big spans varying from 4 m to 15 m, height 3
m to 10 m

Taintor gate
• A radial gate, also known as a Tainter gate has
its water supporting face, made of steel
plates, in the shape of sector of circle,
properly braced and hinged at the pivot. The
gate can thus be made to rotate about fixed
horizontal axis. The load of the gate and water,
etc. is carried on bearings, mounted on piers.
The gate can be lifted by means of ropes and
chains acting simultaneously at both ends or
with the help of power driven winches.

AUTOMATIC GATE
• REYNOLD’S GATE
• It is fixed roller automatic gate, it moves up and
down along vertical face of the spillway.
• The gates are connected to one counter weight by a
system of pulley and chain.
• This counter weight is placed in well and this well is
connected to upstream water and when water
reaches FTL counter weight submerged and losses
its weight and gates slide down allowing flood to
pass over the crest.

Vishweswarya gate:
•
• This gate is fixed roller automatic gate.
• There are 11 sets each having 8 gates. Out of 8 gates, 4 are
heavy and 4 light, 2 heavy and 2 light gates on each side of
the counterweight well. They are connected to each other by
means of systems of pulleys and chains.
• Water enters into the well through inlet pipe when it rises
above FRL which reduces the weight of counterweight and
the heavy gates slide downwards. The light gates which are
connected to heavy gates slide downwards.
• The light gates which are connected to heavy gates get pulled
up creating the opening to pass the flood. When water level
in the reservoir goes below FRL no water enters the well. The
water in the well is drained out.
• The counterweight now sinks down pulling 4 heavy gates up
in a closed position.

.
• 4 light gates which are connected to heavy gates will
come down and close the openings. Thus, the opening
and closing operation of the gate is automatic.
• When the difference in upstream and downstream
water level is more, the flowing water from the spillway
has a very high kinetic energy due to high velocity of
flow. This energy can cause dangerous scour of the
channel bed, hence it is necessary to construct a
structure known as energy dissipater to reduce or
dissipate the kinetic energy of flow, before it enters in
the tail channel.
• It is located near the toe of the spillway and outlet
works.
• If it is omitted then dangerous scour can take place on
downstream side near the toe of the spillway as well as
away from it causing failure of the spillway wall or even
the dam.

ENERGY DISSIPATORS:
• Necessity of Energy Dissipators:
• Energy dissipator in spillway is necessary to reduce
or dissipate the kinetic energy of flow, before it
enters in the tail channel.
• Types of energy dissipaters:
• 1. Hydraulic jump type
• 2. Bucket type
• 3. Jet diffusion type.

.
• Function of an energy dissipater: When water flows
from crest and if the difference in upstream and
downstream water level is more. This causes very
high velocity. This high velocity has a very high
kinetic energy which can scour the bed. Hence,
energy dissipaters help in reducing this kinetic
energy of flow.
• Location of an energy dissipater: It is located near
toe of spillway.
• If energy dissipater will not be provided, it can
cause dangerous scour of the channel bed causing
failure of spillway.

.
• BUCKET TYPE ENERGY DISSIPATORS
• In bucket type energy dissipators flow of water is
directed into air and thus fall occurred away from
toe of dam
• These are more economical than hydraulic jump
type
• The energy is destroyed by a creation of jet and
impact of water on the bed of river and scouring
takes place which is small and away from toe.

Dams and spillways

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