investigation and design of dams
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1. Introduction
Dams
A dam is simply a barrier placed across
a watercourse to prevent or retard the
normal flow of water therein. It is one
of the most ancient of all structuraltypes and scores of thousands of them
have been built in the course of history.
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1. Introduction
Marib Dam, Yemen
Until recently, only a few trained observers had examined thesite of the Marib Dam. This barrier, known as Sudd al-Arim, isranked as the largest of the ancient dams in southern Arabia.
According to one report, it was located on the wadi Sadd (Saba)
near Marib and roughly 320 kilometers (200 miles) north ofAden.
One account described the dam as 3.2 kilometers (2 miles) long,
37 meters (120 feet) high, and 152 meters (500 feet) wide at thebase, with a volume of several million cubic meters of rock. Butanother, much more plausible, version tells of an embankmentonly a fraction as large and composed of earth. This presumably
is the dam whose remains can still be seen today, at a site on thewadi Dhana about 5 kilometers (3 miles) upstream from Marib.
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1. Introduction
Marib Dam, Yemen (Present)
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Some of the more common terms used to describecertain portions of a dam (Fig 1a & Fig 1b) are asfollows:
1. Abutments Either the sloping sides of the valley upon which thedam is built or the actual part of the dam that rests
on this portion of the valley.
2. River orChannelSection
The center portion of the dam that directly overliesthe river channel or that portion of the valley that isso situated.
3. Dam Dam is any artificial barrier and its appurtenantworks constructed for the purpose of holding wateror any other fluid.
4. Earth Dam It is made by compacting excavated earth obtainedfrom a borrow area.
2. Terminology
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5. Gravity Dam It is constructed of concrete and/or masonry and/orlaid-up stone that relies upon its weight for stability.
6. Height It is the vertical dimension from the downstream toeof the dam at its lowest point to the top of the dam.
7. Heel of the
Dam
The upstream portion of the dam where it contacts
the bearing surface (i.e., the ground or rockfoundation).
8. Toe of theDam
The downstream portion of the dam where it contactsthe bearing surface.
9. Crest The top of the dam. If walls are placed along the topof the dam to afford safety to a road or walkway,these walls commonly are called parapet walls.
10. Freeboard The distance between the highest level of water inthe reservoir and the top of the dam.
2. Terminology
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11. Axis of theDam
An arbitrary imaginary line drawn either along theexact center of the plan of the crest or along thecontact between the upstream part of the crest withthe upstream face of the dam.
12. Dam CrossSection
Usually drawn on a vertical plane that is normal tothe dam axis.
13. Galleries Formed openings within the dam. They providemeans for draining water seeping through the face orthe foundation, act as openings to drill grout anddrainage holes, and provide access to equipmentwithin the dam and for observing its performance.
14. Dead-storageWater Surface
The elevation of the reservoir below which waterstays permanently in the reservoir and cannot be
withdrawn. Also includes the silt storage, which isthat portion of the reservoir basin reserved forstoring any silt, which may enter and be deposited.
2. Terminology
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15. Tail Water Water at the downstream base of the dam resultingfrom backup of water discharged through thespillway, outlet works, or powerhouse.
16. MinimumWater Surface
The lowest elevation to which the reservoir can belowered and water still withdrawn by means of theoutlet works
17. MaximumWater Surface
The highest elevation at which water can be stored inthe reservoir without overtopping the dam or beingreleased through the spillway.
18. Appurtenantworks These are structures or materials built andmaintained in connection with dams. These can bespillways, low-level outlet works and conduits.
19. Spillway It is a concrete structure that conveys floodwater
from the valley upstream to the valley downstreamwithout damaging the dam or reservoir walls oreroding the foundation or toe of the dam.
2. Terminology
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20. Auxiliaryspillway
It is a secondary spillway designed to operate onlyduring large floods.
21. EnergyDissipator It is a structure constructed in a waterway whichreduces the energy of fast-flowing water.
22. Low-LevelOutlet
Outlet is an opening at a low level used to drain orlower the water.
23. DiversionTunnel It is a tunnel constructed within the abutment rocksto carry the water of the stream during constructionof the dam.
24. Conduit Conduit is an enclosed channel used to convey flowsthrough or under a dam.
25. Cofferdams They are temporary structures built upstream anddownstream from a dam to increase the stage of thestream so that the water will flow into the diversiontunnel by gravity. The downstream cofferdam
prevents backup of water discharged from the tunnel.These enclose all or part of the construction area sothat construction can proceed in the dry.
26. Cut-off A fabricated structure or a grout curtain placed tointercept seepage flow beneath a dam.
2. Terminology
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2. Terminology
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2. Terminology
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3. Classification Of Dams
3.1 According To Purpose
3.1.1 Stage Control Dams
Diversion Dam
Navigation Dam
Check Dam
3.1.2 Storage Dams
Flood control Dam
Water supply Dam
Hydroelectric power Dam
Sedimentation Dam
Recreation Dam
Groundwater recharge
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3. Classification Of Dams
3.1.3 Multipurpose Dams
3.1.4 Barrier Dams
Levees and dikes-to protect land areas fromoverbank flow
Cofferdams-for temporary dewatering of
construction sites
3.2 According To Material
Concrete Dams
Masonry Dams Earthfill Dams
Rockfill Dams
Steel Dams Timber Dams
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3. Classification Of Dams
3.3 Structural Types
3.3.1 Gravity Dams
Concrete gravity dams-designed so that
water and other loads are resisted by weight
of dam (Fig 2). Concrete arch dams-loads resisted by arch
action carried to abutments.
Gravity-arch dams-loads resisted by
combination of gravity and arch action.
Buttress dams-loads resisted by slab or archaction between successive buttress support.
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3.3.1 Concrete Gravity Dams
Altus Dam Angostura Dam
Olympus Dam
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3.3.1 Concrete Arch Dams
Hungary Horse dam (thick arch)
Horse
Mesa dam(thin arch)
Gibson dam
(thick arch)
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3.3.1 Concrete Buttress Dams
Pueblo dam (massive head buttress)
Bartlett
dam
Lake Tahoe
dam (slab &
buttress)
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3. Classification Of Dams
3.3.2 Masonry Dams
Stone-masonry gravity dams
Stone-masonry arch dams3.3.3 Earth-Fill Dams
Homogeneous embankment dams
Zoned-earth embankment dams-with internalcore of relatively impermeable material
Diaphragm-type embankment dams-with
central core wall of concrete, steel or timber
3.3.4 Rock-Fill Dams Rockfill dam with central earth core Rockfill dam with bituminous concrete core Decked rockfill dam (cement concrete /
bituminous concrete face
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3.3.2 Stone Masonry Gravity Dam
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3.3.2 Earthfill Dams
Anderson Ranch Dam Davis Dam
Pinto Dam
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3. Classification Of Dams
3.3.5 Steel Dams
Steel slab-buttress dams Sheet-steel cofferdams
Cellular-steel cofferdams
3.3.6 Timber Dams Timber slab-buttress dams
Timber crib dams, with rock-filled cribs
3.4 According To Size Of Dam And/Or Reservoir Large Dams
Small Dams
4 Factors Governing Selection Of Type
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4. Factors Governing Selection Of Type
Of Dam
4.1 Site Conditions4.1.1 Geological Factors
Uncertain or variable foundation, Suitable rock in the vicinity-may be obtained
from quarries,
An adequate amount of clay in the vicinity tobe used either as a vertical core or as a
sloping core.
4.1.2 Topography Of Site Gorge: Chord to height ratio (C/H)
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4. Factors Governing Selection Of Type
Of Dam
Wide Valleys: C/H > 6-7* Gravity, buttress, multiple arch, earthfill, rockfill dams
Flat country: Plains* Embankments
4.1.3 Availability Of Materials
Convenient source of earth or aggregate mayindicate earth or masonry dam
Buttress dam requires smallest quantity of
materials
4.2 Hydraulic Factors Spillway requirements
Diversion requirements
Outlet works and penstock
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4. Factors Governing Selection Of Type
Of Dam
4.3 Climatic Effects Spelling of concrete in cold climates disadvantage
of thin arch and buttress dams
4.4 Traffic Factors Crest highways costly for thin arch and buttress
dams
Navigation locks precluded for arch dams
4.5 Social Factors Gravity dams provide greatest safety against
sudden destruction due to earthquake, bombing,
etc., with resultant damage to affectedcommunities
4 Factors Governing Selection Of Type
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4. Factors Governing Selection Of Type
Of Dam
Benefits to be derived may control cost of dam;
temporary dam may have to suffice if benefits aresmall or short-range
Volume of employment, particularly of local labor
Aesthetic considerations.
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5. Dam Site Investigation
5.1 Surface Investigation
5.1.1 Topography
A suitable dam site must exist. The cost of real estate of the reservoir
(including road, railroad, cemetery, and
welling relocation) must not be excessive.
The reservoir site must have adequate
capacity.
A deep reservoir is preferable to shallow one
because of lower land costs per unit of
capacity, less evaporation loss, and lesslikelihood of weed growth.
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5. Dam Site Investigation
Tributary areas which are unusually
productive of sediments should be avoided if
possible.
The quality of the stored water must be
satisfactory for its intended use.
Site from which a considerable quantity of
leakage may occur should be avoided. Sites susceptible to sliding should be
avoided.
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5. Dam Site Investigation
5.1.2 Hydrology
5.1.3 Geology
5.1.4 Hydrogeology5.1.5 Seismicity
5.2 Subsurface Investigations
5.2.1 Subsurface Investigation Methods Geophysical methods
Core drilling
Adits Trenches
Shafts
In-situ Tests Water Pressure Test
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7. Design Of Dams
7.1 Gravity Dams7.1.1 Forces On Gravity Dam
7.1.2 Stability Analysis Of Gravity DamA. Stability Analysis
Overturning
Cracking Sliding7.1.3 Seepage And Leakage
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7. Design of Dams- Gravity Dam
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7. Design Of Dams Embankment Dam
7.2.3 Seepage Analysis And Design
7.2.4 General Problems Of Soil Foundation7.2.5 Typical Design Measures And Seepage Analysis
A. Typical Design measures
B. Seepage Analysis
Excessive Exit Gradient
Excessive Pore Pressure
Excess Seepage Flow
7.2 Embankment Dams
7.2.1 Zoning Of Embankment (Earth Fill And Rock
Fill) Dams And Typical Construction Materials7.2.2 Seepage Through The Dams And Foundation
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7. Design Of Dams
7.2.6 Design Of Different Sections Of Embankment
And Slope Stability Analysis
A. FreeboardTable 5
Freeboard Requirement
Sr.
Nr.
Largest Fetch
(km)
Normal Freeboard
(m)
Min Freeboard
(m)
1 Less than 1.6 1.2 0.92 1.6 1.5 1.2
3 4 1.8 1.5
4 8 2.4 1.85 16 3.0 2.1
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7. Design Of Dams
B. Crest of Dam
Embankment Dams
W = 0.2Z + 3.3 m
where
W = Width of crest in meters
Z = Height of the dam abovestream bed in meters
i f
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7. Design Of Dams
C. Core
Table 6Core Permeability
Sr.
Nr.
Permeability Coefficient
(cm/sec) Typical Soil Value as Core
1 1.01 to 1.01x103 Sands Considerable Leakage
2 1.01 x 103 to 1.01 x 104 Silty Sands Usable with good
control, if some leakagecan be tolerated
3 1.01 x 104 to 1.01 x 106 Silts Little leakage if well
compacted
4 1.01 x 106 and lesser Silty Clay, Clay Impervious
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7. Design Of Dams
D. Shell Slopes and Material
Table 7Tentative Design Slopes Earth Dams
Sr. Nr. Soil Type Upstream Downstream
1 Gravel, Sandy Gravels with core 2.5H: 1V 2H : 1V
2 Clean sands with core 3.0H : 1V 2.5H:1V3 Low Density Silts, Micaceous Silts 3.5H: 1V 3.0H:1V
4 Low Plasticity Clays 3.0H : 1V 2.5H: 1V
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7. Design Of Dams
E. Internal Drainage System
Table 8
Categories of Base Soil Material
Category Percent Finer than the No 200
(0.074 mm) size
1 > 85
2 40 853 15 39
4
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7. Design Of Dams
E. Internal Drainage System
Table 9
Criteria for Filters
S r .
N r .
B a s e S o i l D e s c r ip t io n a n d
P e r c e n t F in e r T h a n N o 2 0 0
( 0 .0 7 4 m m ) s ie v e
F i lte r C r i te r ia
1 F i n e s i lt s a n d c la y s
( m o r e th a n 8 5 % f in e r )
D 1 5 F < 9 D 8 5 B
2 S an d s , S ilt s , C la y s a n d S i lt y &
C la y e y S a n d s(4 0 % to 8 5 % f in e r )
D 1 5 F < 0 .7 m m
3 S i lt y a n d C la y e y S a n d s a n d G r a v e ls
(1 5 % to 3 9 % f in e r )
D 1 5 F < 0 .7 m m
+ ( (4 0 - A )x ( 4 x D 8 5 B 0 .7
m m ) ) / 2 5
4 S a n d s a n d G ra v e ls
( L e s s th a n 1 5 % f in e r )
D 1 5 F < 4 D 8 5 B
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7. Design Of Dams
E. Internal Drainage System
Table 10
D10F and D90F Limits for Preventing Segregation
S r.
N r.
M in im u m D 1 0 F
( m m )
M a xim u m D 9 0 F
( m m )1 < 0 .5 2 0
2 0 .5 1 .0 2 5
3 1 .0 2 .0 3 0
4 2 .0 5 .0 4 0
5 5 .0 1 0 5 0
6 1 0 5 0 6 0
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7. Design Of Dams
E. Internal Drainage System
Table 11
Minimum Thickness of Upstream Blanket forDifferent Materials
Thickness for Given HeadSr.Nr.
Filter0 23m 23 45 m 45 90 m
1 Fine Sand 150 mm 300 mm 450 mm
2 Coarse Sand 225 mm 450 mm 600 mm
3 Gravel 300 mm 600 mm 750 mm
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7. Design Of Dams
Typical Filter Gradation
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7. Design Of Dams
F. Riprap
Table 12
Minimum Riprap Thickness
Riprap Size (m)
3H : 1V Slope 2 H : 1V Slope
Sr.
Nr.
Wave
Height
(m) D50 D100 D50 D100
1 0.5 0.19 0.27 0.21 0.3
2 1.0 0.37 0.55 0.42 0.633 1.5 0.55 0.82 0.63 0.95
4 2.0 0.73 1.10 0.84 1.26
5 2.5 0.92 1.38 1.05 1.58
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7. Design Of Dams
Typical Filter Gradation
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7. Design Of Dams
Table 13
Minimum Factors of Safety
G. Slope Stability Analyses
L o a d i n g
C o n d i t i o n
S h e a r
S t r e n g t h
P a r a m e t e r s *
P o r e P r e s s u r e
C h a r a c t e r i s t i c s
M i n i m u m
F a c t o r o f
S a f e t y
A . E n d o f
c o n s t r u c t i o n
1 . E f f e c t i v e G e n e r a t i o n o f e x c e s s p o r e
p r e s s u r e s i n e m b a n k m e n t a n d
f o u n d a t i o n m a t e r i a l s w i t hl a b o r a t o r y d e t e r m i n a t i o n o f
p o r e p r e s s u r e a n d m o n i t o r i n g
d u r i n g c o n s t r u c t i o n
1 . 3
G e n e r a t i o n o r e x c e s s p o r e
p r e s s u r e s i n e m b a n k m e n t a n d
f o u n d a t i o n m a t e r i a l s a n d n o
f i e l d m o n i t o r i n g d u r i n g
c o n s t r u c t i o n a n d n o l a b o r a t o r y
d e t e r m i n a t i o n
1 . 4
G e n e r a t i o n o f e x c e s s p o r e
p r e s s u r e s i n e m b a n k m e n t o n l y
w i t h o r w i t h o u t f i e l d
m o n i t o r i n g d u r i n g
c o n s t r u c t i o n a n d n o l a b o r a t o r yd e t e r m i n a t i o n
1 . 3
2 . U n d r a i n e d
S t r e n g t h
1 . 3
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7. Design Of Dams
Table 13
Minimum Factors of Safety
G. Slope Stability Analyses
B. Steady-
state seepage
Effective Steady-state seepage under
active conservation pool
1.5
C.Operational
Conditions
Effective orUndrained
Steady-state seepage undermaximum reservoir level
1.5
Effective or
Undrained
Rapid drawdown from normal
water surface to inactive
water surface
1.3
Rapid drawdown form
maximum water surface to
inactive water surface
1.3
D. Unusual Effective or
Undrained
Drawdown at maximum outlet
capacity
1.2
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7. Design Of Dams
Shear Strength Data and Sources Determination of Pore Pressures Methods of Analysis
Slip Surface Configuration
7 2 Embankment Dam Major Sections
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7.2 Embankment Dam Major Sections
Earth and Rockfill-Central Core-Scale B
7 2 Seepage Designs
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7.2 Seepage Designs
7 Embankment Dam Zones - Description
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7. Embankment Dam Zones Description
7. Embankment Dam ConstructionMaterial
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Material
7. Design Of Dams
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7.3 Miscellaneous Problems
7.3.1 Sedimentation In Reservoirs
7.3.2 Problems Associated With Foundation RocksA. Shales
B. Sandstones
C. Carbonate Rocks
D. Evaporites
E. Extrusive RocksF. Intrusive Rocks
G. Metamorphic Rocks