Investigation and Design of Dams

8/22/2019 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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3.1.3 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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3.3 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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3.3.2 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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3.3.5 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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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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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



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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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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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5.1.2 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

7 D i Of D


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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

7 D i Of D


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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


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

7 D i Of D


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7. Design Of Dams


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

7 D i Of D


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7. Design Of Dams


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

7 Design Of Dams


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7. Design Of Dams

Typical Filter Gradation

7 Design Of Dams


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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

7 Design Of Dams


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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

7 Design Of Dams


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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

7 Design Of Dams


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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. Design Of Dams

7. Design Of Dams


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7. Design Of Dams

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

Investigation and Design of Dams

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