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HYDRAULIC STRUCTURES

DAMSby:

Dr. Zahiraniza Mustaffa

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General Content:

• Introduction

– Introduction to Dams

– Dams Classification

• Material classifications

• Concrete Gravity Dam

– Forces (Loads) on the Dam

– Load Combination

– Stability Analysis

• Ancillary Structures

– Spillways etc. (will be covered later)

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Introduction

– What is a dam?

• A dam is a barrier structure placed across a watercourse to store water.

– Why do we need dams?

• To fulfill many functions like water supply (domestic, irrigation & industrial), flood mitigation, hydropower development and irrigation.

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Dam

Energy Dissipator

Structures

Hydraulic jump

Reservoir

Q

Spillway

Typical Layout of a Dam

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Dams Classification

• Dams can be classified in many ways: • Size:

Dams vary in size from a few meters in height to massive structures of over 100 m in height.

– Large Dam (H >15 m or Reservoir Volume > 3 x 106 m3)

– Small Dam

• Purpose:

- Water Supply (domestic, irrigation & industrial), Flood Mitigation, Hydropower and Irrigation Dams.

• Material:

- Earthfill, Rockfill, Gravity (Concrete), Arch, Buttress etc

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Kenyir Dam, Terengganu

(10-11 April, 2004)

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Kenyir Dam, Terengganu

(10-11 April, 2004)

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Dams Classification – Material

• Earthfill (Embankment) Dam

• Rockfill Dam

• Concrete Gravity Dam

• Buttress Dam

• Arch Dam

• Roller Compacted Concrete (RCC) Dam

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Fine material

Coarse material

Filter material

EARTHFILL DAM

• An embankment that uses earth soil (natural materials excavated nearby the area) to provide stability.

• The materials are compacted.

• Impermeable materials at the centre – to prevent seepage

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ROCKFILL DAM

Impervious face

Rock

• An embankment that uses variable sizes of rocks to provide stability.

• A thin membrane (impervious) on its upstream face for water tightness.

• More stable than an earthfill dam. Cheaper than concrete dams.

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CONCRETE GRAVITY DAM

Concrete

• A dam that applies its weight (gravitational forces) for stability.

• Normally in triangular shape (side view).

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ARCH DAM

Concrete

• Narrow in size, in which the abutments are of massive rock of the canyon.

• Is designed to transfer the imposed loads to the adjacent rock walls on either side of the canyon.

• Hard to construct. Cheaper than concrete gravity dams.

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BUTTRESS DAM

Concrete

Buttress

• A hollow gravity dam.

• Buttresses of reinforced concrete rest on the

rock foundation and support a watertight

sloping face of the dam.

• Cheaper than concrete gravity dams.

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Concrete Gravity Dam

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• Concrete gravity dams are designed so that the weight of the dam itself (gravity force) is sufficient to resist overturning by the applied forces.

• The forces that must be considered in the design of a dam are:

1. Weight of the dam

2. Hydrostatic forces (u/s and d/s of the dam)

3. Hydrostatic uplift force

4. Earthquake force

5. Silt force

6. Ancillary forces (roadway etc)

7. Others (ice, waves, wind forces etc)

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ICE JAMS ALONG A RIVER

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ICE JAMS NEAR A BRIDGE

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ICE JAMS

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WFp1

Fp2

Ww

1

2

Forces Acting on a Dam

HW

TW

HW = headwater

TW = tailwater

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RFy

Fx

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1. Weight of Dam (W)

• Necessary to include:

– Weight of the dam, W• The weight of dam per unit (1 m) length,

– Weight of other ancillary structures like gates, bridges, roadways etc.

• The resultant weight acts at the centroid of the dam

i.e. at 1/3 of the dam width, b (from the heel).

Forces on Dam

where, Ac is the area of the dam (side view) and, c is the

specific weight of concrete (24 kN/m3 or 2400 kg/ m3).

(kN/m) cc AW

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b/3

b

W

Heel

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2. Hydrostatic Forces (Fp)

• Sometimes referred to as external hydrostatic

pressure.

• Hydrostatic forces are forces acting at the

upstream and downstream faces of the dam.

• The hydrostatic force, Fp per unit (1 m) length is

given by:

2

2hF w

p

where, w is the specific weight of

water (9.81 kN/m3) and h

is the vertical depth of

water.

(kN/m)

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b’/3

Fp1

Fp2

Ww

1

2

h1 /3

h2 /3

b’

h2

h1

Toe

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• For a vertical surface, Fp is acting

horizontally at 1/3 of the water depth,

measured from the base of the dam.

• For an inclined surface, there are 2 forces

acting on the surface, namely Fp (acts

horizontally) and weight of water,Ww (acts

vertically).

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• Ww is described as follows:

• Its magnitude is equal to the weight of

volume of water per unit (1 m) length

directly above the inclined face of the

dam.

• It is acting through the centroid of the

volume of water, i.e. at 1/3 of b’,

measured from the toe.

www AW

where, Aw is the area of the

water (side view)(kN/m)

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3. Hydrostatic Uplift Force (FU)

• Sometimes is referred to as internal

hydrostatic pressure.

• Hydrostatic uplift force is a force produced by

water (under pressure) in the pores of the

concrete dam and foundation.

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After the reservoir is filled, water will tend to

move/seep from u/s to d/s/.

It will seep into the pores of the concrete

(despite the low permeability of the concrete) and

its foundation.

When the seepage water is stable (resulting

in a saturated condition), a pressure head

gradient will develop along the base of the

dam.

This will give extra pressure force to the

dam!

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For a dam without tailwater (TW) effect:

• FU drops linearly from u/s to d/s; resulting

in a triangular pressure distribution

diagram, decreasing from wh1 to 0.

For a dam with tailwater (TW) effect:

• FU drops linearly from u/s to d/s; resulting

in a trapezoidal pressure distribution

diagram, decreasing from wh1 to wh2 .

How does a pressure head gradient look like?

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FU

b/3

b

h1

w h1

A dam without tailwater (TW)

at downstream section

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FU

b

h1

h2

w h1

w h2

A dam with tailwater (TW)

at downstream section

TW

x

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• The uplift force, FU per unit (1 m) length is

determined by:

• FU is measured at the centroid of the uplift

pressure distribution diagram, measured from

the toe of the dam.

uwu AF

where, w is the specific weight of water (9.81

kN/m3), Au is the area of uplift pressure

distribution diagram.

(kN/m)

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• Is FU good for the stability of the dam?

Why?

• How can we control FU ?

– Constructing cut-offs:

• Grout curtain

• Drainage curtain

– Creating a more impervious zone at the

foundation

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• Grout Curtain

– A line constructed at

the foundation to

block water

seepage from u/s to

d/s of the dam.

– A hole of 4-6cm are

drilled at the heel.

Cement grout is

pumped into the

holes (to seal the

cracks in the rocks).

• Drainage Curtain

– A row of holes

drilled just d/s from

the grout curtain.

– To intercept any

seepage which may

escape past the

grout curtain. The

seepage is collected

in the drain and

flows away by

gravity or pump.

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Grout curtain

Holes

Grout Curtain

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Holes

Drain curtain

Drain Curtain

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Impervious Zone

Impervious

zone

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4. Earthquake Force (Fe)

• When an earthquake occurs, the earth

shakes (vibrates) at an acceleration, a.

• The dam will be accelerated due to the

earthquake with an initial force, Fe but at

opposite direction to a.

• Fe is acting at the centroid of the dam.

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• Fe is given by,

Fe = Ma

• a can be in the range of 0.05g to 0.5g, with

g stands for acceleration due to gravity.

where, M is the mass of the dam and a is the

earthquake acceleration.

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Load Combination

• Not all loads mentioned earlier are considered when designing a dam. Why?

• The load selections are based on below conditions:

– Normal Load Combination (NLC)

– Unusual Load Combination (ULC)

– Extreme Load Combination (ELC)

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Load Combinations

Load Source Qualifications NLC ULC ELC

Primary

Secondary (if applicable)

Headwater

Tailwater

Self-weight

Uplift

Silt

Ice

Exceptional

Earthquake

At DFLAt NFL

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Stability Analysis

• The stability of a dam can be checked by

using the Simple Gravity Method.

• The stability analysis checks:

1. Safety against stresses

2. Safety against sliding

3. Safety against overturning

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Safety Against Stresses

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Let’s talk about stress…

• Stress, .

• Unit of stress = N/mm2

• Two common stresses:

– Tensile stress leads to tension

– Compressive stress leads to compression

Stress =

Pressure?

compression

tension

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Toe

Heel

Tensile stress Compressive stress

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CrushingCracking

Heel Toe

Why are stresses not desired in a dam?

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• There are many stresses acting on a

dam but the focus will be given on

vertical normal stresses, acting on a

horizontal plane.

• Uplift load, Fu is excluded in the stress

determination.

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d/su/s

Stress Diagram at

Dam Foundation

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• At the base of a dam, the normal stresses can

be either tensile or compressive.

• BUT, it is not desired to have any tensile stress

at the heel, so only the compressive stresses

are allowed at BOTH heel and toe, given by:

b

e

b

Fy

heel

61

'

b

e

b

Fy

toe

61

'

concrete

foundation

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where,

Fy’ is the resultant vertical forces above

the plane considered (exclusive uplift),

b is the base width of the dam and e

is eccentricity of the resultant load R (the

horizontal distance from the centre of

the base to the point where R acts) .

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• e is obtained from the equation,

• e MUST be,

if not, u/s will be negative, i.e tensile stress, which

leads to tension at the heel. This will cause

cracking. Not good!

• A good dam design is when the dam is free from

tensile stress at the heel. How to strengthen the

heel from developing tensile stresses?

'yF

Me

6

be

where, is the summation of

moments at toe and is the

summation of all vertical forces

(exclusive uplift).

xM'yF

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b/2b

Lxe

+M

Fx

Fy

R

Fx

Fy

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• Allowable concrete stress, con(allw) :

2000 kPa < con(allw) < 4000 kPa

• Allowable foundation stress, found(allw) :

Foundation Materials Allowable stress, found(all)

(kPa)

Granite

Limestone

Sandstone

Gravel

Sand

Stiff Clay

Soft Clay

4000 – 6000

3000 – 4000

2500 – 3500

300 – 600

200 – 400

200 – 400

50 – 100

Note: Pa = N/m2

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Safety Against Sliding

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• Sliding?

• How would you hold yourself from

sliding if somebody pushed you?

• A dam can resist sliding if the ratio of the

horizontal force, Fx to the vertical force, Fy is

smaller than a safety factor, f . Or,

fF

F

y

x

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Sliding

Worst scenarios that could

happen to a dam!

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• f can be obtained from laboratory analyses

as summarized below:

Materials f

Sound rock, clean and irregular surface

Rock, some jointing and laminations

Gravel and coarse sand

Sand

Shale

0.8

0.7

0.4

0.3

0.3

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Safety Against Overturning

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• Overturning?

• Overturning would occur if the resultant

force, R fell outside the toe.

• But sometimes as R is moving closer to the

toe, the dam already experiences many

failures like crushing, cracking and sliding.

This is explained in the next slide:

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Overturning

Worst scenarios that could

happen to a dam!

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Will cause

overturning

Safe from

overturning

RR

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As R moves closer to the toe (e is closer to

toe), pressure at heel decreases and

pressure at toe increases.

Tension occurs at heel, resulting in a further increase in

uplift pressure, and excessive compressive stresses at

toe result in crushing.

Eventually, before a dam overturns, it experience crushing

(toe), cracking (heel) and increasing in uplift and sliding.

Therefore, a dam is safe from overturning if the criteria

of no tension on the upstream face, the resistance

against sliding, and the quantity of concrete/foundation

is good.

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• A dam can resist overturning if the ratio of the summation of all restoring (+ve) moments to the summation of all overturning (-ve) moments is within the allowable safety factor, fo. Or,

with,

fo 1.5 is desirable, and

fo 1.25 is generally regarded as acceptable.

o

ve

ve fM

M

+ve

M