presentation on gravity dam

7/30/2019 presentation on gravity dam

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CONTENTS INTRODUCTION

TYPES OF DAMS

FORCES ACTING ON GRAVITY DAM STABILITY ANALYSIS

STEPS FOR ANALYSIS OF GRAVITY DAM

REFERANCES


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Introduction

What is a dam ?A dam is a hydraulic structure constructed

across a river or a stream to retain the water.

It prevents the f low of water and accumulatesit in a deep storage reservoir.


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Types of damsBased on materials used :

Concrete dam

Steel dam Earth dam

Timber dam

Based on structural behavior :

Gravity dam

Arch dam

Buttress dam


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Based on hydraulic behavior :

Over flow dam

Non over flow dam

Based on function : Storage dam

Diversion dam

Coffer dam


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GRAVITY DAM

Gravity dams are solid concrete or masonry structuresused to retain the water and maintain their stabilityagainst design loads from the geometric shape and themass and strength of the concrete .

Gravity dam is so proportioned that its own weightresists the forces acting upon it . This type of structureis most durable, solid and required little maintenance.

They may be constructed of masonry or concrete.

They can be constructed on any dam site, where anatural foundation enough to bear the weight of thedam.


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Forces on gravity dam Water pressure

Weight of the dam

Uplift pressure Ice pressure

Wave pressure

Silt pressure

Wind pressure

Pressure due to earthquake


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Water pressure :

It Is the major externalforce acting on a dam.

Pressure intensity variestriangularly, zero at water

surface ,to a value wh at anydepth h below the watersurface.

Force at water pressure,

P = w h / 2where w is unit weight

of water = 1000kg/m3

acts at h/3 from base of

dam


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Weight of the dam :

major resisting force acting ona dam

for convenience cross sectionof a dam is divided into simple

geometrical shapes such astriangles and rectangles.

unit length of dam isconsidered.

W = w1+w2+w3

total weight (W) of dam actsat c.g of its section.


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Uplift pressure :

Uplift pressure is the upwardpressure exerted by water as it seepsthrough the body of the dam or itsfoundation.

Seeping water exerts pressure on

the base of the dam and itdepends upon water head .

Uplift pressure varies with whetherstructure is having drain lines or notand also with section is cracked or

not . Figure shown has drainage holesand not a cracked section.


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Silt pressure :

Silt gets deposited against theupstream face of the dam.

Is code recommends asHorizontal silt and water pressure

is assumed to be equivalent to thatof a fluid with a mass of 1360kg/m3

If h is the height of the siltdeposited, then the force exerted

by this silt in addition to externalwater pressure, can be

Psilt =sat . h . Ka / 2


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Wave pressure : Waves are generated on the surface of the reservoir by the blowing

winds, which cause pressure towards the downstream side. Waves pressure depends upon the wave height.

Hw =

Pw = 20 (hw^2)

It acts at hw/2 above the still water surface.

Ice pressure :

The ice may be formed on the water surface of the reservoir in coldcountries, may sometimes melt and expand.

The magnitude of this force varies from 250 to 1500 KN/m2 dependingupon the temperature variations.


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Earthquake forces : results in inertial forces that include vertical motion, oscillatory

increase, or decrease in hydrostatic pressure (all put force against dam)

As per IS Code, for dams up to 100 m height, the seismic coefficientmethod shall be used for the design of the dams; while for dams over100 m height the response spectrum method shall be used.


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STABILITYANALYSIS1) STABILITY AGAINST OVERTURNING :

If the resultant force cuts the base within the body of dam there

will be no overturning. For safety against overturning

find Sum of all the Resisting moments (MR) and Sum of allOver turning moments (Mo) then

F.O.S = sum (MR)/sum (Mo)

and F.O.S should not be less than 1.5


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STABILITY ANALYSIS2. SLIDING

A dam may fail in sliding at its base.

Sliding will occur when the net horizontal force

exceeds the frictional resistance developed atthat level.

Fs should not be less than 1


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STABILITY ANALYSIS3) COMPRESSION OR CRUSHING

A dam may fail by the failure of its materials.

The compressive stress may exceed the allowable stressand the dam material may get crushed.

4) TENSION

Masonry and concrete gravity dam are usually designedin such a way that no tension is developed anywhere,

because the materials can not withstand sustainedtensile stresses.

If it subjected to such stresses, these materials maycrack.


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STEPS FOR ANALYSIS OF GRAVITY DAM1. Consider unit length of the dam.

Calculate the vertical forces:

weight of dam ,weight of water acting on inclined faces,uplift forcefind sum of these vertical forces


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2. Find out some of horizontal forces :

horizontal component of water forces is

P = w h / 23) Calculate moment due to various forces

4 )By taking over turning moments(Mo) as (+ve) andresisting moments(MR) as (-ve)

check safety against over turning

F. O.S = MR/Mo >= 1.5


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5. Check safety against sliding :

obtain FS against sliding,

it shuld not be less than one

6. Calculation of stresses :

location of resultant force from toe

x = sum (M) / sum (V)

7. Eccentricity

e = (b/2) x

b is base of dam.


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8. Normal stress at the toe

Nt

= sum [V/b(1+6e/b)]

Normal stress at the heel

Nh = sum [V/b(1-6e/b)]

( +ve sign indicates compressive force )


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presentation on gravity dam

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