Seepage: Flow Through Earth damConstruction of flow net:1. Plot earthen dam in suitable scale2. Determine constant (s) and find the equation ofphreatic line and plot it3. Since the pressure in phreatic line is zero potentialdifference is du to elevation head only.

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Piping FailurePiping failure caused by heave can be expected to occur onthe downstream side of a hydraulic structures when the upliftforces of seepage exceed the downward force. It isprogressive phenomenonRemedial MeasureGraded filterTo control piping erosion graded filter is design fulfilling following criteria.

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Consolidation

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What is Consolidation?

When a saturated clay is loaded externally,

saturated clay

GL

The reduction in volume will takes place by expulsionof water from voids over a long time

Compaction VS Consolidation

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COMPACTION

1. Application of dynamic load.

2. Expulsion of air

3. Short Process

CONSOLIDATION

1. Application of static load.

2. Expulsion of water

3. Long Process

Types of ConsolidationThe total compression of a saturated claystrata under excess effective pressuremay be considered as the sum of 1. Immediate compression, 2. Primary consolidation, and 3. Secondary compression.

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1. Immediate compressionThe portion of the settlement of a structurewhich occurs more or less simultaneously withthe applied loads is referred to as the initial orimmediate settlement. This settlement is due tothe immediate compression of the soil layerunder undrained condition and is calculated byassuming the soil mass to behave as an elasticsoil.

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2. Primary consolidationIf the rate of compression of the soil layer iscontrolled solely by the resistance of the flow ofwater under the induced hydraulic gradients,the process is referred to as primaryconsolidation. The portion of the settlementthat is due to the primary consolidation is calledprimary consolidation settlement orcompression.

Change in volume by expulsion of water

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3. Secondary ConsolidationCompression due to the compression andrearrangement of the clay particles andclay layer. It is linear with logarithm ofthe time.

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Basic definitions

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Basic definitions

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Basic definitions

Basic definitions

Normally consolidated clay and overconsolidated clay Normally consolidated clay: A soil is normally

consolidated when it has never been subjected tostress higher than the present stress.

Over consolidated clay: A soil which hasexperienced higher stress in the past than thepresent stress.

Cause of over consolidationI. Removal of the overburden;excavation, erosion, landslide etc.I. Removal of the structureII. Variation in pore water pressure

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Terzaghi spring analogy Valve is opened; The

system is equilibrium.

This is similar to the soilbefore loading

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The valve is closed and thepiston is loaded. Thepressure increase in thegauge is equal to theincreased load.

This is similar to the conditionjust after loading.

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When valve is opened thepiston start to move downand the pore waterpressure is gaugereduced. The applied loadis now share by bothspring and water.

At final stage; pressuregauge shows Ust pressureall load is taken by spring.

The amount of settlement in the spring is depends on thestiffness of spring ( compressibility characteristic of soil)

The rate of settlement depends on the opening ( permeabilityof soil)

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Consolidation testTo determine the compressibility characteristics ofsoil one dimensional consolidation (Oedometer) testis carried out.Objective of test:1. To determine the amount of deformation2. To determine the rate of deformation

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Procedure:1. Sample is placed in the cutting ring in betweentwo porous stone.2. The loading beam is then brought into contactand dial gauge is set at zero.3. When first load of 10kN/m2 is applied reading ofdial gauge is taken at 1/4, ½, 1, 2, 4, 8, 16, 30, 60,120, 240, 1440 mins.4. Now load is doubled and dial gauge reading istaken as in step 3. Load is doubled upto 640kN/m2

5.Unloading is done by removing 3/4th loadand reading is observed as earlier.

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Calculation

Determination of void ratioA. Height of solids method.

B. Change in void ratio method.

h= final height and, e= final void ratio

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Determination of coefficient of consolidation1. Logarithm of time : Casagrande method

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R50=(Rc+R100)/2Tv=(Cv t)/d2

Tv=0.197For 50% consolid-ation

2. Square root time: Taylor method

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Tv=(Cv t)/d2

Tv=0.848For 90% consolid-ation

a b c

Terzaghi theory of one dimensionalconsolidation

Assumptions1. The soil is homogenous2. The soil is fully saturated3. The solid particles and water are incompressible4. The flow is one dimensional5. Darcy’s law is valid6. k and mv remains constant7. There is unique relationship betn void ratio and effectivestress and remain constant

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One Dimensional Consolidation

saturated clay

GL

q kPa

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Settlement computations

eo, vo’, Cc,Cr, p’, mv-oedometer

test

=q

q kPa

H

Two different ways to estimate theconsolidation settlement:

(a) using mv

(b) using e-log v’ plot

settlement = mv H

He

esettlement

o

1

next slide

Settlement computations~ computing e using e-log v’ plot

'

''log

1 0 vo

vocC

e

HS

initial

vo’

eo

vo’+

e

If the clay is normally consolidated,

the entire loading path is along the VCL.

Settlement computations~ computing e using e-log v’ plot

'

''log

1 0 vo

vorC

e

HS

vo’

initialeo

vo’+

If the clay is overconsolidated, and remains so bythe end of consolidation,

e

VCL note the use of Cr

Settlement computations~ computing e using e-log v’ plot

'

''log

1'

'log

1 00 p

voc

vo

pr C

e

HC

e

HS

vo’

initialeo

vo’+

If an overconsolidated clay becomes normallyconsolidated by the end of consolidation,

VCL

p’

e

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