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Presented By

D N Naresh

AGM(PE-Civil)

Effect of Water Logging and Excess pore pressure

in

Dyke Maintenance. 

O&M Conference on SUSTAINABLE GROWTH

Theme – Ash Management Technologies

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MOEF stipulates 

•

Minimum land• Safe design of dyke – No breach/failure

• No leachates- liner

• Effluent discharge TSS less than 100ppm

•

Control of Fugitive dust

Before any construction activity - MOEF clearance is a must

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TYPE OF SECTION and METHOD of CONSTRUCTION

Sub soil exploration & Geo Hydrological studies

Access suitability

& availability of soil

Type of sectionSub soil characteristics

INVESTIGATIONS are taken up after site selection

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CRITERIA FOR SAFE DESIGN.MATERIAL PROPERTIES for deciding type of DYKE and method of CONSTRUCTION

• FOUNDATION

• FILL

NO OVERTOPPING

• ADEQUATE FREE BOARD --- min 1.5m as per IS 10635• SETTLEMENT OF DYKE/FOUNDATION

FOUNDATION STABILITY

• SHEAR STRESSES should be less than SHEAR STRENGTH

SEEPAGE LINE WITHIN d/s FACE of DYKE

• SEEPAGE ANALYSIS

TO PREVENT PIPING ACTION IN FOUNDATION & EMBANKMENT

• SLOUGHING & TOE SOFTENING

• INTERNAL DRAINAGE like (SAND CHIMNEY, SAND BLANKET, ROCK TOE ,TOE DRAIN) is provided---as per IS:9429

•  

SLOPES TO be PROTECTED AGAINST

•   WAVE ACTION,WIND ACTION,RAIN WASH ,VELOCITY OF FLOW

---as per IS 8237

STABILITY OF SLOPES

 – STABILITY ANALYSIS

 – ALLOWABLE FACTORS OF SAFETY (FOS) --- as per IS 7894

 – STATIC >= 1.5, DYNAMIC >= 1.0

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Design

• Stable slopes

• Internal drainage

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Provision of internal drainage

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Dyke under construction

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Water logging

Signs of Distress :Water Stagnation

Water logging

Due to water logging

there is softening of base

sloughing

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Effect of water logging

…. 

Pore Pressure

WATER LOGGING LEADS TO SLOUGHINGWILL WEAKEN THE BASE

LOSS OF SHEAR STRENGTH

REDUCE PASSIVE RESISTANCE

WILL REDUCE FoS

WILL TRIGGER THE SLIDING FAILURE

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Before saturation

During saturation

24 hours of saturation

Cleavages & split seen

on cores

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Effect of water logging

on safety of dyke

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Control Measure

• Do not allow water logging/inundation in and

around the dyke

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Maintaining DRY CONDITIONS D/S

STORAGE LAGOON-I STORAGE LAGOON-I I

SLURRY PIPE LINE

OVERFLOW LAGOON

SPILLWAY

WATERESCAPE

STRUCTURE

 ASHPIPE LINE TO PLANT

WATERRECIRCULATION

N  AT U R  AL W  AT E R  B O D Y 

DISCHARGE CHANNEL

TAPPING FOR

RECIRCULATION

* *

*  ASH FILLING SHALL BE CARRIED OUT

NEAR THE WATER ESCAPE IN THEINITIAL PERIODS

XX

DYKE ALIGNMENT

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Effect of Excess pore pressure

exit to Pore pressure

No proper exit to Pore pressure

exit to Pore pressure blocked

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Present Practice of assessing the pore water……Piezometers 

• Monitoring dykesreacting quickly to inadequate performance or

to danger signals is a continuing

critical aspect for dyke safety.

• Instrumentationdata will fore tell of any abnormal behavior

quick response to abnormal behavior

can prevent failures in many cases.

i f

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Location of instruments to measure pore water

Location of 

Piezometers

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Present Practice of assessing the pore water……Piezometers 

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Seepage /piping seen on the d/s slope Slippage of slope due to pore pressure

Need to have data of seepage path along the dyke

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Need for State of art

GPR….. 

Based on the forgone discussion there is an urgent needto get pore pressure on the dyke section on continuumbasis for better management of ash disposal

In order to assess the pore water pressure across thedyke on continuum basis can be obtained from GroundPenetrating radar(GPR).

This is a non destructive test which uses electromagneticwaves to image the water table/seepage path in thedyke section.

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Basic Principle……… 

Photographs: Georadar Inc.

Different Dielectric Constant

Hyperbola – Image Formation

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Field Example- Pipes……… 

Buried PipeBuried Pipe

Trench

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Field Operation……… 

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Application of GPR in Ash Dyke

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Conclusions…….  Water logging in and around the dyke - not desirable for safety of the dyke

excess pore pressure leads to slippage of slopes/piping

there is need to detect seepage path along the dyke instead of discretelocations

GPR - Quick assessment of seepage path in non-destructive manner

GPR - Helps in prediction of seepage path in ash dyke indicating the behaviorof dyke.

• Highly effective for R&M Projects.

• Recommended as a tool

for dyke inspection, monitoring and maintenance

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Signs of healthy dyke 

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SUSTAINING GROWTH

WITHOUT LOSS OF GENERATION THRU SAFE ASH DISPOSAL

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• BACK UP SLIDES

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Dielectric Constants of Various

Materials

• Clay 2.8-3.0

• Sand 5.0-5.5

• Air 1.0

• Glass 5.0

• Quartz 5.0

• Slate 7.0

• Mica 9.0

• Water 88

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Depth Determination……… 

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Need for continuous profile of pore pressure

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S TORA GE L A GOON- I S TORA GE L AGOON- I I

SLURRY PIPE LINE

OVERFLOWLAGOON

SPILLWAY

WATERESCAPE

STRUCTURE

 ASHPIPE LINE TO PLANT

WATERRECIRCULATION

N  AT U R  ALW  AT E R B O D Y 

DISCHARGE CHANNEL

TAPPING FOR

RECIRCULATION

* *

*  ASHFILLING SHALLBE CARRIEDOUT

NEARTHE WATERESCAPE INTHE

INITIALPERIODS

XX

DYKE ALIGNMENT

Water Recirculation

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Maintaining Zero Discharge

STORAGE LAGOON-I STORAGE LAGOON-II

SLURRY PIPE LINE

OVERFLOWLAGOON

SPILLWAY

WATERESCAPE

STRUCTURE

 ASHPIPE LINE TO PLANT

WATERRECIRCULATION

N  AT U R  AL W  AT E R B O D Y 

DISCHARGE CHANNEL

TAPPING FOR

RECIRCULATION

* *

*  ASH FILLING SHALL BE CARRIED OUT

NEAR THE WATER ESCAPE IN THE

INITIAL PERIODS

XX

DYKE ALIGNMENT

RAMAGUNDAM

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ASH POND MAINTANANCE 

PRACTICES

RAMAGUNDAM 

ENSURING CLEAR FLOW OF

WATER FROM TOE DRAINS AND

SLOPE DRAINS

ENSURING HEALTHINESS OFDOWNSTREAM AND UPSTREAM

SLOPES OF DYKE

REGULAR MONITORING OF

PIEZOMETER READINGS

MONTHLY ASH DYKE INSPECTION

BY CROSS FUNCTIONAL TEAM

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Typical section of healthy dyke

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MANAGEMENT OF ASH DISPOSALDESIGN,CONSTRUCTION , ROUTINE CHECKS & SAFETY ASPECTS

D.N.NARESHPE-Civil

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ISSUES RAISED IN OPERATING STATIONS

• SEEPAGE FROM BODY OF DYKE

• WATER LOGGING ON d/s SIDE

• SETTLEMENT

• OVER TOPPING/BREACH

• RAT HOLES

• EROSION OF u/s AND d/s SLOPE

• PIPING

• DELAY IN PLACING AWARD/COMPLETION OFCONSTRUCTION LEADING TO OVER LOAD ON THE DYKE

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WATER LOGGING

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T i l ti f h lth d k

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 Typical section of healthy dyke

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BASIC FEATURES

•TO BE DESIGNED for MOST CRITICAL CONDITIONS.

•ENSURE FoS AGAINST THE ULTIMATE LOADS WHICH THE STRUCTURE IS LIKELY TO

FAIL