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है”ह”ह

IS 14954 (2001): Distress and Remedial Measures in Earthand Rockfill Dams - Guidelines [WRD 9: Dams and Spillways]

Is 14954:2001

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Indian Standard

DISTRESS AND REMEDIAL MEASURES IN EARTHAND ROCKFILL DAMS — GUIDELINES

ICS 93.160

@ BIS2001

BUREAU OF INDIAN STANDARDSMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

AZfgu.st2001 Price Group 5

Dams and Reservoirs Sectional Committee, WRD 9

FOREWORD

This Indian Standard was adopted by the Bureau of Indian Standards, afier the draft finalized by the Dams andReservoirs Sectional Committee had been approved by the Water Resources Division Council.

Conservation of water is the most important prerequisite for proper management of water resources and hence,large number of earth and rockfill dams have been constructed in India, most of them afier’independence. Whilethese dams have contributed to the development of the nation, some of them also pose as potential hazard becauseof catastrophic damage which can take place if the reservoir water stored behind the dam is suddenly released dueto any failure or accident. The safety of the dam is thus of paramount importance and any deterioration or disorderobserved in the dam, therefore, warrants immediate suitable strengthening. Strengthening may sometimes be necessaryto restore the stipulated utility of the dam and reservoir. Strengthening may-also be required for upgrading thestability of the dam due to inadequacies in original design, deficiencies in construction, ageing, increase in storage/spillway capacity/seismicity or modification in codal design requirement. However, remedial measures imply restorationof original state and stability of dam after it has experienced some distress like breach, slope failure, settlements,crack formation, piping, subsidence/upheaval etc.

The main object of strengthening the dam through remedial measures is not only to improve its safety alone butalso to restore it fully to meet the designed requirements and estimated benefits.

There is no 1S0 standard on the subject. This standard has been prepared based on indigenous manufacturers’data/practices prevalent in the field in India.

The composition of the committee-responsible for formulating this standard is given in Annex A.

For the purpose of deciding whether a particular requirement of this standard is complied with, the final value,observed or calculated expressing the result of a test or analysis, shall be rounded off in accordance withIS 2:1960 ‘Rules for rounding off numerical values ( revised )’. The number of Significant places retained in therounded off value should be the same as that of the specified value in this standard,

Is 14954:2001

Indian Standard

DISTRESS AND REMEDIAL MEASURES IN EARTHAND ROCKFILL DAMS — GUIDELINES

1 SCOPE

This standard deals with the various causes of distress,detecting methods and implementation of the remedialmeasures for earth and rockfill darns. The distress andremedial measures pertaining to concrete/asphalt facedrockfill dams are not covered in this standard. Theremedial measures for the works relating to increasein spillway capacity and construction of fuse plugor breaching secton are also not covered in this standard.

2 REFERENCES

The following Indian Standards contain provisionswhich through reference in this text, constituteprovisions of this standard. At the time of publication,the editions indicated were valid. All standards aresubject to revision, and parties to agreements basedon this standard are encouraged to investigate thepossibility of applying the most recent editions of thestandards indicated below:

IS No. Title

6066:1994 Pressure grouting of rockfoundations in river valleyprojects — Recommendations( second revision)

7894:1975 Code of practice for stability analysisof earth dams

8826:1978 Guidelines for design 6f large earthand rockfill dams

9296:1979 Guidelines for inspection andmaintenance of dam and appurtenantstructures (first revision )

3 GENERAL

The remedial measures best suited for a dam dependupon many factors. The type of treatment should beselected/adopted on the basis of techno-economicconsiderations to ensure the safety and restorationof embankment within stipulated period. The typesof treatments discussed in the standard are overalland general and should be considered together withspecific instructions that might have been stipulatedduring design, construction or maintenance.

4 CAUSES FOR DISTRESS IN DAMS

4.1 An evaluation of the existing rules of the art andany modification of the criteria used to evaluate the

1

safety may result.in disagreement with standard actuallyfollowed in design and construction of the dam.

4.2 Distress in the dam generally takes place onaccount of the following:

a)

b)

c)

d)

e)

Natural factors like ageimg, earthquake,unprecedented rains and floods, etc;

Design inadequacies and deficiency;

Imperfections and deficiencies duringconstruction, operation and maintenance;

Incorrect estimation of design parameters suchas density, shear parameter, permeabi Iity,etc;and

Non-observance of design requirements withregard to construction of structures onupstream and downstream within specifiedzones.

5 TYPES OF DISTRESS IN DAMS

5.1 Types of distress most frequently abserved inembankment dams are as follows:

a) “Distress due to deteriorations in thefoundations, and

b) Distress due to deteriorations in the dam body.

5.2 Distress Due to Deteriorations in the Foundations

5.2.1 Percolation and Internal Erosion

The main cause of the deficient behaviour is due to(a) inadequacies ofexisting-seepage control system,namely seepage barriers and drainage system; and(b) excessive pore pressure or -degradation in thefoundation.

5.2.2 Loss of Shear Strength

Saturation of the soil may result in high value of pore-pressure and reduction in shear strength of thefoundation which create disorders in and the form ofdownstream slips degradation of foundation materialetc.

5.2.3 Deformation and Subsidence

Differential settlement in the foundation may lead toexcessive seepage, high exit gradient through the strataof the foundation itself or opening/cracks developmentin the cut-off.

Is 14954:2001

5.3 Distress Dueto Deteriorations inthe Dam Body

5.3.1 Percolation and Internal Ero.~ion

The distress is mostly associated with the following:

a) Deficiency in construction with respect tomoisture content control, earth placement andcompaction particularly at junctions of zonesand method dealing with frost effects;

b) Unsatisfactory and sub-standardconstruction of the impervious core and otherwater barrier systems; and

c) Use of undesirable construction materialswithout proper compliance to designparameters and without adequate precautions,in respect of tine clays, organic clay,dispersiveclays, silt and fine uniform sands, soluble soilswith boulders, decomposed rocks, weatheredrocks, soils with unsuitable grain-size-distribution, etc; and

d) Inadequacies of drainage system and filters.

5.3.2 Slope Protection Works

The distress in the slope protection works is generallyattributed to:

a)

b)

c)

d)

e)

f)

material decomposition or weathering;

unforeseen actions or actions of exceptionalmagnitude such as precipitation or waves inreservoir;

differential movements and/or slips of theprotection measures at upstream;

inadequate protective drainage filter layersbelow revetment and improper packing ofrevetments;

splashing due to wave action, in case ofinadequate free board, resulting in thedownstream slope erosion; and

Presence of too small size of stones.

5.3.3 Dl~ferentia[ Movements

The differential movements including uneven loadtransfer, cracking, arching and hydraulic fracture inthe dam body are liable to occur due to unsuitableembankment materials, improper construction methods,poor compaction and, heterogeneity in foundationstrata.

5.3.4 Interface Between Embankments and Concrete/Masonry/Steel Structures/A ~oining Embankments

Improper contact at interface leads to:

a) Preferential path of seepage associated with2

internal erosion; and

b) Differential movements.

5.3.5 Upstream Slips

The upstream slips are mostly associated with thefollowing:

a)

b)

c)

d)

e)

9

Inadequate shear strength of the foundation;

Inadequate strength of the embankmentmaterials;

Poor construction;

Excessive pore pressure;

Sudden drawdown;

Liquefaction.

and

5.3.6 Downstream Slips

The downstream slips are mostly associated with thefollowing:

a) Inadequate shear strength of the foundation;

b) Inadequate strength of the embankmentmaterials;

c) Poor construction;

d) Excessive pore pressure;

e) Percolation and/or internal erosion;

f) Liquefaction; and

g) Saturation due to sustained rainfall.

6 DETECTION METHODS

6.~ Direct Observation and Monitoring

Direct observation is one of the most effective methodsof detecting the following deteriorations in foundationand dam body:

a)

b)

c)

d)

e)

f)

Deformation and land subsidence;

Excessive seepage and internal erosion;

Slope protection;

Differential movements;

Upstream slip?; and

Downstream slips.

Embankment dam should be monitored and examinedfor cracks, leakages, saturated areas or wet spots,springs, sinkholes, evidence of piping, erosion,excessive growth of vegetation, frost, heave, crestalignment, bulging or depression of slopes and berms,animal burrows, and deterioration ofrip-rap or @her

slope protection materials. The major areas needingattention in inspection have been listed in IS 9296which may be referred to for guidance.

6.1.1 Under-Water Observation

Under-water observations with video equipment andremotely operated vehicles ( ROV ) or divers forinspecting inaccessible areas under-water may be usedfor detecting the deterioration in foundation andreservoir side of dam.

6.2 Measurement

Periodic and systematic measurements in respect ofseepage, phreatic level, uplift, pore pressure, turbidity,horizontal displacement, vertical displacement andrainfall may also be used in detecting the deteriorationin foundation and dam body.

6.3 Investigation

Test results on soil samples from dam and foundat~onand chemical and physical analysis of seepage canbe used to investigate the deterioration in respect ofshear strength, seepage and internal erosion of thefoundation and dam body.

7 ‘REMED[AL MEASURES

7. I Strengthening, repairs and remedial measures arenot amenable to standardization and should be carefullyselected depending upon (a) the risk element asinfluenced by the height of the dam, reservoir volumeand potential loss to life and properties, etc, (b) theeconomic value of water stored, (c)nature of foundationstratum, and (d) materials and methods used inconstruction of the dam. Measures generally usedare as follows:

1) Monitoring distress level;

2) Seepage control measures;

3) Construction and/or repair of drains filtersand relief wells;

4) Strengthening by grouting or other methods;

5) Filling of fractures, cavities and sink holes;

6) Construction or repair of slope protection;

7)

8)

9)

10)

11)

12)

13)

Reconstruction of deteriorated zones;

Upstream stabilization methods;

Downstream stabilization methods;

Raising of dam;

‘Increase in spillway capacity;

Construction of fuse plug/breaching section;and

Lowering of reservoir level.

Is 14954:2001

Construction methods which may be used forrehabilitation are given in 7.2 to 7.10.

7.2 Treatment for Control of Excessive Seepage

The main objective of treatment is to restore the watertightness of the dam and foundation. The method oftreatment may differ for foundation, dams with upstreammembrane, dams with impervious core andhomogeneous dams.

7.2.1 Foundation

7.2,1.1 Rock

Vepending upon the nature of rock, grouting maybedone either from the upstream or top of the damensuring the formation of continuous barrier in therock along with core. The grouting may be carried outwith ordinary portland cement or chemicals( see IS 6066 ).

7.2.1.2 Soil

During the service time of reservoir it is usuallyimpractical to provide a positive cut-offor open partialcut-off through out the length of the dam as a remedialmeasure. However, a full cut-off maybe adopted byway of providing any one of the following:

a) R,C.C. diaphragm;

b) Plastic diaphragm;

c) Sheet piles; and

d) Alluvial grouting.

Sometimes clay blanket constructed by suitablyspreading imperviousmaterial upstream under minimumwater condition in the reservoir combined withdownstream relief wells may be the most appropriatesolution.

7.2.2 Dams with Impervious Core and HomogeneousDams

Problems related with seepage through the dam bodyhaving homogeneous section or core may beeffectively tackled using the following measures:

a) The seepage at about FRL and above ismainly as a result of siphon effect over thedam body and may be eliminated by extensionof core up to at least one metre above MWLby open excavation and construction ofcontinuous additional core above the existingone according to specification,

b) Seepage through homogeneous section maybe reduced by providing upstream geo-membrane lining with suitable cover ofprotective layer (see Fig. 1),

3

Is 14954:2001

I!

1.

2.

3.

c)

d)

e)

AnchoringofLDPEFilminTrenches 4. 90 cm ThickEarth CoverExistingSlope Profile 5. 30 cm ThickStone PitchingLDPEFilm350 MicronThickness

FIG. I TYPICALDETAILOF LAYINGOFLDPE FILMONTHEUPSTREAMSLOPE

Construction of a cenlent-bentonite/p lasticor concrete diaphragm wall through oradjacent to the core of the dam,

Cement-bentonite or chemical grouting, and

NO-FE— Proper care is to be exercised to avoidreduction in the etTiciemcy of downstream filter

arrangements due to grouting.

Removing and rebuilding the damaged partsof the dam including core with generouslyprovided filter layers with very precisespecifications in respect of the filter.

7.3 Drains and Filter Construction or Repair

The main aim of the remedial measure is to collect theseepage water inside the dam and through foundationand provide it a safe exit through the drainage system,

7.3.1 Perched Water

To drain the perched water or high phreatic lines atdifferent places leading to local slushiness in thedownstream slope or piping, the following methodsmay be adopted:

a)

b)

Constructing vertical drains in the downstreamslope of the dam section going down andjoining the extended horizontal filters. Thiswill drain the perched water into the existingdownstream drainage system of the damsection (see Fig. 2),

Providing a new filter section with berm at theappropriate level of the downstream sectioncovering the entire area of seepage. Thissect ion wiII serve the purpose of downstreamIoilding and safe exit to the seeping water,

c)

d)

e)

Providing an inclined chimney filter andtransverse filter drain for the seepage indownstream and covering the chimneysection by constructing additionaldownstream section of dam (see Fig. 3),

Providing filter and rock toe at the downstreamtoe and improve drainage, and

Combination of above measures may also be

7.3.2

adopted on account of safety and economicgrounds.

Sand Boils and Slushy Conditions onDownslreom of Datn

The main cause of sand boils and slushy conditionson the downstream of dam is the excess hydrostaticpressure and seepage through deep pervious strataunderlying the dam. The severity of under-seepageboth in respect of excessive hydrostatic pressure andseepage-flow is dependent upon head of water,permeability of substratum and characteristics of theupper strata of downstream portion. This problem maybe tackled by properly designed and constructeddownstream loading berm with suitable invertedfilter, drainage trench, upstream clay blanket with orwithout relief wells or their suitable combination.

7.4 Strengthening by Grouting

Multiple-row-grout-curtain may be used for sealingdeep permeable alluvium in foundation. However, dambody grouting may also be done if need arises. Specialcare has to be taken in working out the allowablegrouting pressure and grout intake to avoid thepossibility of further cracks in the core and rise ofpore water pressure within the dam endangering its

4

Is 14%4 :2001

320

310:

300

1. VerticalDrains 4. ExtendedHorizontalFilter

2. Filter 5. ExistingDownstreamDrainageSystem

3. CleanGravel

FIG.2 TYPICALDETAILSOFVIXUCALDRAINSINTHEI)owr-wrrwmISLOPIZ

I?-@

1.

2.

3.

4.

5.

6.

ExistingProfile 7. HydraulicGradient Line with Water Level ( for

Proposed Profile ReservoirConditionA)

VerticalStand Pipe Piezometer 8. StableSlopeSuitableto SiteConditions

EarihfillCompactedinLayers 9. HydraulicLinewith Water Level ( for Reservoir

Cut LineConditionB)

Cut Line10. 1.5mThickSand Filter

11, Cross FilterDrains

FIG. 3 TYPICALDETAILOF INCLINEDCHIMNEYFILTERINTHEDOWNSTREAMSLOPE

safety. Grouts, usualIyclay-cement-benton ite mixtureswith certain additives and reagents, have to be adjustedto suit varying local conditions based on extensivelaboratory and field tests to optimise the mixproportions with regard to stability, bleeding, fluidity,lubricity and strength, Clay-gels and silica-gels maybe used but resins which are too expensive shouldbe used judiciously depending on the necessity.

7.5 Filling in Fractures and Cavities

The main reasons for fractures and cavities are poor

5

quality materials used for construction and defectiveplacing of embankment or inefficient drainage system.The methods described in 7.5.1 to 7.5.3 are generallyemployed for strengthening.

7.5.1 Removing the zone of cavities by openexcavation and refilling the cavities with the dammaterial by placing, dumping, spreading, moisteningand compacting as per specification adopted for damconstruction.

7.5.2 Filling the cracks with thick slurry consisting

Is 14954:2001

primarily of natural clayey soil. Thoroughlymixed slurryis pumped with low headpump under gravity head.Before pumping any slurry, the cracks are washed withwater under pressure. Slurry filled into the cracks areallowed to dry for several dayslweeks. In case ofappreciable shrinkage of the slurry due to drying, theprocess is repeated. Finally the cracks should be sealedat the surface by trenching filling and recompactingwith appropriate soil to a depth of O.3to 0.6 m.

7.5.3 Sealing of sink holes in the reservoir or in theupstream of dam section by encircling the area by sheetpiling and filling the hole with dense mixture ofappropriate materials such as sand, gravel, pebble,boulder and rock fragments etc and then compactingas per specifications adopted for dam construction.

7,6 Repair of Slope Protection

7.6.1 The following measures are generally adoptedto repair slope protection (see Fig. 4):

3“0””’”1r’”’”

c

A.

B.

c.

a)

b)

c)

d)

e)

Partial repair with the same material andmethods as adopted in the original damconstruction,

Enlarging the upstream protection by dumpingselected rockfill along the entire upstreamslope,

Enlarging the upstream protection zone bydumping selected rockfill over the platformconstructed out of dumped coarse fractjonedrockfill,

Replacement of slope protection, and

Partly enlarging the upstream protection fromdumped coarse fractioned rockfill.

7.7 Reconstruction of Deteriorated Zones

Some times it becomes inevitable to reconstruct thedam section in part or whole length due to severe rain

-1 t--’””” ‘inIll

-——— ——— ——— —

D

I

v— — —__——-—— .

.—— ——. ———E

PartialRepair D. ReplacementofSlope Protection

Enlarging E. PartlyEnlargingfromDumpedCoarse Fractioned

EnlarginefromDumpedCoarse FractionedRockfill RocldN

FIG. 4 DIFFERENTWAYS OFREPAIRINGANDSTRENGTHENINGUPSTREAMSLOPEPROTECTION

cuts, bulges, slope slides, settlement, unwarrantedhuman and animal activities etc. In such cases, a freshdam section bwed on IS 8826 and 1S7894, be evolvedand fitted over the existing dam section in mosteconomical methods.

7.8 Upstream Slope Stabilization Methods

Following methods may be employed for upstreamslope stabilization:

a)

b)

c)

d)

Flattening of the slope and/or reloading onthe upstream toe. This may be done byconstructing large width rockfill berm at lowerlevel. Properly designed flatter slope can thenbe constructed from this lower level berm tothe top of the dam,

Stabilizing the upstream slope by groutingwith sand cement grout,

Restoration of failed slopes using geogrids,and

Providing additional earthfill with horizontalfilters within.

7.9 Downstream Slope Stabilization Methods

The following methods may be employed fordownstream stabilization:

a)

b)

c)

d)

e)

o

g)

Installing drainage system,

Flattening of slope,

Reinforcing of earth,

Stone pitching,

Providing an effective rain-water drainagesystem,

Providing good and effective turfing, and

Employing any other treatment suggestedin 6.3.

Is 14954:2001

7.10 Raising of Dam Crest

Raising of dam crest maybe required on free boardconsideration. For raising a dam crest, strengtheningof the profile by downstream loading may be done.While taking up the upstream strengthening for raisingthe dam crest, necessary precautions should be takenin respect of maintaining the reservoir outlet, removalof embankment protection, possible presence ofsediments etc. In case of a zoned dam, the prolongationand strengthening of the core should be given dueconsideration.

8 PLANNING, DESIGN AND EXECUTION OFREMEDIAL WORKS

8.1 Design Procedure

For any.enlargement, alteration, rehabilitation, repairor abandonment of existing structures or facilities,the original design documents and all availableconstruct ion and operation records should be carefullystudied.

If the rehabilitation would require substantial structuralmodification or ifbaiic assumptions and environmentalconditions which form the basis of the original designhave considerably changed, the whole structure shouldundergo a new stability analysis.

8.2 Principles and Criteria of Design

Designing of any part or whole of any component ofthe embankment dam should be done as per currentrelevant Indian Standards. Before starting detaileddesign work, the actual condition of those parts orcomponents of the structures to be enlarged, modifiedor repaired should be carefully surveyed a~ddocumented.

8.3 A close watch should also be kept to study theeffectiveness of the treatment. All necessary data andprocedures adopt’ed should be properly documented.Regular inspection of the distressed area, dependingupon its importance, should be carried out.

“.

Is 14954:2001

ANNEX A

( J’oreword)

COMMI’ITEE COMPOSITION

Dams and Reservoirs Sectional Committee, WRD 9

Chairman

DR B. K. MITTAL

Members

CHIW EN(iINI:BJt( BHAKRA~AM)

DIRIXTOJ?( D~SI~N ) B&B DENGNDIWCTOkATE( Alternate)

SHRI S. P. KAUSHISH

SHRI T. S. MURTHY( Alternate)

DIRECTOR

SHRIA. K. DHAVAN( Alternate )

SHUI R. M. KHATSURIA

SHIU P. B. DIiOLALIKAk( Alternate)

DIRECT(JR[ CMDD-NW&S ]

DIRLCTORRiIS~RVOiR( Alternate )

SHRI M. K. NARASIMHAI.YA

SHRI S. S. NARANG~ Alternate )

SHJUG. K. KAISTHA

SHWR. N. SINCiH( AJternate )

CHIEF ENWN~~R ( M~DIUM & MINOR ) & ADDITIONALS~CRI:TA~Y

SUIWRINTENIXNGENGINEER( CDO ) ( Alternate )

HEAD OF THE CIVIL ENGINEERINGDEPARTMENT

CHIW ENCNNiZ~R[ 1&CAD ]SUPJHiINT~N~INCiENGINEER( DAMS ) ( Alternate )

SHJUA. DASCiUPTASHRI H. P. CHAK~ABARTI( Alternate )

CHIW ENGINWR ( DAM DIZWN )

SUIWINTENOINGENCiINHiRDAM DESIGNCIRCLE 1( Alternate)

CHIEFENGINEER( RSDD )

DIRECTORDAMS ( RSDD ) ( Alternate)

SUIILI~INTI>NKIINGENGIN~JiR( MD )EXJ+CUTIVEENCHNEJR( MD-4) ( Alternate)

CHIW ENCiINK~R( PRCJJECTS)DIR~CTO~( ENci~ ) ( Alternate )

SHW A. K. RISHI

DIRI:CTOR( DAMS ) ( Alternate)

SHRI D. G. KADKADE

SHRI NAR~NLJRASINGH( Alternate )

SHRI P. R. MALTI KARJUNA

SHRI S. M. CHHJBJ( Alternate )

Representing

Central Water Commission, New Delhi

Bhakra Beas Management Board, Nangal

Central Board of Irrigation and Power, New Delhi

Central Soil and Material Research Station, New Delhi

Central Water and Power Research Station, Pune

Central Water Commission, New Delhi

Consulting Engineering Services ( I ) Ltd, ‘New Delhi

Geological Survey of India, Shillong

Narmada and Water Resources Department, Government of Gujarat,Gandhinagar

Indian Institute of Technology,NewDelhi

Irrigation Department,Governmentof Andhra Pradesh

Irrigation wtd Water Ways Dkectorate, Government of West Bengal,Kolkata

Irrigation Department, Government of Uttar Pradesh, Roorkee

Irrigation Department, Government of Punjab, Chandigarh

Irrigation Department, Government of Maharashtra, Nasik

Irrigation Department, Government of Harytma, Chimdigarh

Water Resources Department, Goverment of Madhya Pradesh,BhopaI

Jaiprakash Industries Ltd, New Delhi

Kartmtaka Power Corporation Limited, Bangalore

( Continued on page 9 )

8

Is 14954:2001

( Continued from page 8 )

Members

SHRI GEORGECHERIYAN

SHRI M. S. BJSARIA

SHRI R. D. VARANGAONKAR( AIterna(e )

StiRI K.S. NAGARAJA

SHRI UTPALBORA

DR S. M. SETH

DR P. K. MHAPAT~( Alternate )

ENGINEERIN CHIEF

CHIEFENGINEER( Alternate)

SHRI L. K. BANSAL

SHRI S. S. SETHI

Director & Head ( WRD )

Representing

Kerala State Electricity Board, Thiruvananthapuram

Gammon India Ltd, Mumbai

National Hydroelectric Power Corporation Ltd, Faridabad

North Eastern Electric Power Corporation Ltd, New Delhi

National Institute of Hydrology, Roorkee

Public Works Department, Government of Tamil Nadu, Chen nai

Tehri Hydro Development Corporation, Noida

Director, General, BIS ( Ex-officio Member)

Member-Secretary

SHRIB. K. SINHA

Joint Director ( Water Resources Department), BIS

9

Bureau of Indian Standards

BIS is a statutory institution estabhshedunderthe Bureau of Indian Standards Act, 1986 to promoteharmonious development of the activities of standardization, marking and quality certification of goods andattending to connected matters in tie country.

Copyright

BIS has the copyright of all its publications. No part of these publications maybe reproduces in any form withoutthe prior permission in writing of BIS. This does not preclude the free use, in the course of implementing thestandard, of necessary details, such as symbols and sizes, type or grade designations. Enquiries relating tocopyright be addressed to the Director (Publications), BIS.

Review of Indian Standards

Amend,rnents are issued to standards as the need arises onthebasis of comments. Standards are also reviewedperiodically: astandard~along with amendments is reafilrmed when such review indicatetihat no changes areneeded; if the review indicates that changes are needed, it is taken up for revision. Users of Indian Standardsshould ascertain that they are in possession of the latest amendments or edition-by referring to the latest issueof ‘BIS Catalogue’ and ‘Standards: Monthly Additions’.

This Indian Standard has been developed from Doc : No. WRD 9 ( 125 ).

Amendments Issued Since Publication

Amend No. Date of Issue Text Affected

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