इंटरनेट मानक Disclosure to Promote the Right To Information Whereas the Parliament of India has set out to provide a practical regime of right to information for citizens to secure access to information under the control of public authorities, in order to promote transparency and accountability in the working of every public authority, and whereas the attached publication of the Bureau of Indian Standards is of particular interest to the public, particularly disadvantaged communities and those engaged in the pursuit of education and knowledge, the attached public safety standard is made available to promote the timely dissemination of this information in an accurate manner to the public. “जान1 का अ+धकार, जी1 का अ+धकार” “प0रा1 को छोड न’ 5 तरफ” Mazdoor Kisan Shakti Sangathan Jawaharlal Nehru “The Right to Information, The Right to Live” “Step Out From the Old to the New” IS 6955 (2008): Subsurface exploration for earth and rockfill dams - Code of practice [WRD 5: Gelogical Investigation and Subsurface Exploration] “!ान $ एक न’ भारत का +नम-ण” Satyanarayan Gangaram Pitroda ““IInnvveenntt aa NNeeww IInnddiiaa UUssiinngg KKnnoowwlleeddggee”” “!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता हहहहै””ै” Bhartṛhari—Nītiśatakam “Knowledge is such a treasure which cannot be stolen” IS 6955 :2008 ~ 3TR i1<cR ~ ~ et ~ ~Cf)T~-~~ y;rtterur ) ( Qt5C11 Indian Standard SUBSURFACE EXPLORATION FOR EARTH AND ROCKFILL DAMS - CODE OF PRACTICE ( First Revision ) ICS9:;.()~() t BIS ~008 B UREA u OF INDIA!' STA l'lDARDS \lA~AK BHAV."'. 9 BMIADLR SHAll ZAFAR MARG )\EWDELIII II()(J02 December ~008 Price Group6 C;eologicallnvestigationsand Subsurface Exploration SectionalCommittee, WRD 05 FOREWORD This Indian Standard (First Revision) was adopted by the Bureau of Indian Standards, after the draft finalized by the Geological Investigations and Subsurface Exploration Sectional Committee had been approved bythe Water Resources Division Council. Thisstandard was first published in 1973. The present revision isproposed to retlect the experiencegained on the subject since then. Earthand rocktill dams have been constructedsince early ages. The dams built inolden days, were generally of lowtomedium heights. With increasingheights ofdams and fasterrates ofconstruction, there isagreater need for proper investigations and design based on the latest developments in the fields ofsoil and rock mechanics. Animportantrequisite forproperdesign isadequate investigation. Subsurfaceexplorations form an important part ofthese investigations. It hasbeenassumed informulating this standard that the executionofits provisions isentrusted to appropri atelyqualified and experienced people, forwhose guidance ithas been formulated. Forthepurpose ofdecidingwhether aparticularrequirementofthis standard iscompliedwith, the final value, observed or calculated, expressing the result ofa test, shall be rounded offin accordance with IS 2 : 1960 .Rulesforrounding offnumerical values (revised)'. Thenumberofsignificantplacesretained inthe rounded off value should be the same as that ofthe specified value in this standard. IS6955 :2008 Indian Standard SUBSURFACE EXPLORATION FOR EARTH AND ROCKFILL DAMS - CODE OF PRACTICE ( First Revision) I SCOPE 3.2 Subsurface explorations in connection with an earth and/orrockfill dam would coveraspecifiedarea 1.1 Thisstandardgivesguidanceon the type, extent around thedam site and willbecarriedtoaspecified and details of subsurface explorations needed in depth. Acompleteprogrammeofexplorationshould connection with earth and rocklill dams. It is not be able to give information regarding the following possible to lay down the required extent of points: explorationtocoverall types ofcases.The standard provides guidelines for planning the exploratory a) Types ofdifferent soil and rock massesthat work through various stages of the project exist in the foundation and abutments. development. These recommendations may have to b) The location, sequence,thicknessand areal be modified for individual projectsdepending upon extentofeachsoil/rockstratum, includinga the site conditions and other conditions peculiar to descriptionand classificationofthesoil and each project, such as, height, importanceofthe dam their structure, stratification in the and the heterogeneity offoundation formations. undisturbed state, significant geological or 1.2 Theterm subsurfaceexploration.asused herein, other structural features, such as. buried covers all types of exploration connected with channels, seams, joints, fissures, and determinationofthe natureand extentofsoil and/or mineral and chemical constituents. rock belowthe naturalground surfaceat/or nearthe c) The depth to and typeofbedrockas well as dam site. the location, sequence, thickness, areal 1.3 This Code does not however cover the types extent, attitude, depth of weathering, and methods of exploration for materials of soundness, •description and classification construction for earth and rockfill dams, such as, ofrack ineachrockstratumwithin thedepth soil, rock and material for riprap protection. These ofexploration. will be covered by a separate Code on subsurface d) The characteristics of the ground water, explorationforconstructionmaterials. includingwhetherthe watertableisperched 2 REFERENCES ornormal,directionofflowofgroundwater, depthofand pressure inartesianzones,and The standards listed inAnnex Acontain provisions quantity of dissolved salts present in the which, through reference in this text, constitute ground water. provisions of this standard. At the time of publication, the editions indicated were valid. All e) Engineeringand index propertiesoftheover standards are subject to revision and parties burdenand physicalcharacteristicsofrocks. to agreements based on these standards are t) Seismo-tectonicset upofthe projectregion. encouragedto investigatethe possibilityofapplying the most recent editions ofthe standards indicated 4 STAGESOFEXPLORATION inAnnexA. 4.1 The extentoffoundationexplorationrequired for 3 GENERALCONDITIONS a dam ofgiven size varies greatly from site to site depending on the subsurface conditions and cannot 3.1 The type and extent ofexploration should be beadequatelyvisualizedinadvance. Theexploration commensurate with the size and importance ofthe generally proceeds in stages, the details of each projectand willdepend upon the sizeofthedam and stage growing out of the one before. It normally the type offoundation. Theseshould neitherbe too follows a learn-as-you-go procedure in which little, resulting inadequate data, nor too much, characteristicsofthesubsurfacesoilsand conditions resulting inexcessivecostand timefor investigation are developed in progressivelygreaterdetail as the (seeIS 15662). IS6955 :2008 explorationproceeds. 4.2.2.2 Choice ofmethods 4.2 Explorations can be generally sub-divided into Fordams upto 30 mheight exploration bytrial pits, four stages as in4.2.1 to 4.2.4. trenches and drill holes should be sufficient. For dams upto and aboveI00 m height and above, 4.2.1 Reconnaissance or Pre-feasibility Stage additional exploration by drifts and shafts may be This should comprise of selection of suitable required depending upon the geological complexity alternative sites on the basis of regional and local ofthe site. geology, topographic expression and anticipated 4.2.2.3 Spacing oftest pits/drill holes depth to bed rock and impermeable strata. This will consist of photo-geological interpretation and a Fordams lessthan30 minheight,exploration bypits general field inspection by qualified engineering at aspacingof250 mto 300 m depending upon the geologists and engineers for an assessment ofthe nature ofthe foundation material may be necessary. overall aspects of the geology of the site and Fordams over 30 m inheight, the spacing between foundation conditions. On the basis ofinformation drill holes may be decided so as to have minimum gathered at this stage, an evaluation is made about 5 numbers of drill holes with particular attention the depth and characteristics of foundation strata, being given for adequate coverage to deeper which would serve as a basis for initial planning of sections. Inbetweendrill hole locations,trial pits or theprogrammesoffieldwork, which willbroaden or auger holes would besufficient(see IS4453). add tothe existingknowledge ofthesite conditions 4.2.2.4 Location and the methods and scope of investigations and testing. Exploratory holes, pits and auger holes may be locatedalongtheaxis ofthe dam fordam upto 100m 4.2.2 Preliminary Investigation or Feasibility Stage height. For dams greater than 100 m, however, additional lineofholesmay be necessarydepending 4.2.2.1 Objectives and methods ofexploration ongeologicalconditions(see IS4453). During this stage, necessarydata for formulation of 4.2.2.5 Depth ofexploration the project would be collected. The coverage of exploration should be adequate for examination of Ingeneral,thedepth uptowhich explorationsshould thefeasibility, which includesestimationofthe cost be made depends upon the following factors: andevaluationofthe benefits.This stage wouldalso a) Depth ofoverburdenand depth upto which include studies for preliminary choice of the weathering of bed rock has progressed. alignment aswellastheheight ofthedam.Thisstage Exploration should be carriedto a depth to of exploration includes the following types and locate all weak and compressible or methods ofinvestigations: otherwise undesirable layers in the a) Explorationbytest pits,trenches,drifts and foundation, such as buried channels. shafts; b) At the preliminary investigation stage, the b) Exploration bygeo-physical methods; depth would be generally guided by the permeabilitycharacteristics ofthe strata. It c) Exploration by drilling using coring and maybesufficientto explore uptoadepthof non-coring methods or by other boring I I methods; 3" or 2"ofthe hydraulic headat the location d) Determination ofthe depth to water table ofthedam ifrockisfoundatshallowdepths and evaluation of field permeability; I I oflessthan 3"to 2"ofthe hydraulic head. If observation of temperature, pressure and dischargeofspringsmetatthesurfaceor in I I exploratory borings, trenches, etc; the depth to rock is larger than - or - the 3 2 hydraulic head, one or two drill holes may e) Field penetration and field density tests in betaken down to 10minto the in-siturock. overburden; and 4.2.3 Detailedinvestigation or DPR stage t) Laboratory tests on representative samples and undisturbed samples for th e 4.2.3.1 In this stage of investigation, all data determination of engineering and index required for detailed design and preparation of properties of the overburden material! constructiondrawingsshouldbecollected.Closeco bedrock. ordination is essential between the work of the 2 IS6935 :2008 organizations.for exploration. geology and design. particularattentionbeinggivenfor adequate The design engineer and the geologist should be coverage to deeper sections. Half the closely associated with the exploration and they number ofholes should be taken to depths should-berequiredto preparean outlineotthescope equal to the hydraulic head and the and extentofexploration.Whilethe details wouldbe remainingto halfthe hydraulic head or to a left to those incharge ofexploration, the designer depthto proveacontinuous impervioussoil and the geologist should participate inthe choiceof or rock or such strata that can be rendered the method of investigation and the equipment. so impervious by treatment. The depths to that they can appreciate the limitations ofthe data which exploration should be continued in obtainedthrough field work. the impervious mediumor medium that can be rendered impervious bytreatment should 4.2.3.2 Investigations at this stage would comprise be decided on the basis of design the following: considerations. a) Intensiveexploration byadditionaldrilling c) For damsabove 100 m inheight.three lines and pitting (trenches. adits and shafts. of holes should be drilled at locations as where found necessary) ofthe foundation! per design considerations. The depth of abutment to determine spatial distribution theseholesshouldbe equal to the hydraulic and characteristics of different types of head. In addition. trenches to explore the foundation materials in relation to specific foundation sequence inthe riverbed section design features; and for collection of undisturbed samples b) Use of geophysical methods to define in may berequired. greater detail the subsurface conditions. For adam more than 100 m height. three or such as the depth to bed rock or depth to (Pore drifts on either bank with cross cuts water table in specific sections ofthe dam arc recommended. However. the length of base. During this stage. usc of bore hole the main driftand the crosscutswill depend geophysical methods. such as electric upon the dimension ofthe structure (dam logging.GPR. tomography.etc (as and when section across and along the river flow) at required) may be found advantageous to that particular elevation. meaning thereby define particular characteristics of over that the length of the main drift and cross burden and bed rock: cutswill be more nearthe baseand reducing c) Definingofgeohydrologicalcharacteristics gradually towards the top ofthe dam. The of the foundations and its environmerus length ofthe drift should be decided by the through pumping in or pumping out tests engineering geologist as per design as dictated by site conditions; requirement. which in tum depend on the height of the dam and quality of the d) Ascertainingthe groutabilityoffoundations rockmass. These drifts can be utilized for through trial groutingofspecified reaches: various purposes, namely, inspection, drainage and grouting at a later stage. The e) Specialfield tests like blastingtests and field groutability of foundation through trial shear tests, where found necessary: and grouting of the specific section. in a set f) Seismo-tectonicset upofthe projectregion. pattern, should also be tested during this stage. 4.2.3.3 Depth andspacing ofholes 4.2.4 Construction Stage a) For dams less than 30 m in height. one additional lineofholes(inadditionto those During this stage. large scale geological indicated in 4.2.1) as per design investigations are carried out to delineate all considerationsmaybenecessary. The holes geological and structural features exposed on the should be suitably staggered to provide foundation ofthe dam and itsappurtenantstructures informationat 30 m intervals. Thedepth of andtoapprisethe constructionanddesign engineers 1/3 ofthese holes may be kept equal to the regarding rock mass structure and any special geo hydraulic headofthe dam. structural features. like. fault zones,defonnedJweak zones.brecciazones,shear.etc.whichrequiredental b) For dams 30 m to 100 m in height. two or any other suitable treatment, like. grouting for additional rowsofholes wouldberequired. strengtheningrockmassandmakingthemwatertight. The spacing between drill holes may be The shear zones. deformed zones or weak zones 1/IOth of the length of the portion with shouldbemarkedwith colourpainton the foundation 3 IS6955:1008 a) Seismic refraction,and surface. The final foundation grade geological mapping should be done on I:I00 scale w.ith I.m b) Electricalresistivity. contour interval ingridpattern(2mx2m)usinghigh 6.4.1 Seismic Refraction Method precision survey equipment like total station. The foundation surface should be properly cleaned by Earthvibrationsset upartificiallybyexplosionsform airorwaterjetbeforetakingupgeologicalmapping, the basis ofthis method. The earth waves travel in which guides the foundation preparation and all directions through the ground and are refracted treatment. or reflected back to the surface by lower rock formation throughwhichtheytravelwithadifferent 5 METHODS OF EXPLORATION velocity than through the overburden. The time of 5.1 The following categories of methods may be arrival ofthese wavesat any pointon the surfaceof usedforsubsurfaceexplorationforearth androckfill thegroundisrecordedbyaspecialseismographand dams: thetimeoftravelfrom theexplosionpointtothepick up point is thus determined. This information a) Geophysical, enablesdeductionsto be drawnregardingnatureand b) Pits and trenches, depth ofunderlyingformations. c) Borings(augerboringandcore drilling), d) Shafts and drifts, and 6.4.1.1 This method may be used to determine the e) In-situ tests. depth to bedrock,the dip in specialcasesand other data regardingtheunderlyingrockformationswhich 5.1.i Geophysical explorations enable gaining of are useful for designing foundations for dams, such knowledge of properties of subsurface strata by as dynamic modulus from shear wave velocity and inference from measured rates oftransmission of vibration characteristics of the foundation. These electriccurrentorseismic waves. In-situtestsenable studies may be required in special cases of weak directmeasurementofpropertiesintheground. The rocks andhighdams (see IS 15681). rest are means ofvisual examination, collection of samples and for performance of laboratory tests 6.4.2 Electrical Resistivity Method thereon. Thisgenerallyusesfour electrodesatequaldistances 6 GEOPHYSICAL OBSERVATIONS along a straight line. An electric current is passed 6.' Geophysical methods can, under appropriate between the outer two electrodes and is precisely conditions,be used toobtaininrelativelyveryshort measured byamilliammeter.Thepotentialdifference time, informationregardingthenature ofthevarious between the innertwo electrodes is measured using strata and their position and depths of change. null point typeofcircuit.Fromthedataobtained,the However,since itisnotadirectmeasurement,borings electricalresistivityiscalculated. have to be made for correlation inorderto interpret 6.4.2.1 The valueofapparent resistivitychangesat correctly the geophysical data. each change of strata and since, in general, the 6.2 The geophysical methods are not adequate in distancebetweenthe electrodes isequal to depth of themselves as tools for subsurface investigation. layer beingmeasured, it is by this method possible Whereas they permit a fast coverage ofthe entire tomeasurespecificresistanceto differentdepthsby area at lowcost.and the processisnot hampered by varyingtheelectrodespacing.With someknowledge presenceofboulders,etc, which generally produce of the local geology and ofthe typical values for obstruction in boring, the correct interpretation of different strata, it is possible to determine the geophysicalobservationsisdifficult,particularly in thicknessand depth ofthe different strataby use of areas ofirregularformationsand irregulardepths of one or more methods of interpretation, namely strata and steep topography. It is, therefore, mathematical analysis, empirical methods, inter imperativethat interpretationfromgeophysicalwork correlation with curves and correlation with model be confirmedby borings. experiments.Asregardsdams,thismethodcan be employed for locating bedrockand watertable (see 6.3 Geophysical investigations should always be IS15736). carried out with proper equipment by properly trained and experienced investigators, because 7 EXPLORATIONBYPITS accuracy inobservations and interpretation ofdata isveryessential forarriving at reliable conclusions. 7;. Deep trilll pits (see IS 4453) may be used to investigateopen fissures,ortoexplore zonesofweak 6.4 Theprincipal ormainmethods applicableincase rocks, which would break up in the core barrel and ofexplorationsfordams are: are incapable ofbeing recovered in tact. In case of 4 IS6955 :2008 dams, open pits are useful forinvestigatingthenature SINo. ForSoils For Rocks ofoverburden in foundation area. (I) (2) (3) 7.2 At the surface the excavated material shall be c) Shortdrilling placed in an orderly manner around the pit and (seeISJ892) markedstakesshallbedrivento indicatedepthofpit iii) Wash boring from which the material came, in orderto facilitate (seeIS 1892) loggingand sampling. iv) Rotarydrilling (seeIS1892) 7.3 The levelofthewatertable andthe level, location and rate ofseepage flow in the test pit should be' 9.3 Auger Boring date-wiserecorded. 9.3.1 Post-HoleAuger 8 EXPLORATIONBYTRENCHES Hand-operatedpost-holeaugers, 100mmto300 mm 8.1 Explorationbytrenches(see IS4453)isuseful in indiameter,can beused for exploration uptoabout6 providing acontinuousexposureofthegroundalong m.However, with theaidofthetripod,holes upto25 a given line or sections. They are best suited for m depth can be excavated. Depth of auger shallow explorations (3 m to 4.5 m) on moderately investigationsare limitedbygroundwatertableand steepslopes,forexample,abutmentofdams. by the amountand maximumsize ofgravel.cobbles and boulders.ascomparedtosize ofequipmentused. 8.2 The profile exposed by these trenches may represent the entire depth ofsignificant strata inan 9.3.1.1 Mechanically operated augers are also abutmentofadam. However, theirshallowdepth may availableand are particularlysuitable. where a large limit explorations to the upper weathered zone of numberofholesare to be made,or ingravellysoils. foundations. Machine driven augers are ofthree types and are given below: 8.2.1 Trenchingpermitsvisualinspectionofthesoil strata, which facilitates logging ofthe profile and a) Helical augers 75mmto400 mmindia selection ofsamples. It also aids in obtaining large b) Disc augers Up to I050 mm india undisturbedsamplesfor testing. Trenches insloping c) Bucketaugers Upto I200 mm india ground have the advantage ofbeing self-draining. 9.3.1.2 Anaugerboringismade byturningtheauger 8.3 The level of the water table and the level. to the desired distance into the soil, withdrawing it locationand the rate ofseepage, ifmet with,should and removingthe soil for examinationandsampling. bedate-wiserecorded. The auger is inserted in the hole again and the processisrepeated. Holesare usuallyboredwithout 8.4 The length and spacing will be determined on additionofwaterinloose,moderatelycohesivemoist the basisofheightand lengthofdam and geological soil. But inhard dry soilsor cohesionless sands,the complexities. introductionofasmallamountofwaterintothe hole 9 EXPLORATIONBYBORINGS will facilitatethedrillingand sampleextraction. 9.1 Borings provide the simplest method of 9.3.2 Shell andAugerBorings subsurface investigation and sampling. They may Pipe casing or shell is required in unstable soil in be used to indicate the subsurface stratum and to which the bore hole collapses, and especiallywhere collectsamplesfrom eachofthestrata. the boringisextendedbelowthe groundwaterlevel. The insidediameterofthe casingshould beslightly 9.2 Borings may be made by several methods larger than the diameterofthe auger used. Borings depending upon the nature of subsoil strata, as up to 200 mm diameter and 5 m depth can be done detailedbelow: with manual operation.Powerwinch isrequired for SINo. ForSoils For Rocks deeper borings.The casing isdriven to a depth not (I) (2) (3) greaterthan the top ofthe next sampleand iscleaned out by meansofthe auger. i) Post holeauger Percussion boring (seeIS1892) 9.4 CoreDrilling ii) Shell and Rotarydrilling auger boring (seeIS1892) 9.4.1 Coredrillingshould bedone inaccordancewith (seeIS 1892) IS6926(seealso IS4078 and IS 4464). a) Mud-rotarydrilling 9.4.2 Theaccuracyand dependabilityoftherecords b) Coredrilling (seeIS 6926) 5