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है”ह”ह

IS 6955 (2008): Subsurface exploration for earth androckfill dams - Code of practice [WRD 5: GelogicalInvestigation and Subsurface Exploration]

IS 6955 : 2008

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Indian Standard

SUBSURFACE EXPLORATION FOR EARTH ANDROCKFILL DAMS - CODE OF PRACTICE

( First Revision )

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C;eologicallnvestigations and Subsurface Exploration Sectional Committee, 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 SectionalCommittee had been approved by the Water Resources Division Council.

This standard was first published in 1973. The present revision is proposed to retlect the experience gainedon the subject since then.

Earth and rocktill dams have been constructed since early ages. The dams built in olden days, were generallyof low to medium heights. With increasing heights of dams and faster rates of construction, there is a greaterneed for proper investigations and design based on the latest developments in the fields of soil and rockmechanics. An important requisite for proper design is adequate investigation. Subsurface explorations forman important part of these investigations.

It has been assumed in formulating this standard that the execution of its provisions is entrusted to appropri­ately qualified and experienced people, for whose guidance it has been formulated.

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, shall be rounded off in accordance with IS 2 : 1960. Rules for rounding off numerical values (revised)'. The number of significant places retained in the roundedoff value should be the same as that of the specified value in this standard.

IS 6955 : 2008

Indian Standard

SUBSURFACE EXPLORATION FOR EARTH ANDROCKFILL DAMS - CODE OF PRACTICE

( First Revision)

I SCOPE

1.1 This standard gives guidance on the type, extentand details of subsurface explorations needed inconnection with earth and rocklill dams. It is notpossible to lay down the required extent ofexploration to cover all types of cases . The standardprovides guidelines for planning the exploratorywork through various stages of the projectdevelopment. These recommendations may have tobe modified for individual projects depending uponthe site conditions and other conditions peculiar toeach project, such as, height, importance of the damand the heterogeneity of foundation formations.

1.2 The term subsurface exploration. as used herein ,covers all types of exploration connected withdetermination of the nature and extent of soil and /orrock below the natural ground surface at/or near thedam site.

1.3 This Code does not however cover the typesand methods of exploration for materials ofconstruction for earth and rockfill dams, such as,soil, rock and material for riprap protection. Thesewill be covered by a separate Code on subsurfaceexploration for construction materials.

2 REFERENCES

The standards listed in Annex A contain provisionswhich, through reference in this text, constituteprovisions of this standard. At the time ofpublication, the editions indicated were valid. Allstandards are subject to revision and partiesto agreements based on these standards areencouraged to investigate the possibility ofapplyingthe most recent editions of the standards indicatedinAnnexA.

3 GENERAL CONDITIONS

3.1 The type and extent of exploration should becommensurate with the size and importance of theproject and will depend upon the size of the dam andthe type offoundation. These should neither be toolittle, resulting inadequate data, nor too much,resulting in excessive cost and time for investigation(see IS 15662).

3.2 Subsurface explorations in connection with anearth and /or rockfill dam would cover a specified areaaround the dam site and will be carried to a specifieddepth. A complete programme ofexploration shouldbe able to give information regarding the followingpoints:

a) Types of different soil and rock masses thatexist in the foundation and abutments.

b) The location, sequence, thickness and arealextent of each soil /rock stratum, including adescription and classification of the soil andtheir structure, stratification in theundisturbed state, significant geological orother structural features, such as . buriedchannels, seams, joints, fissures, andmineral and chemical constituents.

c) The depth to and type of bedrock as well asthe location, sequence, thickness, arealextent, attitude , depth of weathering,soundness, • description and classificationof rack in each rock stratum within the depthofexploration.

d) The characteristics of the ground water,including whether the water table is perchedor normal, direction offlow ofground water,depth of and pressure in artesian zones, andquantity of dissolved salts present in theground water.

e) Engineering and index properties of the overburden and physical characteristics of rocks.

t) Seismo-tectonic set up of the project region.

4 STAGESOFEXPLORATION

4.1 The extent of foundation exploration required fora dam of given size varies greatly from site to sitedepending on the subsurface conditions and cannotbe adequately visualized in advance. The explorationgenerally proceeds in stages, the details of eachstage growing out of the one before. It normallyfollows a learn-as-you-go procedure in wh ichcharacteristics of the subsurface soils and conditionsare developed in progressively greater detail as the

IS 6955 : 2008

exploration proceeds.

4.2 Explorations can be generally sub-divided intofour stages as in 4.2.1 to 4.2.4.

4.2.1 Reconnaissance or Pre-feasibility Stage

This should comprise of selection of suitablealternative sites on the basis of regional and localgeology, topographic expression and anticipateddepth to bed rock and impermeable strata. This willconsist of photo-geological interpretation and ageneral field inspection by qualified engineeringgeologists and engineers for an assessment of theoverall aspects of the geology of the site andfoundation conditions. On the basis of informationgathered at this stage, an evaluation is made aboutthe depth and characteristics of foundation strata,which would serve as a basis for initial planning ofthe programmes offield work, which will broaden oradd to the existing knowledge of the site conditionsand the methods and scope of investigations andtesting.

4.2.2 Preliminary Investigation or FeasibilityStage

4.2.2.1 Objectives and methods ofexploration

During this stage, necessary data for formulation ofthe project would be collected. The coverage ofexploration should be adequate for examination ofthe feasibility, which includes estimation of the costand evaluation of the benefits. This stage would alsoinclude studies for preliminary choice of thealignment as well as the height of the dam. This stageof exploration includes the following types andmethods of investigations:

a) Exploration by test pits, trenches, drifts andshafts;

b) Exploration by geo-physical methods;

c) Exploration by drilling using coring andnon-coring methods or by other boringmethods;

d) Determination of the depth to water tableand evaluation of field permeability;observation of temperature, pressure anddischarge of springs met at the surface or inexploratory borings, trenches, etc;

e) Field penetration and field density tests inoverburden; and

t) Laboratory tests on representative samplesand undisturbed samples for thedetermination of engineering and indexproperties of the overburden material!bedrock.

2

4.2.2.2 Choice ofmethods

For dams upto 30 m height exploration by trial pits,trenches and drill holes should be sufficient. Fordams upto and above I 00 m height and above,additional exploration by drifts and shafts may berequired depending upon the geological complexityof the site.

4.2.2.3 Spacing of test pits/drill holes

For dams less than 30 m in height,exploration by pitsat a spacing of250 m to 300 m depending upon thenature of the foundation material may be necessary.For dams over 30 m in height, the spacing betweendrill holes may be decided so as to have minimum5 numbers of drill holes with particular attentionbeing given for adequate coverage to deepersections. In between drill hole locations, trial pits orauger holes would be sufficient (see IS 4453).

4.2.2.4 Location

Exploratory holes, pits and auger holes may belocated along the axis ofthe dam for dam up to 100 mheight. For dams greater than 100 m, however,additional line of holes may be necessary dependingon geological conditions (see IS 4453).

4.2.2.5 Depth of exploration

In general, the depth up to which explorations shouldbe made depends upon the following factors:

a) Depth ofoverburden and depth up to whichweathering of bed rock has progressed.Exploration should be carried to a depth tolocate all weak and compressible orotherwise undesirable layers in thefoundation, such as buried channels.

b) At the preliminary investigation stage, thedepth would be generally guided by thepermeability characteristics of the strata. Itmay be sufficient to explore up to a depth ofI I

3" or 2" ofthe hydraulic head at the location

of the dam ifrock is found at shallow depths

I Iofless than 3" to 2" of the hydraulic head. If

I Ithe depth to rock is larger than - or - the

3 2hydraulic head, one or two drill holes maybe taken down to 10m into the in-situ rock.

4.2.3 Detailed investigation or DPR stage

4.2.3.1 In this stage of investigation, all datarequired for detailed design and preparation ofconstruction drawings should be collected. Close co­ordination is essential between the work of the

organizations. for exploration. geology and design .The design engineer and the geologist sho uld beclosely associated with the explora tion and theyshould-be required to prepare an outline otthe scopeand extent of exploration. While the details would beleft to those incharge of exploration, the designerand the geologist should participate in the choice ofthe method of investigation and the equipment. sothat they can appreciate the limitations of the dataobtained through field work .

4.2.3.2 Investigations at this stage would comprisethe following :

a) Inten sive exploration by addition a l drillingand pitting (trenches. adits and sha ft s.where found necessary) of the foundation!abutment to determine spatial distributionand characteristics of different types offoundation materials in relation to specificdesign features;

b) Use of geophysical methods to define ingreater detail the subsurface conditions.such as the depth to bed rock or depth towater table in specific sections of the dambase. During this stage. usc of bore holegeophysical methods. such as electriclogging. GPR. tomography. etc (as and whenrequired) may be found advantageous todefine particular characteri stics of overburden and bed rock:

c) Defining of geohydrological characteristicsof the foundations and its en vironmerusthrough pumping in or pumping out testsas dictated by site conditions;

d) Ascertaining the groutability of foundationsthrough trial grouting of specified reaches:

e) Special field tests like blasting tests and fieldshear tests, where found necessary: and

f) Seismo-tectonic set up of the project region .

4.2.3.3 Depth and spacing ofholes

a) For dams less than 30 m in height. oneadditional line of holes (in addition to thoseindicated in 4.2 .1) as per designconsiderations may be necessary. The holesshould be suitably staggered to provideinformation at 30 m intervals. The depth of1/3 of these holes may be kept equal to thehydraulic head of the dam.

b) For dams 30 m to 100 m in height. twoadditional rows of holes would be required .The spacing between drill holes may be1/IOth of the length of the portion with

3

IS 6935 : 2008

particular attention being given for adequatecoverage to deeper sections . Half thenumber of holes should be taken to depthsequal to the hydraulic head and theremaining to half the hydraulic head or to adepth to prove a continuous impervious soilor rock or such strata that can be renderedimperv io us by treatment . The depths towhich exploration should be continued inthe impervious medium or medium that canbe rendered impervious by treatment shouldbe decided on the basis of designconsiderations.

c) For dams above 100 m in height. three lineso f holes should be drilled at locations asper design considerations. The depth ofthese holes should be equal to the hydraulichead. In addition . trenches to explore thefoundation sequence in the river bed sectionand for collection of undisturbed samplesmay be required.

For a dam more than 100 m height. three or(Pore drifts on either bank with cross cutsarc recommended. However. the length ofthe main drift and the cross cuts will dependupon the dimension of the structure (damsection across and along the river flow) atthat particular elevation. meaning therebythat the length of the main drift and crosscuts will be more near the base and reducinggradually towards the top of the dam. Thelength of the drift should be decided by theengineering geologist as per designrequirement. which in tum depend on theheight of the dam and quality of therockmass . These drifts can be utilized forvarious purposes, namely, inspection,drainage and grouting at a later stage. Thegroutability of foundation through trialgrouting of the specific section. in a setpattern , should also be tested during thisstage.

4.2.4 Construction Stage

During this stage . large scale geologicalinvestigations are carried out to delineate allgeological and structural features exposed on thefoundation of the dam and its appurtenant structuresand to apprise the construction and design engineersregarding rock mass structure and any special geo­structural features. like. fault zones, defonnedJweakzones. breccia zones, shear. etc. which require dentalor any other suitable treatment, like. grouting forstrengthening rockmass and making them water tight.The shear zones. deformed zones or weak zonesshould be marked with colour paint on the foundation

IS 6955 : 1008

surface. The final foundat ion grade geologicalmapping should be done on I: I00 scale w.ith I. mcontour interval in grid pattern (2m x 2m) using highprecision survey equipment like total station. Thefoundation surface should be properly cleaned byair or water jet before taking up geological mapping,which guides the foundation preparation and

treatment.

5 METHODS OF EXPLORATION

5.1 The following categories of methods may beused for subsurface exploration for earth and rockfilldams:

a) Geophysical,b) Pits and trenches,c) Borings ( auger boring and core drilling),d) Shafts and drifts, ande) In-situ tests.

5.1.i Geophysical explorations enable gaining ofknowledge of properties of subsurface strata byinference from measured rates of transmission ofelectric current or seismic waves. In-situ tests enabledirect measurement of properties in the ground . Therest are means of visual examination, collection ofsamples and for performance of laboratory teststhereon.

6 GEOPHYSICAL OBSERVATIONS

6.' Geophysical methods can, under appropriateconditions, be used to obtain in relatively very shorttime, information regarding the nature ofthe variousstrata and their position and depths of change.However, since it is not a direct measurement , boringshave to be made for correlation in order to interpretcorrectly the geophysical data .

6.2 The geophysical methods are not adequate inthemselves as tools for subsurface investigation.Whereas they permit a fast coverage of the entirearea at low cost. and the process is not hampered bypresence of boulders, etc , which generally produceobstruction in boring, the correct interpretation ofgeophysical observations is difficult, particularly inareas of irregular formations and irregular depths ofstrata and steep topography. It is, therefore,imperative that interpretation from geophysical workbe confirmed by borings.

6.3 Geophysical investigations should always becarried out with proper equipment by properlytrained and experienced investigators , becauseaccuracy in observations and interpretation of datais very essential for arri ving at reliable conclusions.

6.4 The principal or main methods applicable in caseofexplorations for dams are:

a) Se ismic refraction, and

b) Electrical resistivity.

6.4.1 Seismic Refraction Method

Earth vibrations set up artificially by explosions formthe basis of this method. The earth waves travel inall directions through the ground and are refractedor reflected back to the surface by lower rockformation through which they travel with a differentvelocity than through the overburden. The time ofarrival of these waves at any point on the surface ofthe ground is recorded by a special seismograph andthe time oftravel from the explosion point to the pick­up point is thus determined. This informationenables deductions to be drawn regarding nature anddepth of underlying formations .

6.4.1.1 Th is method may be used to determine thedepth to bedrock, the dip in special cases and otherdata regarding the underlying rock formations whichare useful for designing foundations for dams, suchas dynamic modulus from shear wave velocity andvibration characteristics of the foundation . Thesestudies may be required in special cases of weakrocks and high dams (see IS 15681).

6.4.2 Electrical Resistivity Method

This generally uses four electrodes at equal distancesalong a straight line . An electric current is passedbetween the outer two electrodes and is preciselymeasured by a milliammeter. The potential differencebetween the inner two electrodes is measured usingnull point type ofcircuit. From the data obtained, theelectrical resistivity is calculated.

6.4 .2.1 The value of apparent resistivity changes ateach change of strata and since, in general , thedistance between the electrodes is equal to depth oflayer being measured, it is by this method possibleto measure specific resistance to different depths byvarying the electrode spacing. With some knowledgeof the local geology and of the typical values fordifferen t strata, it is possible to determine thethickness and depth of the different strata by use ofone or more methods of interpretation , namelymathematical analysis , empirical methods, inter­correlation with curves and correlation with modelexperiments. As regards dams, this method can beemployed for locating bedrock and water table (seeIS 15736).

7 EXPLORATION BY PITS

7;. Deep trilll pits (see IS 4453) may be used toinvestigate open fissures, or to exp lore zones ofweakrocks , which would break up in the core barrel andare incapable of being recovered in tact. In case of

4

IS 6955 : 2008

dams, open pits are useful for investigating the natureof overburden in foundation area.

SI No.

(I)

For Soils

(2)

For Rocks

(3)

9 EXPLORATION BY BORINGS

8 EXPLORATION BY TRENCHES

8.4 The length and spacing will be determined onthe basis of height and length of dam and geologicalcomplexities.

9.1 Borings provide the simplest method ofsubsurface investigation and sampling. They maybe used to indicate the subsurface stratum and tocollect samples from each ofthe strata.

9.2 Borings may be made by several methodsdepending upon the nature of subsoil strata, asdetailed below:

75 mm to 400 mm in diaUp to I 050 mm in diaUp to I 200 mm in dia

Short drilling(see IS J892)Wash boring(see IS 1892)Rotary drilling(see IS 1892)

a) Helical augersb) Disc augersc) Bucket augers

c)

iii)

iv)

9.3 Auger Boring

9.3.1 Post-Hole Auger

Hand -operated post-hole augers, 100 mm to 300 mmin diameter, can be used for exploration up to about 6m. However, with the aid of the tripod, holes up to 25m depth can be excavated . Depth of augerinvestigations are limited by ground water table andby the amount and maximum size of gravel. cobblesand boulders. as compared to size of equipment used .

9.3.1.1 Mechanically operated augers are alsoavailable and are particularly suitable. where a largenumber of holes are to be made, or in gravelly soils .Machine driven augers are of three types and aregiven below:

9.3.1.2 An auger boring is made by turning the augerto the desired distance into the so il, withdrawing itand removing the soil for examination and sampling.The auger is inserted in the hole again and theprocess is repeated. Holes are usually bored withoutaddition ofwater in loose, moderately cohesive moistsoil. But in hard dry soils or cohesion less sands, theintroduction ofa small amount of water into the holewill facilitate the drilling and sample extraction.

9.3.2 Shell and Auger Borings

Pipe casing or shell is required in unstable soil inwhich the bore hole collapses, and especially wherethe boring is extended below the ground water level.The inside diameter of the casing should be slightlylarger than the diameter of the auger used. Boringsup to 200 mm diameter and 5 m depth can be donewith manual operation. Power winch is required fordeeper borings . The casing is driven to a depth notgreater than the top of the next sample and is cleanedout by means of the auger.

9.4 Core Drilling

9.4.1 Core drilling should be done in accordance withIS 6926 (see also IS 4078 and IS 4464).

9.4.2 The accuracy and dependability of the records

For Rocks

(3)

Percussion boring

Rotary drilling( see IS 1892)

For Soils

i) Post hole auger(see IS 1892)

ii) Shell andauger boring(see IS 1892)

a) Mud-rotary drillingb) Core drilling

(see IS 6926)

(I) (2)

8.2.1 Trenching permits visua l inspection of the soilstrata, which facilitates logging of the profile andselection of samples. It also aids in obtaining largeundisturbed samples for testing. Trenches in slopingground have the advantage of being self-dra ining.

8.3 The level of the water table and the level .location and the rate of seepage, if met with , shouldbe date-wise recorded.

7.2 At the surface the excavated material shall beplaced in an orderly manner around the pit andmarked stakes shall be driven to indicate depth of pitfrom which the material came, in order to facilitatelogging and sampling.

7.3 The level ofthe water table and the level, locationand rate of seepage flow in the test pit should be 'date-wise recorded.

8.1 Exploration by trenches (see IS 4453) is useful inprov iding a continuous exposure of the ground alonga given line or sections. They are best suited forshallow explorations (3 m to 4.5 m) on moderatelysteep slopes, for example, abutment of dams.

8.2 The profile exposed by these trenches mayrepresent the entire depth of significant strata in anabutment ofa dam. However, their shallow depth maylimit explorations to the upper weathered zone offoundat ions.

SI No.

5

IS 6955 : 2008

furnished by diamond drilling depend largely uponthe size of the core in relation to the kind of materialdrilled , the percentage of core recovery, thebehaviour during drilling and the experience of thedrill crew. Largest practicable diameter core shouldbe obtained. Recovery of core is much more importantthan rapid progress in drilling the hole. When drillingin soft materials, the water circulation should bereduced or stopped entirely and the cores berecovered dry.

9.4.3 Detailed history of mechanical operation ofdrilling including observations on the loss of returnwater and its reappearance, difficulties encounteredand time taken in these difficult areas and in areas ofcore loss should be included in the drilling report.

9.4 .4 Percolation tests under specified pressuresshould be done in drill holes using packers. as thedrilling progresses.

9.4.5 Completed holes should be capped to preservethem for use in ground water level observations oras grout holes or for re-entry, if it is later founddes irable to deepen the hole.

10 EXPLORATION BYDRIfTS

10.1 Drifts or tunnels (see IS 4453) are normallyemployed to explore at depth the cont inuit y orcharacter of subsurface formations. They are mostfrequently used for the investigation of fault or shearzone , buried channels and suspected places ofweakness in dam foundation, abutments and beneathsteep slopes or back of cliff like faces to determinethe extent of weathering, slump zone and bed rockconfiguration in areas of fossil valleys .

10.1.1 They are also used for taking undisturbedsamples of rock for tests in the laboratory and forperform ing in-situ tests like the plate bearing testand flat jack tests to determine the modulus ofelasticity and deformation of rock formations andshear. etc, required to study the properties of therock.

10.2 Logging and sarnpliag of exploratory driftshould proceed concurrently with excavationoperation. They should be mapped giving direct ionof dip, fault zones, shear zones and seams. etc. asdetailed in IS 4453.

10.3 Level. location and piezometric heads ofseepage flows. ifany, occur should be recorded date­wise.

II EXPLORATION BYSHAFTS

11.1 Shafts (see IS 4453) are vertical holes and arenormally employed to reach a particular point at a

6

great depth . either to extend the exploration belowriver bed by means of drifts for dam foundation orfor explorating locations of structures. such as gates,underground diversion tunnels. penstocks. etc . Theyalso provide continuous exposures of the groundalong the direction of shaft.

12 IN-SITU TESTS

12.1 In-situ field tests are those , in which thematerial is tested without actual removal of thematerial from its existing position. The in-situ testsapplicable to earth and rockfill dams are the following:

a) Strength tests:I) Deep penetration tests, and2) Shear tests.

b) Measurement of density of foundationmaterial,

c) Permeability tests, and

d) Blasting tests.

12.1.1 The necessity and the number of each typeof test to be conducted depend on the foundationmaterial and its degree ofvariability.

12.2 Strength Tests

12.2.1 Deep Penetration Tests

12.2.1.1 These tests [see IS 2131, IS 4968 (Part I),IS 4968 (Part 2) and IS 4968 (Part 3)] consist ofmeasuring the resistance to penetration under staticor dynam ic loading of different shaped tools. Thetests are empirical and have been developed fromexperience. They should be performed carefully inthe prescribed manner.

12.2.1.2 Static and dynamic penetration tests in boreholes or direct provide a simple means of comparingthe results of different bore holes on the same siteand for obtaining an indication of the bearing valueofthe soils, and of the state of densification of non­cohesive soils. Correlation between number of blowsobtained in standard (dynamic) penetration tests (seeIS 2131) and between penetration resistance in stat icpenetration test with bearing capacity and relativedensity of non-cohesive soils are given in severalpublications (see a/so IS 6403). While these can beused as guides . a better method would beto do actualcalibration of the apparatus, or at least use more thanone method for comparison.

12.2.1.3 Where dams are to be founded on sandydeposits. these tests are suitable methods fordetermining in-situ densities at depth, which is vitalfor assessment of settlements, and of potentialitiesof liquifaction under earthquakes.

12.2.1.4 The number of tests should be fairly largeenough to cover the entire critical foundation area.

12.2.2 Shear Tests

12.2.2.1 Vane shear tests

These tests (see IS 4434) measure the in-situ strengthof cohesive soils, which are too soft or sensitive forsampling.

12.2.2.2 Large shear tests

These tests may be necessary under specialfoundation conditions when large specimens of thefoundation material are to be tested for bettersimulations offield conditions and representation ofthe material which is not possible under laboratorytests . Special loading and observation set up arerequired in such tests on the same principles as thosefor laboratory tests.

12.3 Permeability Tests

12.3.1 Permeab ility ofa soil rockmass is the propertywhich governs the rate at which water can flowthrough unit area under unit hydraulic gradient. Aknowledge ofpermeability is necessary in estimatingseepage through the foundation and in determiningany foundation treatment that may be needed.

12.3.1 Permeability is usually determined by, in-situpumping in and pumping out tests [see IS 5529 (Part I)and IS 5529 (Part 2) ] in wells . The tests are of greatimportance in case of earth dams, particularly wherethe foundation is not sufficiently impervious, andhence , they should be performed in sufficient numbercovering the entire area. In rock foundations also,water loss tests are done in sufficient number ofexploratory drill holes.

12.4 Measurement of Density or FoundationMaterial

12.4.1 In-situ density of foundation material is usedin stability analysis. It also affords information onthe state ofcompaction and to decide whether furthercompaction is needed.

12.4.2 The sand density method is used todetermine the in-situ density by excavating a holefrom a horizontal surface, weighing the materialexcavated and determining the volume of the holeby filling it with calibrated sand [see IS 2720(Part 28)]. Other methods for the determination ofin-situ density are the core cutter method [seeIS 2720 (Part 24] and the rubber balloon method [seeIS 2720 (Part 34)]. The water content [see IS 2720(Part 2)] of the soil at the place of determination ofin-situ density is needed to calculate the dry densityof the soil.

7

IS 6955 : 2008

12.4.3 This test is applicable to very shallow depthsonly, or to the depth s of pits and trenches. where thetests are performed at their bottoms . The densitydetermination at depth sho u ld be made fromundisturbed samples obtained from depths. or bydeep penetrat ion tests in non-cohesive soils. In caseof dams, surface tests have hardly any sign ificance;hence this may be performed in pits and trenches .

12.5 Blasting Test

12.5.1 Blasting test is often performed infoundations of saturated loose non-cohesive soilsmainly for assessment of the likely chances ofliquifaction and settlement in the event ofearthquake, and also as prototype test for studyingthe efficacy of blasting as means of compaction ofnon-cohesive soils, where compaction is considerednecessary or desirable .

12.5.2 The test consists in blasting charges ofdifferent strength and at different depths andmeasuring induced accelerations, pore pressure riseand settlements or heave at d ifferent po ints . Nostandards can be laid down for the deta iledprocedure and strengths and depths of charge, andthese factors have to be decided taking intoconsiderat ion the past work on the subject andcharacteristics of the particular site and the proposedstructure. This is a special investigation, which maynot be needed in all cases.

13 SAMPLING

13.1 The methods employed for enabling collectionofsamples for visual examination and for performanceof laboratory tests thereon have already beendescribed in 9 to 13.

13.2 To take undisturbed samples from bore holes ,properly designed sampling tools shall be used .These differ for cohesive and non-cohesive soils andfor rocks. Special samplers like piston samplers andlor the freezing or grouting techniques may have tobe employed in cases where samples are to becollected from cohesion less sand which cannot besampled by ordinary equipment and methods ,particularly those existing below ground water table.

13.3 Sufficient quantity of representativeundisturbed samples for foundation exploration shallbe collected for carrying out the necessary tests.

13.4 While boring small diameter bore holes infoundation area of dams, the total material recoveredas core should be collected and stored in core boxes(see IS 4078). Samples of soil and rock should becollected and preserved in sealed pint jars to preservetheir natural water content . Samples should berepresentative of the material as it is found in the area.

IS 6955 : 2008

13.5 In the exploration of the materials infoundations. the excavation from which are insubstantial quantities and may be used inembankment construction, samples should becollected representative of each stratum in a volumesufficient to provide about 35 kg of material passinga 4.75 mm IS Sieve. Material smaller than 75 mmshould not be removed from this sample.

13.6 Samples collected in the process of routineexplorations are not as a rule satisfactory fordetermination of properties of soil or rock enmassein its natural condition. For this purpose, samplesshould be collected of material unaffected byseasonal climatic influence from large diameter bore­holes (100 mm to 150 mm diameter minimum) or fromthe bottom of open pits.

13.6.1 Bore hole samples should be 300 mm to600 mm long and open pit samples 250 mm to 300 mmcubes . Every effort should be made to preserve suchsamples as nearly in their natural condition aspossible.

14 LABORATORY EXAMINATION ANDTESTING OF SAMPLES

14.1 The samples of soils and rocks collected asdescribed in 13.1 to 13.5 should be examined andtested in the laboratory for determining theirengineering properties. The various tests that areusually necessary are given in 14.2 to 14.5.

14.2 Tests for Soils

14.2.1 Visual and Manual Examination

This would give general description of the soil orrock in terms of colour, consistency, structure,lithological type, etc, to help in general classificationofthe material.

14.2.2 Natural Moisture Content

It helps in assessment of foundation pore pressures[seeIS 2720 (Part 2)].

14.2.3 Liquid and Plastic Limits

Liquid and plastic limits are semi quantitativemeasures of water absorption qualities ofclay. Theygive an indication of the cohesiveness of the soils,and are also useful in soil classification [seeIS 2720 (Part 5)].

14.2.4 Specific Gravity

Specific gravity indicates a basic characteristic ofthe soil and is useful in calculating several of the soilparameters [see IS 2720 (Part 3/Sec I and Sec 2)].

8

14.2.5 Particle Size Distribution

A knowledge of particle size distribution is of use forsoil classification in understanding the foundationfeatures, such as density, permeability andsusceptibility to liquefaction [see IS 2720 (Part 4)].Material must be well graded for dam constructionand have a grain size distribution depending uponrock strength and tendency to breakage.

14.2.6 Bulk Density

Knowledge of bulk density is essential for computingstability.

14.2.7 Permeability

A knowledge of permeability ofdifferent foundationstrata is essential for estimating general seepage loss,piping danger and grouting requirements. It is alsoessential for the, design of under seepage controlmeasures. Ratio ofhorizontal to vertical permeabilitycan indicate the degree of homogeneity and isotropyof the granular foundation material.

14.2.8 Consolidation Characteristics

These are required for estimating the magnitude andrate of settlement due to consolidation of soil andfor assessment of pore pressure development duringconstruction. One dimensional consolidation test[see IS 2720 (Part 15)] is also used for determiningthe additional consolidation which occurs in a soil,placed at a particular moisture content when it getssaturated. A series of such tests at different moisturecontents helps determine the appropriate placementmoisture percentage. Pore pressure development canbe calculated from data of one dimensional test, ordirectly by use of three dimensional consolidationtest.

14.2.9 Swelling Tests

Swelling tests are useful for clays particularly thoseof montmorillonite family to assess likely pressuresthe clay would exert on saturation. These testsshould be conducted at the lowest moisture contentthat may be obtained in the field.

14.2.10 Strength Characteristics

Strength characteristics of soil may be determinedby unconfined compression test (see IS 2720 (Part10)],direct shear test [seeIS 2720 (Part 13)]and triaxialshear test. Unconfined compression test is generallysuitable for rock samples for determination offoundation strength. Strength characteristics ofundisturbed soil samples from foundations areusually determined by triaxial shear test or direct sheartest (different types being used for differentconditions of stability analysis).

14.2.11 Compaction Test

May be required for comparison with in-situ densities[see IS 2720 (Part 7)].

14.2.12 Density Index (Relative Density)

For cohesion less soil to assess the degree ofcompaction ofthe soil in-situ [see IS 2720 (Part 14)].

14.2.13 Mineralogical Composition

By differential and X-ray difraction studies. May berequired for expansive soils combined with low heightdams.

14.2.14 Chemical Analysis

Chemical tests may be performed on one or twotypical soil samples to determine soluble salt content[see IS 2720 (Part 21)], calcium carbonate content [seeIS 2720 (Part 23)] and organic matter content [seeIS 2720 (Part 22)].

14.2.15 Dispersive Test

Presence of dispersive clays leads to disperse orflocculation in the presence of water which becomesturbid as dispersion progresses. Soil classes haveto be evolved on this and necessary precautionsundertaken for use of the same material.

14.3 Tests for Rock

14.3.1 Petrographic Study

Petrographic study of the rock done by petrographerhelps to evaluate the stability of the constituentminerals under conditions of prolonged saturationof the foundation material (see IS 1123). These maybe required in special cases.

14.3.2 Shear Strength Tests

Shear strength tests may be required in the case ofweak and layered rock foundations.

14.3.3 Specific Gravity and Porosity

This would indicate the state of denseness of therock (see IS 1122).

14.3.4 Water Absorption

This test determines the capacity of rock forabsorbing water (see IS 1124).

14.4 Chemical Analysis

Chemical tests may be performed on one or twotypical rock samples to determine soluble saltcontent, calcium carbonate content and organicmatter content.

9

IS 6955 : 2008

14.5 Water Analysis

Chemical analysis of river water and ground waterincluding determination ofpH value (set' IS 3025) maybe done to assess the effects ofwater. such as corrosion.on underground. or other hydromechanical installationsand leaching of salts from the foundation strata ordeposition of salts from the percolating waterunderground .

IS RECORDING AND REPORTING OF DATA

15.1 General

Information collected from the explorationsmentioned should be recorded and presented in aconcise and systematic manner. suitable forconvenient use, in the form of maps, subsurfacesections, etc. The locations of sections and pointsof exploration should be clearly indicated on a map.Pits, trenches, drifts, shafts, different types of boreholes, etc, should be indicated on location maps usingsuitable symbols as given in IS 7422 (Parts I to 5).

15.1.1 The scales used for maps should be inaccordance with IS 15686.

15.2 Logging of Pits, Trenches and Holes

15.2.1 Location

Every pit, trench and hole should be definitely locatedon a map by being tied to a co-ordinate grid system.The top elevations should be recorded. as also theinclination of the inclined holes.

15.2.2 Identification

The holes, pits, etc, should all be numbered normallyin the order in which they are drilled and with suitablesymbols as given in IS 7422 (Parts I to 5).

15.2.3 Logs

A standard and exhaustive log form should be usedgiving as much information as possible (see IS 4453and IS 4464)

15.2.4 Description ofSoils

The soils should be described in the logs and in therecords according to IS 1498.

15.2.5 Description of Rock Cores

The description of the rock core should include itstypical name followed by data on its lithologic andstructural features, physical conditions, and any specialgeologic, mineralogic , or physical features pertinent tointerpretationofthe subsurface conditions (see IS4464).

IS 6955 : 2008

15.3 Subsurface Sections

Sections showing subsurface conditions believed toex ist should be pr epared . The loc at ions of thesections should be selected in a manner such thatthe informat ion is presented in the be st possible

manner. With d ifferent informati on lik e type andnature of subsurface mater ial. natural moisturecontent, den sity, permeabil ity, etc, shown for differentstrata, th e secti o ns pr esent ve ry useful data fordesign stud ies .

ANNEX A(Clause 2)

LIST OF REFERRED INDIAN STANDARDS

IS No.

2: 1960

1122 : 1974

1123: 1975

1124 : 1974

1498: 1970

1892 : 1979

2131: 1981

2720

(Part 2): 1973

(Part 3/Sec I) :1980

(Part 3/Sec2) :1980

(Part 4) : 1985

(Part 5) : 1985

(part 7) : 1980

(Part 10): 1991

tnt,

Rules for round ing off numericalva lues (r evised)

Method of test for determinationof true specific gravity of naturalbu ilding stones (first revision)

Method of id entification ofna tura l building stones

Method of test for determinationof water absorption, apparentspecific gravity and porosi ty ofnatura l building sto nes (fi rs trevision)

Classification and identificationof soils for general engineeringpurposes (first revision)

Code of practice for subsurfaceinv est ig at ions for foundations(fir st revision)

Method for standard penetrationtest for soils (first revision)

Method of test for soils :

Determination of water content(second revis ion)

Determination ofspecific gravity,Section 1 Fine grained soil s (firstrev ision)

Determination ofspecific gravity,Section 2 Fine , medium andcoarse grained soils

Grain size analysis (s econ drevision)

Determination of l iquid andplastic limit (second revision)

Determination of water content­dry density relation using lightcompaction (second revision )

Determination of unconfinedcompressive st rength (se condrevis ion)

10

IS No. Title

(Part 13) : 1986 D ire ct s hea r te st (s ec on drevision)

(Part 14) : 1983 Determination of den sit y index(relative dens ity) of cohesionlessso ils (first revision)

(Part 15): 1965 Determination of consolidationproperties (first revision )

(Part21 ) ; 1977 Determinat ion of total solubleso lids (first revisi on)

(Part 22) ; 1972 Determinat ion of organic mailer(first revision )

(Part 23) : 1976 Determination of calciumcarbonate (first revision)

(Part24) ; 1976 Determination of cationexchange capacity

(Part 28): 1974 Determination of dry density ofsoils in place , by the sandreplacement method (firstrevision)

(Part 29) ; 1975 Determination of dry density ofsoil s in-place by the core-cutter

(Part 34) ; 1972 Determination of dry density ofsoils in-place by rubber-balloonmethod

3025 : 1964 Method of sampling and test(physical and chemical) for waterused in industry

4078 ; 1980 Code ofpractice for indexing andstorage of drill cores (firstrevision )

4434 : 1978 Code of practice for in-situ vaneshear test for soils (first revision)

4453 : 1980 Code of practice for subsurfaceexploration by pits, trenches ,drifts and shafts (first revision)

4464: 1985 Code of practice for presentationof drilling information and coredescription in foundation

IS No .

4968

(Palt I) : 1976

(Part 2) : 1976

(Part 3) : 1976

5529

(Part 1) : 1985

(Part 2) : 2006

6403: 1981

6926 : 1996

Title

investigation (first revision)

Method for subsurface soundingfor soils:

Dynamic method using 50 mmcone without bentonite slurry(first revision)

Dynamic method using cone andbentonite slurry (first revision)

Static cone penetration test (firstrevision)

Code of practice or in-situpermeability test

Test in overburden (first revision)

Code of practice for in-situpermeability tests: Part 2 Tests inbedrock (second revision)

Code of practice fordetermination ofbearing capacityof shallow foundations (firstrevision)

Diamond core drilling - Siteinvestigation for river valleyprojects - Code ofpractice (firstrevision)

II

IS No.

7422

(Part I) : 1974

(Part 2) : 1974

(Part3) : 1974

(Part 4) : 1985

(Part 5) : 1992

15662 :2006

15681 :2006

15686: 2006

15736: 2006

IS 6955 : 2008

Titl e

Symbols and abbreviations foruse in geological map s, sectionsand subsurface exploratory logs:

Abbreviations

Igneous rocks

Sedimentary rocks

Metamorphic rocks

Line symbols for formationcontacts and structural features

Geological exploration for gravitydams and overflow structures ­Code ofpractice

Geological exploration bygeophysical method (seismicrefraction) - Code of practice

Recommendations for prepara­tion of geological andgeotechnical maps for river valleyprojects

Geological exploration bygeophysical method (electricalresistivity) - Code of practice

Bureau of Indian Standards

B1S is a statutory institution established under the Bureau oflndiun Standards Act, 1986 to promote harmoniousdevelopment of the activities of standardization, marking and quality certification of goods and attending to

connected mailers in the country.

Copyright

BIS has the copyright ofall its publications. No part of these publications may be reproduced in any form withoutthe prior permission in writing ofBIS. This does not preclude the fret: use, in course of implementing the standard.of necessary details, such as symbols and sizes. type or grade designations. Enquiries relating to copyright be

addressed to the Director (Publ ications). BIS.

Review oflndian Standards

Amendments an: issued to standard s as the need arises on the basis of comments. Standards are also reviewedperiodically: a standard along with amendments is reaffirmed when such review indicates that no changes areneeded; if the review indicates that changes an: 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 issue of'BIS Catalogue' and 'Standards: Monthly Additions' .

This Indian Standard has been developed from Doc: No. WRD 05 (451) .

Amendments Issued Since Publication

- - . ._---- _._ ---_. . - - _.__._- - - - - - _._ - - - -- - - - - - - - - - - - - - - -Amendment No. Date of Issue Text Affected

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