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FOUNDATIONS OF HIGH RISE BUILDINGSDr.S.R.KULKARNI, Bharati Vidyapeeth University

Subsurface Geological Exploration 5. Observations and precautions during drilling:

Introduction:

Geological conditions at tf:\ebuilding foundations arean important factor in the design of this structure.Detailed geological investigations have therefore tobe carried out to find out the subsurface geologicalconditions before the design and the constructionwork is taken in hand. Numerous cases are onrecord in which neglect of geological conditions hasgiven rise to all sort of difficulties and has led toavoidable delays and increases in cost of projects.On the other hand ~ thorough understanding ofgeological condition has made possible large,economies in the execution of many projects.

Procedure:

The preliminary geological investigations are aimedat finding out the nature and structure of the rocks atthe work site, and to obtain correct information aboutthese the investigations have to be carried outsystematically.

Subsurface Exploration:

Type of Drilling Equipment

Three types' of equipment are currently available;.single tube, double tube and triple-tube. Single tubeunit is mostly used which needs to be prohlblted.Most of the NITS, these days, provide for doubletube but in practice it is not implemented. Even forvery large projects costing erores of Rupees singletube drilling is being adopted. In some regions, tripletube drilling may be needed where weak roeks suchas shale, volcanic breccias, Tachylytes, tuffs, tuffbreccias, mudstones, siltstones, laterites, weaksedimentary rocks, slates, phyllites, fault zones,.shear zones, crush zones, folded rocks ete occurs.In such zones only triple tube must be used.

Location of Bore Holes

Bore holes as far as practicable, shall be located atevery comer of the building and one at the centre. Ifarea of the building is small minimum three boreholes should be taken.

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To ensure that drilling data are not misinterpretedand also that valuable' data are not lost certainprecautions have to be taken and observations haveto be recorded carefully during drilling.

As drilling data cannot be interpreted withoutknowledge of local geology drilling should not beundertaken before establishing local geology bysurface geological work. The locations of drilledholes and depth to which they are to be taken mustbe decided taking into account the local geological·structure and the nature of the infonnation required.Not taking into account the local geological,',conditions while deciding the depths of drill holesmay result into the driU holes not yielding all· .the·information required.

As soon as core is removed from the core barrel allpieces of core' must be immediately numberedserially, their depths and serial numbers painted onthem and their lower ends marked so as to ensurethat all pieces are. kept in correct order exactlyrepresenting the depth from which they have beenobtained. Lengths of all pieces of core must bemeasured and recorded in the Daily Drill Report.Core must be immediately placed in core boxesmade-according-to-standarq-specification , and tallthe necessary information such as the name of theproject, location of drill hole, drill hole number, etc. ,must be painted on the box.

Quality and quantity of Returning drill water

To ensure that drilling data are not misinterpretedand also that valuable data are not lost, certainprecautions have to be taken during drilling andobservations carefully recorded. All the water that isfed into the drill comes back to the surface if, therocks being drilled through, are water tight. If,however, the drill is passing through pervious rocksthe water will leak into them and will not return to thesurface. This drill water loss may be complete orpartial depending on the nature of the rocks. As drillwater loss indicates a leaky zone all drill water

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losses must be carefully recorded during drilling.Observing carefully the colour of drill water isimportant. Rate of drilling of each run gives in-.valuable information. Experience shows that theseimportant requirements are usually neglected. It isalways important to know exactly where weak zonesoccur and what their nature is. But, routine drillingprocedures will not provide adequate information onthis vital point. In such cases another hole close toprevious one is to be drilled in short runs in weakzone. Another alternative is to carry out nearly drydrilHng at a very slow rate.

8. Minimum depth of drilling

Each and every be drill hole must be drilled minimum5 to 6m in rock with minimum core recovery morethan 80% and R.O.D more than 70%.

9. Length &Number of Pieces of Core

In hard but jointed rock the core recovery may bevery good, and consideration of the core recoveryalone will lead to the conclusion that the rock is good.This, however, may be wrong, as because of its.fragmented condition, the rock will not be good fromthe engineering point of view.

10. .Preservation of Core Pieces

Preservation of cores of rocks that disintegrate onexposure to atmosphere:

The cores of some soft &weak rocks such as shales,chlorophaeitic basalts and red or black tachylytesand volcanic breccias with lava matrix of these typesof basalt, shales, mudstones, slates, phyliites,Jaterites disintegrate on exposure to atmosphere butas they are sound rocks when occurring in situ theygive good core when drilled through. However,because of their tendency to disintegrate whenexposed to atmosphere, the core disintegrates withthe time in the core box and falls to a looseincoherent powder. Then when the core is examinedsome days after extraction heaps to loose powderare seen in .the core box, and it is concluded thatloose material occurs at the level at which theseheaps of loose powder are seen in the core. But theconclusion that powdery material indicates theoccurrence 9f loose material is obviously wrong as

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the core was quite sound when extracted, indicatingthat . it had come from sound rock, and haddisintegrated in the core box. As it is thedisintegration of core in the core box that creates thewrotig impression that loose materials occurs at thelevel at which powder is seen, special precautionshave to be taken during drilling to prevent suchdisintegration oftha core in the core box.

For this purpose, core of such rocks when raisedto the surface-shouldblt covered with a thin layerof wax by Immediately Immersing It In moltenwax for preventing Its exposure to atmosphere.

Ease or dlfflculty of drilling and the speed ofdrilling at different depths must be carefullynoted and recorded.

To cool the drill water is continuously pumped in~othe inside of the bit. This water goes down the hole toits bottom and rises to the surface on the outside of .bit. This water is called drill water. Drill water bringswith it to the surface the rock cuttings produced bythe cutting action of bit. While drilling through hardrock the amount of cuttings is small and the returningdrill water is quite clear. If, however, soft material isbeing drilled through it is pulverized by the violenceof drilling. and the chuming action of the drill forms asuspension with the drill water. Therefore, no core isobtained as the soft material is pulverized and risesto the surface in suspension in the drill water.Because of the presence of the soft material insuspension in such cases the drill water becorr 'turbid. Turbidity of returning drill water theretrindicates that the drill Is passing through sonmaterial which will not give core. In such cases theturbid drill water must be collected and thesuspended matter allowed settling. As no core willbe obtained this suspended material will be the only .indication available about the nature of the softmaterial, and it should therefore be preserved in. suitable containers and kept in the core box at the .appropriate place corresponding to the depth from

which it comes.

All the water th,!t is fed into the drill comes back to the """surface if the rocks being drilled through are 'watertight. If, however the drill is passing through """

previous rocks the water will leak into them and willnot return to the surface. this is called drill water loss .~hich may be complete or partial depending on thenature of the rocks. As drill water loss indicates aleaky zone all drill water losses must be carefullyrecorded during drilling.

,,- 12. FractureSurfaces

r- NatlHal Fractures and Mechanical Fractures:

Core would normally break along pre-existingdivisional planes only. However due to vibrations.during drilling, particularly with a defective machineor defective operation, core may also break even atplaces where joints do not exist. Such mechanicalfractures do not indicate any weakness in rock asjoints do, & therefore these fractures may be ignoredwhile considering number & length of pieces of core.It is therefore necessary to distinguish betweenfractures due to jointing & mechanical fractures,which can be done by examining fracture surfaces.As joints have plane surfaces joint fractures willshow plane surfaces. Also water circulating alongthe natural divisional planes will have brought aboutdecomposition of the rock along joints or may havedeposited dissolved material on the joint face. Jointfractures will therefore show decomposition, or astain of iron oxides, or a coating of iron oxides orsilica or calcium carbonate. Amechanical fracture onthe other hand will have clean rough surfaceswithout any stain or coating or decomposition. Incase of broken core therefore it is necessary toexamine all fracture surfaces & to ignore mechanicalfractures.

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If fragmented rock has come from fault zone fracturesurfaces will show parallel scratches because of theenforced movements of rocks against each other.

Mechanical Fractures:

Core would normally break along preexistingdivisional planes only. However, due to vibrationsduring drilling, particularly with a defective machine.or defective operation, core may also break even atplaces where joints do not exists. It is necessary todistinguish between fractures due to jointing andmechanical fractures, which can be done byexamining the fracture surfaces.

13. Corelogs and Lithologs

For obtaining from the core the required informationuseful for engineering purposes it has to beinterpreted taking into account the various pointsdiscussed above. To be able to do this its relevantcharacters have to be recorded and for this purpose,core properly arranged in a core boxes according todepth is examined in detail, and the observations arerecorded in the standard form. This is called corelogging and the,record core log. During core loggingevery core piece has to be examined individuallyand all information about it. such as length, type ofrock, state of preservation, structural features,nature of the fracture at the ends;' "etc. has to berecorded in the successive draws and the corerecovery in each. In addition it records theobservations carried out during' drilling, such ascolour or turbidity of drill water, loss of drill water,etc., and result of percolation tests. Thus the core logserves as the basic record containing all field dataobtained by drilling. Core logging arid preparation oflithologs graphic logs require not only geologicalexpertise of higher order, ~ut also interpretinggeological data for engineering purposes, .and.hence these should not be attempted by anyoneexcept an experienced engineering geologist.

14. Interpretation

The purpose of drilling is not just to find out thegeological condition the location of each drill holeisolated from others, but also to workout thegeological structure of the entire worksiteconsidered as a unit. However, such interpretationcan be done only if the local geology is adequatelyknown. Interpretation attempted without knowledgeof local geology is likely to prove misleading andtherefore before undertaking drilling local geologymust be correctly workout by surface geologicalwork. Also this interpretation requires knowledge ofhigh degree and therefore it should not be attemptedby anyone except the trained geologist conversantwith local geology .

15. RQO, JFI and SCR :

Core recovery <toes not take into account thefrequency of joints, which is a major short coming. To

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overcome this other methods of describing the stateof the core have been devised. One is RQO (RockQuality Designation), for which recovery iscalculated by excluding the aggregate length ofpieces less than 10em in length between joints. Toovercome certain deficiencies in RQO the JointFrequency Index (JFI) is calculated. this Is thenumber obtained bydividing recovery by the number'of joints in' the draw. Sometimes Solid CoreRecovery (SCR) is also used. For computing allthese it is of vital importance to distinguish betweenjoints & mechanical fractures, & these must becalculated only by a competent engineeringgeologist.

QUANTITATIVE CLASSIFICATION SYSTEMS OF. THE ROCK MASS - GUIDELINES ROCK MASS

RATING (RMR) FOR PREDICTINGENGINEERING PROPERTIES

16. SCOPE:

This standard covers the procedure for determiningthe class of rock mass based on geomechanicsclassification system.

17. PROCEDURE:

Toapply the geomechanics classification system, agiven site 'should be divided into number of.geological structural units in such a way that eachtypeof rock mass present in the area iscovered.Thefollowing geological parameters are determined foreachstructural unit:

a) Uniaxial compressive strength of intact rockmaterial(lS 8764),

b) Rockquality designation [IS 11315(Part11)],

c) Spacingof disconttnuitles [IS 11315(Part2)],

d) Condition of discontinuities [IS 11315(Part4 )],

e) Groundwater condition [IS 11315(Part8)] and

f) Orientation of discontinuities [IS 11315(Part1)].

17~1 Uniaxial Compressive Strength of Intact Rock .Material(qc)

The strength of the intact rock material should beobtained from rock cores in accordancewith IS9143

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or IS 8764 or IS 10785as applicable based on s.conditions. The rating based on unlaxtcompressive strength and point load strength at.given InAnnex B (Iteml).Howeverthe use ofunlaxicompressive strength Is preferred over the poinload indexstrength.

17.2 Rock QualityDesignation(RQO)

Rock quality designation (RQD) should bdetermined 'S specified in IS 11315 (Part 11). thEdetail of rating are givenInAnnex B (Item II).

17.3 Spacing of Discontinuities

The term discontinuity covers joints, beddings 01

foliations, shear zones,minor fault, or other surfacesof weakness. The linear distance betwee: '"adjacent discontinuities should be measured allsets of discontinuities. The details of ratings argiven inAnnex B.

17.4 Condition of Discontinuities

This parameter includes roughness of discontinuit)surfaces, their separation, length or continuityweathering of the wall rock or the planes ofweakness, and infilling (gauge) material. The detai~of rating are given inAnnex B. The description of theterm used in the classification is given in the I~11315(Part4)and IS11315(PartS) .

DATASHEET FORGEOMECHANICALCLASSIFICATION OF ROCK MASSES (RM~'

I. STRENGTHOF INTACTROCK MATERIAL (M

Compressive Point Load RatingStrength Strength

Exceptionally strong >250 >8 15

Very strong 100-250 4-8 12Strong 50-100 24 7

Average 25-50 1-2 4

Weak 10-25 Use of uniaxial 2com ressive

Very weak 2·10 strength iseferred

Extremely weak <2 0

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l u. ROCK QUALITY DESIGNATION (RQD)

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ROD Rating

Excellent 90-100 20

Good 75-90 17

Fair 50-75 13

Poor 25-50 - 8

Very poor <25 3

III. SPACING OF DISCONTINUITIES

Spacing, m Rating

Very wide >2 20

Wide 0.6-2 15

Moderate 0.2-0.6 10

Close 0.06-0.2 8

Very close <0.06 5

NOTE·

r: If more than one set of discontinuity is present andthe spacing of discontinuities of each set varies,consider the' set with lowest rating.

IV. CONDmON OF DISCONTINUITIES

IVery rough Rough <WId I Sf19hUyrough Slicken sided I Smm thick scltandu~ sligllty I and moderately wall rock gauge Smmwee!heled weathered ! kl highly surface Of 1- I wide

! wall roct, tight wall rock ! weathered walt Smmlhick I continuousI and surface, i rock surface. gauge or 1- ' discontinuity

I discorilnuous. separation I separation<1 mm Smmwldeno separation <1mm i aperlng.

~COOtinuous ._-

so 25 !2O 10 0I

. 'i. GROUND WATER CONDITION

i Inflow per 10m none <10 10-25 25-125 >125: tunnel length,!Oitrelmin)I Joint water 0 0-0.1 0.1-0.2 0.2-0.5 >0.5: pressure/major I i! Principal !!stressiGeneral Completely Damp Wet Dripping Flowing!description dry I!Rating 15 10 7 4 0

V.ORIENTATION OF DISCONTINUITIES

Oriel)tation of slope/foundation axis

Average strike direction .

Average dip direction .

ADJUSTMENT FOR JOINT ORIENTATION

Strike and dip vety FaYOll'able Far Un- ! Vety (Un-orientation ofpnls F8WInbIe flMlurable I favourablefor Raft foundation I )

0 ·2 -7 -15 I ·35

Slopes

Use slope mass rating (SMR) as per IS 13365 (Part 3)

VI. ROCK MASS RATING (RMR)

NET SAFE BEARING PRESSURES BASED ON RMRCLASSIFICATION No.

Descriptio Very Good Far Poor Veryn of rock aood POOl'

RMR 100-81 80-61 60-41 40·21 . 20-0

qns(t/m2) 400 300-200 150-100 80-50 40-0'or < 40

CLASSIFICATION & CHARACTERISTICS OF ROCKS

1. Characteristics of rock0. "

Parent strength of rock stratum is materially affectedby characteristics like weathering, hardness, jointspacing and bedding and rock quality designation(ROD). Broad recommended descriptions of these'characteristics are given in tables below. The extentto which these characteristics will affect the parentstrength of the rock will vary from case to case andwill have to be decided upon based on engineeringjudgment to access the factors of safety to beallowed for arriving at the allowable bearingpressures. These decisions should be taken as perthe advice of the engineering geologist.

Table 2 -Weathering

Rock generally fresh, joints stained,

some joints may show clay if open

crystals in broken face show bright.

Rock rings under hammer if

crystalline.

Fresh Rock fresh, crystalsbright, few joints mayshow slight staining.Rock rings underhammer if crystalline.

Very slight

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Note: For specific projects involving only a limited numberof rock types, subdivisiOn of major groupings may bedesirable. Numerical or alphabetical subscripts·may be used to identify such subdMsions.

Table 4.- Joints and Bedding In Rock

Spacing Joints Bedding RemarksLessthan 50 an Very dose Very thin50mmto 300mm dose ThinJOOmmt01m Moderatelyaose Medium1mto3m Wide Thick Massively beddedMorethan 3m VerytMde Very thick Massively bedded

Note: joint spacing refers to the distance normal to theplane of the joints of a single system "set" of jointswhich are parallel to each other or nearly so. Thespacing of each "set" should be described, ifpossible to establish.

Table 5 • Rock Quality Designator (RQD)

RQO Diagnostic Description

Exceeding90% Excellent

90-75 Good

75-50 Fair

50-25 Poor

Lessthan25 Verypoor

1. ROD should always be given in percentage.Diagnostic description is intended primarily forevaluating problems with tunnels or excavations inrock.

If ROD is to be determined, double tube N size corebarrel with non-rotating inner barrels must be used.

18. FOUNDATIONS

INTRODUCTION

Foundation is the structural elements that transfer theloadsfrom the building or individual columns to the earth.If these loads areto be properly transmitted, foundationsmust be designed to prevent excessive settlement orrotation, to minimize differential settlement and to provideadequate safety against sliding and overturning. The typeof foundation used in a given situation depends on a

number of factors such as soil strata, bearing capacity offoundation material, type of structures, type of loads;permissible differential settlement, and economy etc.'Foundation may be broadly classified into two categories:

1) Shallow foundations

2) Deep foundations

18.1 SHALLOW FOUNDATION

A shallow foundation transmits the loads to the strata atshallow depth. It is termed shallow if it is laid at a depthequal to or less than its width. Shallow foundation islocated just below, the lowest part of the wall or a columnwhich they support. Foundations are structural membersmade of brick work, masonry or concrete that is used totransmit the load of the wall or column such that the load isdistributed over a large area. Shallow foundation includesspread or isolated footing, strip footing, combined footing,strap or cantilever footing, mat or raft foundations.

SPREAD OR ISOLATED FOOTING.

A spread or isolated or pad footing is provided to supportan individual column. A spread footing is-eir~lar, squareor rectangular slab of uniform thickness. Sometir:nes, it isstepped or haunched to spread the load over a large area.

STRIP FOOTING

A strip footing is provided for a load bearinq wall. A strip ~.footing is also provided for a row or columns.which are soclosely spaced that their spread foo~·ngs overlap or nearlytouch each other. In such a case, it is more economical toprovide a strip footing than a number of spread footings inone line. A strip footing is also known as continuousfooting.

COMBINED FOOTING

A combined footing supports two columns. It is used when .the two columns are two close to each other that their ..individual footings would overlap. A combined footing isalso provided when the property line is so close to onecolumn that a spread footing would be eccentricallyloaded when kept entirely within the property line. Bycombining with that of an interior column, the load isevenly distributed. A combine footing may be rectangularor trapezoidal in plan.

STRAP OR CANTILEVER FOOTING

A strap or cantilever fo<?~ingconsists of two isolated

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footings connected with a structural strap or a lever. Thestraps connect the two footing such that they behave asone unit. The strap simply acts as connecting beam anddoes not take any soil reaction. The strap is designed as arigid beam. The individual footings are so designed thattheir combined line of action passes through the resultantof the total load .A strap footing is more economical than acombined footing when the allowable soil pressure isrelatively high and the distance between the columns islarge.

MAT OR RAFT FOUNDATIONS

A mat or raft is a thick reinforced concrete slab whichsupports all the load bearing walls and column loads of astructure or a portion of structure. A mat is required whenthe loads are heavy and the soil is very weak or highlycompressible. A mat is more economical than individualfooting when the total base area required for the individualfootings exceeds about half the area covered by thestructures. The matfoundations considerably reduces thedifferential settlement. Like all other shallow foundations.a mat must be safe against shear failure and thesettlement should be within the allowable limits. As raftare generally at some depth below the ground surface. alarge volume of soil is excavated and. therefore. the netpressure on the soil is considerably reduced. Anadvantage of this reduction in the pressure can be takenwhile designing a raft.

18.2 DEEP FOUNDATIONS

When the soil at or near the ground surface is not capableof supporting a structures. deep foundations are requiredto transfer the loads to deeper strata. Deep foundationsare therefore. used when the soil surface is unsuitable forshallow foundations. and a firm stratum is so deep that itcannot be reached economically by shallow foundations.A deep foundation is generally much more expensive thana shallow foundation. It should be adopted only when ashallow foundation is not feasible. The most commontypes cf deep foundations are piles. piers and caissons.

18.3 PILE FOUNDATIONS.

Pile foundations consisting of vertical structural memberswhich is driven or cast into the ground by suitable means.The piles are usually provided in groups for moststructures. The piles may be subjected to vertical loads.horizontal loads or both.

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Pile foundations are used in the following conditions:

1. When the strata at or just below the ground surfal .•.highly compressible and very weak to support 1.•••

load transmitted by the structures.

2. Pile foundations are required for the transmissio ...•.the structural loads through deep water.

3. Pile foundation are used to resist the horizonforces in addition to support the v~rtical loads..••

earth retaining structures and tall structures that ~subjected to horizontal forces due to wind aearthquakes.

4. Piles are used for the foundation of some structure,such as transmission towers. offshore platfomwhich are subjected to uplift.

5. In case of expansive soils. such as black con". Isr.which swells or shrinks as the water contechanges piles are used to transfer the load below:"active zone.

A pile may be classified based on classified based on orofthe following criteria:

a) Material of construction- timber, steel, concre--composite materials

b) Shape-Cylindrical. tapered or under ream

c) Mode of load transfer-end bearing. friction. tensioi

d) Method of construction-Cast in situ reinfore=concrete. precast reinforced concrete. prestresseconcrete.

e) Method of installation- bored. driven. vibrated.

18.4 Drilled Piers and Caisson foundations

A drilled pier is a large diameter concrete cylinder built i_the ground. For construction of a drilled pier. a lal1diameter hole is drilled pier and a bored pile is basically c,the size. Generally. bored piles are of diameter less thaor equal to O.6m.The shafts of size larger than O.6mar-generally designated as drilled piers. A drilled pier is ~type of deep foundation. constructed to transfer hea.axial or lateral loads to deep stratum below the groun,-surface.

A caisson is a tYpe of foundation of the shape of a holleprismatic box, which is built above the ground level anc_then sunk to the"J;equired depth as a single unit. It is •

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watertight chamber used for laying foundations underwater as In rivers, lakes, harbour, etc. The caissons are offollowing types:

1.) Open caissons

2.) Pneumatic caissons

3.) Floating caissons

19. FAILURE OF FOUNDATION:

Foundation failures on solis

Foundation failure-can be attributed to several things.Most commonly foundation failure Is caused by themovement of expansive and highly plastic soils beneathdifferent sections of the foundation footings. Thismovement can be In the form of shrinkage, which causessetUement, or expansion, which causes heave. When dry

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is true, and soils swells. Officially any structure movementis known as differential settlement.

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In addition to expansive soils, subsurface peat, which hasa low bearing capacity and deteriorates over time, canalso cause differential settlement.

Other soil types such as sand and silt also have lowerthanrequired bearing capacities.

Poor. drainage from yard run off and gutter downspoutsdischarging at the base of the foundations is among othercauses. Excess moisture around the foundation cancause the soil to become over saturated and lose bearingpressure or the strength to support weight. When this

, -'happens, structures settle or sink into the ground.'.r'-

Seismic forces create various problems in foundationsuch as settlement of structures, sliding, over tuming,liquefaction, heaves etc. A major earthquake produces astrong ground movement in the subsoil, consequentlyunderground and subsurface structure supported onground mass will be induced to move and take dynamicforces. Their action in the foundation structure may beestimated knowing the subsurface behavior. The generalgeology of the affected areas is important and thestartigraphy of the upper part of the subsoil comprises ofsoft sediments where the strong ground motions takeplace should be determined.

When the seismic waves coming from the zone of

generation hit the Inner phase of the firm ground with theunconsolidated sediments to types of body waves are

.originated, namely Irrotational and pure shear waves. Theirrotational waves travel with high velocity then the shearwaves, and for translation require changes in the volumeof soil.Therefore,ln saturated soil they produce high porepressures and eventually if the acceleration is high in.noncohesive soil liquefaction may be take place at the' .'ground surface, the shear waves do not produce thevolume change in the soil during there propagation;however high shear stresses may developed runningover the shear strength of the soil. From the aboveconsideration two types of problems may be considered;

1) Problem Induced by irrotational waves;

2) Problem Induced by transverse waves.

Irrotational seismic waves

The .stability of the foundation the cohesion less finesediments with very low cohesion may be affected bydilation, or irrotational waves. In a saturated soil the porepressure originated during a strong ground motion maybe high al1ri the shear strength may be completely lostand liquefaction of the soil may take place. Thephenomenori is detected at the ground surface by boilsappearing after the earthquake. Never the less even withsmall acceleration the shear strength is reduced to a pointthat the bearing capacity is affected and the foundationssupported on the type of soil may suffer partial or totalfailure. The high pressure induced by strong earthquakemay cause failure of the structural foundation on firm soil.

Shear seismic waves

The transverse or pure shear waves propagate from theinterface at the firm ground into unconsolidatedsediments producing important shear distortion in the soilmass. Deep foundation and the under ground installationmay be strong stressed when large displacement areobserved by the shear waves. The behaviors of the soilmay be estimated with the knowledge of the dynamicshear modulus of elasticity and shear wave velocity.Based on the above it is essential to carry out thesubsurface investigation, this will include thedetermination of grain size, the consolidation and theshear strength charactertstlcs, permeabilitycharacteristics of the soil as well as the previous stress

history.

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/. 20. Liquefaction of foundation solis underearthquake

Introduction:

Liquefaction is a phenomenon in which a saturatedcohesion less soil looses strength during an earthquakeand acquires a degree of mobility sufficient to permit asignificant movement. ~en sand is not saturated theearthquake may cause significant compaction andsubsidence. Catastrophic failures have occurred byliquefaction of cohesion less soil during recentearthquakes.

Site and soil conditions:

Because the foundation must be capable of adequatelysupporting a structure in an economical manner it isimperative that there be proper geotechnicalinvestigation. This geotechnical investigation to beprovide information about the soil types beneath the siteand their physical characteristics (strength,compressibility, permeability etc) the investigation shouldprovide economical and feasi~le altematives for thesupport of the structure. In the seismic environmentgeotechnical investigation would also need to evaluatebehaviour of the supporting soils under earthquakeexcitation, predict the consequences for thestructure andrecommend the foundation types.

Not only it is important to investigate the soil conditions,the general site conditions also need deep scrutiny. Theinvestigation should indicate the features near thefeatures near the building area and also distant features.Important nearby site features include water levels.Topographical features and the presence of otherstructure both above and below the ground for examplethe presences of fault, folds, fractures shear zones.Cavernous rocks, weak beading planes, dykes etc. willhave the influence on the structure.

Causes of Liquefaction

Soil liquefaction during an earthquake leads to loss ofstrength and stiffness of soil. This could result in thesettlement of the structure, causes land slides, overturning, precipitate failures of earth dams, or cause othertypes of hazards. The soil liquefaction hasbeen observedto occur most often in loose saturated sand deposits.During strong earthquake shakinp the loose saturated

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sand deposits will have a tendency to compact and thusdecreases in volume. If these deposits can not drainrapidly, there will be an increase in pore water pressure.Effective stresses in the sand deposits are equal to thedifferencebetweenthe overburden pressureand theporewater pressure. With increasing oscillations, the porewater pressure can increase to the value of overburdenpressure.Since the shear strength of a cohesion lessSOil'is directlyproportional to the effective stress, the sandwillnot haveany shearstrength, it is now in the liquefied state.Sand boils appearing at the ground surface during anearthquakeareevidence that liquefaction hasoccurred.

Factors known to Influence Liquefaction:

Both laboratory investigation and observations of fieldperformances have shown that the significant factors orwhich liquefactionpotential ofa soil depositdepends are:

i) Soil type, Ii) Relative density or void ratio, Iii) Initialconfining pressure, iv) Intensityof ground shaking andv)Durationof groundshaking

Intensity and Duration of Ground Shaking:

For a soil in a given condition and under a given confiningpressure, the vulnerability to liquefaction during anearthquake depends on themagnitude of the stressesorstrains induced in it by earthquake. These in turn arerelated to the intensity of ground shaking. Duration ofground acceleration also affect the susceptibility toliquefaction, naturally susceptibility increases withincreasingduration.

Liquefaction -Induced ground failures and effects .0",Structures

If a soil becomes liquefied and looses its shear strength,groundfailures may result. If there are structures foundedover or near ~~e soil deposits, they may be damaged.Youd has classified ground failures caused byliquefactionintothree categories .

1) 'Lateral spreading

2) Flow failures

3) loss of BearingCapacity

lateral spreading isa movementof surficialsoil layersinadirection parallel to the ground surface, which occurswhen there is a loss of shearing strength in a subsurfacelayerdue to liquefaction. lateral spreading usuallyoCCUrs

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on very gentle slopes with a slope ofless than 5%. If thereis a differential lateral spreading under a structure, therecan be sufficient tensile stresses developed in thestructore to tear it apart. Flexible buildings have beenobserved to withstand extensional displacements betterthan stiff or brittle buildings.

Mitigation and Liquefaction Hazard by SiteModification

There are site modification methods, which can reducesusceptibility of the soils beneath a site to liquefaction.Tllesemethods are summarized below.

Methods to mitigate soil liquefaction by site modification

1) Excavation and replacement of liquefiable soils.

Excavation and engineered compaction of existingsoils

Excavation and engineered compaction of soilsimproved with additives

Excavation of e~sting soils and replacement withp~perly compacted and non liquefiable soils

2) Densifications of in situ soils ~.

Compaction. piles B. Vibratory probes C. Vibrofloatation D. Compaction grouting E. Dynamic'compaction or impact densification

3) In situ improvement of the soils by alterationsMixing in situ soils with additives

Removing in situ soils by jetting and replacementwith no liquefiable soils

.i) Grouting or chemical stabilization

--;tlgatlon of liquefaction Hazard by Drainage

Dewatering systems may reduce potential for liquefactionby removing the water from those layers. which couldliquefy. Also the resulting increase in effective overburdenpressure will add to the resistance of, the soils againstliquefaction. If total dewatering of a site is not practical,providing some means of drainage may Olitigate theproblem. Drainage allows for the rapid dissipation ofexcess pore pressures in the potentially liquefiable soillayers. If the pore pressures can be relieved quickly, theeffective stresses will not be decreased significantly andSoil ~iII retain most of its shear strength not allowingliqUefaction to occur. Vertical gravel drains placed in a grid

pattern may be able to accomplish this. Vibroreplacement also utilizes this principle, as the coarsegrained stone column will be very permeable incomparison with the surrounding soils.

Foundation of structures on Rocks

Until now foundation problems associated with soils havebeen discussed. It is general misconception that there willbe no problems for foundations on rocks. But there arelarge no cases when structures founded on rocks, fresh ormoderately weathered, have failed. This problem will bemore aggravated in highly seismic areas. The rocks likeshale, slates, phylites, tuffs, volcanic breccias with redtachylytic basalt matrix, volcanic breccias with tuff matrix,black, red, green, tachylyte and the geological structureslike fault zones, shear zones, crush zones, folds, limestones with solution cavities, and areas where miningactivity is in progress etc. pose a lot of problems· forfoundations. Some rocks in dry condition may haveadequate strength, but soften with water.and loose therestrength to a great leading· to . uneven settlement.Difference in the degree of weathering and deteriorationin dry and wet condition also leads 10 settlement of thestructures. Structures constructed 'on unstable slopes.with slope greater than dip of bed will fail when majorshocks occur. Therefore during investigations due caremust be taken while drilling not only through soils but alsothrough weathered rocks and fresh rocks. All the featuresof rock like mineral composition, texture, state ofpreservation of rock masses i.e. degree of weathering.structure of rock masses like strike, dip, joints, beddingplanes lamination, lineation, foliations, plane ofschistocity, folds, faults, shear zones, crush zones,tension cracks, dykes, solution cavities and caves,spheroidical weathering, sheeting etc must be recordedcarefully and taken cognizance of while deciding the(S.B.C.), type offoundation during the design of structure,in general and in earthquake prone areas in particular.

21. Land and Sliding

Landslide:

Earthquakes may trigger landslides or other forms ofslopes instability. Slope failures may occur as a result ofdevelopment of excess pore pressures which will reducethe shear strength of the soil or cause loss of strengthalong bedding or joints in the rock materials. Earthquakes

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may also cause shallow debris slides in areas with high,steep slopes. These slides can be minor or quite major.The 1970 debris avalanche triggered by the Peruvianearthquake of May 31, 1970 buried the towns of YungayRanrahirca, in which 18000 lives were lost Carefulconsideration should be given to structures, which aresighted in a location that could directly or indirectly beaffected by some form of slope instability.A very carefulGeotechnical andGeological investigationwill be neededto determine if such hazards exist and if there are anypracticalmeans of mitigation.

Earthquake Induced slope Instability

While scientist are busy, analyzing the recent majorearthquakes, that have occurred in recent years in Indiae.g. the one at Latur. Uttar Kashi and Jabalpur, whichhave taken a high toll of human life and have alsothreatened the structures. Engineers are, at the sametime engaged in research and developmental activity formakingsafe a seismic structure. Studies carried out after22nd May 1977 earthquake of Jabalpur have gatheredevid~nces to prove that large scale upliftment ofgeological terrain takes place near and around epicenteras an immediate effect of seismic activity. This causesmaximum damage to human life and structures due todifferential upliftment.Where the earthquakes occur insparsely populated areas or uninhabited areas, thephenomenon of differential upliftment goes unnoticed asit does not manifests itself immediately through damageto manmade structures but causes damage to stability ofthe slopes of the' hilly mountains and ridges to becomeseriously unstable. Recent major landslide of Gandhawairegions are in fact connected with the earthquake. Theyhave their genesis in the previous seismic activities thathaveoccurred there.

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22. REFERENCES

1) GupteR.B.,Preparationof Projects and EnginaerinGeology,P.W.D.handbook chapter VI(part II) 1980,1994.

2) Indian seismicity and past earthquakes-earthquakreslsitance design and construction otbuildings(August 2001). Indian Institute 0

Technology,Kanpur.

3) IndianStandards ofEarthquakeEngineering.

4)15: 6926:1973 Code of practice for diamond core.drilling for site Investigationforriver valley projects.

5) IS: 5313:1969guidefor coredrillingobservations.

6) IS: 4078:1967 Code of practice for Indexing andstorageof drill cores.

7) IS: 4464:1967 Codeof practicefor presentingOT driJinformation and core description in foundationinvestigation.

8) IS: 13365 (part 1)-1998Quantitative ClassificatioSystemsof Rockmass- Guidelines

9)' IS: 12070-1987 (Reaffirmed 1995) Codeof practicefor Designand Construction of Shallow foundation.on Rocks

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