DESIGN OF A PILE GROUP BY USING SOIL PROPERTIES

1DESIGN OF A PILE GROUP BY USINGSOIL PROPERTIES12D.SATYANARAYANA , S.CHANDRAKALA1CIVIL DEPARTMENT.2CIVIL DEPARTMENT.(SRI VASAVI ENGINEERING COLLEGE PEDATADEPALLI,W.G.Dt. ANDHRA PRADESDH PINCODE-534101)1 Piles are used to transfer the load beyondthe zone of possible moisture changes incollapsible soils.INTRODUCTIONA pile foundation is a slenderstructural member made of steel, concrete orwood, which is either driven into the soil orformed in-situ by excavating a hole and filling itwith ndra 104kala@gmail.comAbstractThe silty clay soil is a type of soil whichis not capable of supporting a structure, deepfoundations are required to transfer the loads.The most common types of deep foundations arePiles, Piers and Caissons.By using the soil properties such asLiquid limit, Saturated density, Optimummoisture content etc,. The pile foundation canbe designed.A pile is a slender structural membermade of steel, concrete or wood, which is eitherdriven into the soil or formed in-situ byexcavating a hole and filling it with concrete.Application of pile foundation: If the plan of the structure is irregularrelative to its outline and loaddistribution, it can be used to reducedifferential settlement. It can resist horizontal forces in additionto support vertical loads for earthquakeprone areas & tall buildings. Piles are used for foundations of somestructures such as, transmission towers,off-shore platforms which are subjectedto uplift.CLASSIFICATION OF PILE FOUNDATION:Based on the function: End bearing pileTension or uplift pileCompaction pileFender pile and dolphinsAnchor pileFriction pileBatter pileBased on materials and composition: Concrete pileTimber pileSteel pileComposite pileREVIEW OF LITERATUREPile Foundation: Pile is one of the types ofdeep foundations, which are used whensurface soil is unsuitable for shallowfoundation. A pile is a slender membermade of steel, concrete or wood. It is eitherdriven into soil or formed in-situ byexcavating a hole and filling it with

2concrete. These are mostly used for thefoundations in case of expansive soils suchas black cotton soil and collapsible soilssuch as loess.Design of pile groupMost of the pile foundations consist not of asingle pile, but of a group of piles, Which act inthe double role of reinforcing the soil, and alsoof carrying the applied Load down to deeper,stronger soil strata failure of the group mayoccur either by the failure of individual pile oras failure of the overall block of soil thesupporting Capacity of group of y less than the sum of the capacitiesthe individual piles comprise the group. Groupaction in piled foundation could result inNecessity of pile groups:Pile groups are used whenl) Column load is heavy2) Method of installation of piles is bydriving.failure or excessive settlement' even thoughloading tests made on a single pile haveindicated satisfactory capacity in all cases theelastic and consolidation settlements of thegroup are greater than those of single pilecarrying the same working load as that on eachpile within the group. This is because the zoneClassification of pile groups:Pile groups are classified as:l) Free standing pile group2) Piled foundationof soil or rock which is stressed by the entiregroup extended to a much greater width anddepth than the zone beneath the single pile.Free standing pile group:These are used where the foundationsoil is expansive in Nature pile cap does nottransfer any of the column loads directly tofoundation soil.Piled foundation:As pile cap is made to rest on theground surface, it helps in transfer of part ofcolumn load directly to foundation soil theseare use in expansive soils.

3Pile groups in cohesion less soils:Pile spacing and pile arrangement:In certain types of soil,especially in sensitive clays, the capacity ofindividual Piles within the closely spacedgroup may be lower than for ficant effect, this may be ignored indesign. Instead the main worry has been thatthe block capacity of the group may be lessthan the sum of the individual piles capacities.As a thumb rule, if spacing is more than 2 - 3pile diameters, then block failure is mostunlikely it is vital importance that pile group infriction and cohesive soil arranged that evendistribution of load in greater area is achieved.Large concentration of piles under the center ofthe pile cap should be avoided. This could leadto load concentration resulting in localsettlement and failure in the pile cap. Varyinglength of piles in the same pile group may havesimilar effect for pile load up to 300kn; theminimum distance to the pile cap should be100mm. For higher than 300kn, this distanceshould be more than 150 mm.In general, the following formula may beused in pile spacing:As per IS CODEEnd-bearing: S 2.5 dFriction piles: S 3.0 dPiles in loose sand: S 2.0 dWhere:d assumed pile diameters pile centre to centre distance (spacing)For driven piles embedded in cohesion lesssoils, the capacity of the large Equivalent pile(block) will be almost always greater than thesum of the capacities of individual piles, inview of the densification that occurs duringdriving. Consequently, for design, the groupcapacity is taken as the sum of the individualpile capacities or the product of the number ofpiles in the group and the capacity of theindividualpile.Thisprocedureisnotapplicable, if the pile tips rest on Compressiblesoils such as clays; in such cases, the pile groupcapacity is governed by the shear strength andcompressibility of clay soil, rather than on theCharacteristics of the cohesion less soil. d by boring a hole of requireddiameter and depth and pouring in of concrete.Boring is accompanied invariably by somedegree of loosening of the soil. In view of this,the group Capacity of such piles will besomewhat less than the sum of individual pileCapacities typically-about two-thirds of it. Itmay also be taken as the sum of Individual pilecapacities approximately.Pile groups in cohesive soils:When piles are driven into clay soils, there willbe considerable re-moulding especially whenthe soil is soft and sensitive. The soil betweenthe piles may also have since compactioncannot be easily achieved in soils of such lowpermeability.Boredpilesaregenerallypreferred to driven piles in such soils.

4However, if driven piles are to be used, spacingSettlement of Groups of Piles:of piles must be relatively large and the drivingThe computation of settlement of groups ofso adjusted as to minimize the development ofpiles is more complex than that for apore pressure. The mode of failure of pileSingle pile.groups in cohesive soils depends primarilySettlement of pile groups in sandsupon the spacing of piles. For smallerSettlements of pile groups are found to bespacing’s, 'block failure' may occur, in othermany times that of a single pile. The ratio, Fg,words, the group capacity as a block will beof the settlement of a pile group to that of aless than the sum of individual pile capacities.single pile is known as the group settlementFor larger spacing’s, failure of individual pilesratio.may occur; or, it is to say that the groupFg Sg/Scapacity is given by the sum of the individualWhere Fg group settlement ratio,pile capacities, which will be smaller than theSg settlement of pile group. andstrength of the group acting as a unit or aS total settlement of individual pileblock. The limiting value of the spacing forVesic has obtained the relationwhich the group capacities obtained from thebetween Fg and B/d, where B is the width oftwo criteria-block failure and individual pilethe pile group (centre to centre of outer mostfailure- are equal is usually considered to bepiles), and d is the diameter of the pile (Onlyabout 3 piles –diameters.pile groups ,square in plan, are considered).These results have been obtained for mediumdense sand. For sands with other Densityindices the results could be different.

5Settlement of pile groups in clayThe total pressure may be assumed to beThe equation for consolidation settlement maydistributed on a slope of 2 vertical to 1be used treating the pile group as a block orHorizontal, for the purpose of computationunit. The increase in stress is to be evaluatedof increment of stress, in an approximateappropriately under the influence of the load onmanner.the pile group. When the piles are embedded ina uniform soil (friction and end-bearing piles),the total load is assumed to act at a depth equalLABORATORY TESTS AND RESULTSto two-thirds the pile length. Conventionalsettlement analysis procedures assuming theCharacteristics of soil considered in PileBoussinesq or Westergaard stress distributionfoundation design:are then applied to compute the consolidationFor design the of pile foundation wesettlement of the soil beneath the pile tip.considered some of the properties of soilWhen the piles are resting on a firmer stratumsuch asthan the overlying soil (end-bearing piles), theLiquid limittotal load is assumed to act at the pile tip itself.Plastic limitIf the piles are embedded into the firmer layerOptimum moisture contentin this case, the load is assumed to beCo-efficient of cohesiontransmitted to a depth equal to two-thirds of theEo value To find the characteristics of soilembedment from the top of the firmer layer.we have done some tests of the soil. TheyThe rest of the settlement analysis procedure isareapplicable.Liquid Limit:We have conducted Casagrande’s liquidlimit test for a sample of soil.

6Compaction factor:From compaction factor testwe can obtain the values of Optimum moisturecontent, Bulk density, Dry density, d of thesoil.We have obtained a liquid limit of 25%fromthe test.Plastic Limit:Plastic limit is the water content belowwhich the soil stops behaving as a plasticmaterial.The values obtained from compaction factortest are:OMC 8%Bulk density, 26.22Dry density, d 24.28 33Type of soil:The plastic limit of soil obtained is 20%.Accordingtotheplasticitychart,corresponding to the values of liquid limit

7 25% and plastic limit 20%, the soil isinorganic clay with medium plasticity.Name oftheexperimentLiquid limittestPlastic limittestCompactionfactor testResultTerminologyLiquid limit25%Plastic limit20%OMC,,8%,26.2224.28 33,

8DESIGNA pile group to be installed in a saturated siltclay deposit [ CU 40 KN / M 2 ,L.L 2.5%,G 2.7, NMC 8%, d 24.28 Kg / M 26.22 Kg / M 3Q p 40 9 APAP Area of the shaft that effective in3developing in skin frictiois designed and estimatedthe settlement of pile groups assuming water 2d4 A p 0.40 24AP table at 2m depth below ground level isperformed.For piles in saturated silt clayCU 40 KN / M 2For u 0Assume dia. Of pile 40 mmAP 0.125 M2NC 9Q p 40 X 9 X 0.125Nq 1Q p 45.239 KNThe ultimate load capacity of the pile groupaccording to static formula can be calculated asQu Q p Q sQs Ca X AsC a Skin friction developed between the pilesurface and clayQu Ultimate failure loadQ p Load carrying capacity of pileQs Shaft resistance developed by frictionbetween the pile and shaftQ p CN c APC cohesion co-efficient of clayN C Bearing capacity factor for deep foundation[depends onDratio, varies from 6 to 9 ]BCa c ' Addition factor 0.3 (at 6 meters depth)c ' Average cohesion along the length of theshaft ( 50 KN/M2 )Consider c ' 40 KN/M2As area of the shaftAS 2 r lAS 2 0.200 6