Pile Foundations SUBMITTED FOR PARTIAL FULLFILLMENT OF B.TECH BY PIYUSH KUMAR IN CIVIL ENGINEERING FROM GAUTAM BUDDHA TECHNICAL UNIVERSITY,LUCKNOWGOEL INSTITUTE OF TECHNOLOGY AND MANAGEMENT LUCKNOW
Pile Foundations BS8004 defines deep foundation with D>B or D>3m. Pile foundation always more expensive than shallow foundation but will overcome problems of soft surface soils by transferring load to stronger, deeper stratum, thereby reducing settlements. Pile resistance is comprised of End bearing Shaft friction For many piles only one of these components is important. This is the basis of a simple classification
End Bearing PilesEnd bearing pile rests on a relative Most of the piles usedfirm soil . The load of the structure is in Hong Kong are endtransmitted through the pile into this bearing piles. This isfirm soil or rock because the base of because the majority ofthe pile bears the load of the new developments arestructure, this type of pile is called on reclaimed landend bearing pile PILES SOFT SOIL ROCK
Friction PilesIf the firm soil is at a considerabledepth, it may be very expensive touse end bearing piles. In suchsituations, the piles are driventhrough the penetrable soil for somedistance. The piles transmit the loadof structure to the penetrable soil bymeans of skin friction between thesoil. PILES SOFT SOIL
Types of Pile The pile installation procedure varies considerably, and has an important influence on the subsequent response Three categories of piles are classified by method of installation as below: Large displacement piles They encompass all solid driven piles including precast concrete piles, steel or concrete tubes closed at the lower end Small displacement piles They include rolled steel sections such as H-pile and open-end tubular piles Replacement piles They are formed by machine boring, grabbing or hand-digging.
Loads applied to Piles V M Combinations of vertical, horizontal and moment loading H may be applied at the soil surface from the overlying structure For the majority of foundations the loads applied to the piles are primarily vertical For piles in jetties, foundations for bridge piers, tall chimneys, and offshore piled foundations the lateral resistance is an important consideration The analysis of piles subjected to lateral and moment loading is more complex than simple vertical loading because of the soil-structure interaction. Pile installation will always cause change of adjacent soil properties, sometimes good, sometimes bad.
Modes of failure The soil is always failure by punching shear. The failure mode of pile is always in buckling failure mode.
Total and Effective Stress Analysis To determine drained or undrained condition, we may need to consider the following factors: Drainage condition in the various soil strata Permeability of soils Rate of application of loads Duration after the application of load A rough indicator will be the Time Factor (Tv=cvt/d2)
Displacement Pile (A/D) Advantage DisadvantagesPile material can be inspected for May break during drivingquality before drivingConstruction operation affect by Noise and vibration problemsground waterCan driven in very long lengths Cannot be driven in condition of low headroomConstruction operation not Noise may prove unacceptable.affected by ground water Noise permit may be requiredSoil disposal is not necessary Vibration may prove unacceptable due to presence of sensitive structures, utility installation or machinery
Replacement Pile (A/D) Advantage DisadvantagesLess noise or vibration problem Concrete cannot be inspected after installationEquipment can break up practically Liable to squeezing or neckingall kinds of obstructionsCan be installed in conditions of low Raking bored pile are difficult toheadroom constructNo ground heave Drilling a number of pile groups may cause ground loss and settlement of adjacent structuresDepth and diameter can varied easily Cannot be extended above ground level without special adaptation
Ultimate capacity of axially load single pile in soilEstimated by designer based on soil data and somewhat empiricalprocedures. It is common practice that the pile capacity be verifiedby pile load test at an early stage such that design amendment canbe made prior to installation of the project piles. The satisfactoryperformance of a pile is, in most cases, governed by the limitingacceptable deformation under various loading conditions. Thereforethe settlement should also be checked.
Basic Concept QuThe ultimate bearing capacity (Qu )of a pilemay be assessed using soil mechanicsprinciples. The capacity is assumed to be thesum of skin friction and end-bearingresistance, i.eQu =Qb+Qs-W ……………………….(1) QswhereQu total pile resistance,Qb is the end bearing resistance and WQs is side friction resistanceGeneral behaviourShaft resistance fully mobilized at small pile Qbmovement (<0.01D)Base resistance mobilized at large movement(0.1D)
Loading Loading Qu Qu QS QB QB QS Settlement Settlement Behaviour of Frictional Pile Behaviour of End Bearing Pile Piles founded on dense soils Piles founded on strong stratum Important to adopt good Not much benefit in enhancing base construction practice to enhance resistance shaft friction and base resistance Important to adopt good Shaft and base grouting useful in construction practice to enhance enhancing pile capacity shaft friction Shaft grouting useful in enhancing pile capacity
Ultimate Limit State Design QTQDES = QB/FB + Qs /Fs –W……(2) dWhere FB and FS is the factor of safety ofcomponents of end bearing strength and shaft hofriction strengthQU = QB + Qs–W……(3) D QsQb=Ab[cbNc+Po(Nq-1)+γd/2Nγ+Po] -WpWhere Ab is the area of the base , cb is the Wcohesion at the base of the pile, Po is theoverburden stress at the base of the pile and dis the width of the pile. QB
End Bearing ResistanceAssumptions1. The weight of the pile is similar to the weight of the soil displaced of the pile => Wp=AbPo2. The length (L) of the pile is much greater than its width d => Wp=AbPo+ Abγ dNγ/23. Similarly for φ>0, Nq approximately equal to Nq-1Qb=Ab[cbNc+Po(Nq-1)+γd/2Nγ+Po] –Wp=> Qb=Ab[cbNc+PoNq]
End Bearing resistance for Bore pile in granular soilsDue to the natural of granular soil, the c ’ can be assumed equation tozero. The ultimate end bearing resistance for bored pile in granularsoils may be express in terms of vertical effective stress, σ’v and thebearing capacity factors Nq as :QB=AB Nq σv’Nq is generally related to the angle of shearing resistance φ’. Forgeneral design purposed, it is suggested that the N q value proposedby Berezantze et al (1961) as presented in Figure ?? are used.However, the calculated ultimate base stress should conservativelybe limited to 10Mpa, unless higher values have been justified by loadtests.
Shaft Friction ResistanceThe ultimate shaft friction stress qs for piles may be expressed interms of mean vertical effective stress as :qs =c’+Ksσv’tanδsqs =βσv’ (when c’=0)WhereKs= coefficient of horizontal pressure which depends on the relative densityand state of soil, method of pile installation, and material length and shapeof pile. Ks may be related to the coefficient of earth pressure at rest,K0=1-sinφ as shown in Table 1.Qv’ = mean vertical effective stressσs’ = angle of friction along pile/soil interface (see table2)β= shafte friction coefficient (see Table 3)Qs = pLqsWhere p is the perimeter of the pile and L is the total length of the pile
Driven pile in Granular soils The concepts of the calculation of end-bearingcapacity and skin friction for bored piles ingranular soils also apply to driven piles ingranular soils. The pile soil system involvingeffects of densification and in horizontal stressesin the ground due to pile driving. In Hong Kong,it is suggested that the value of q b be range from16 to 21Mpa.
Bored pile in ClaysThe ultimate end bearing resistance for piles inclays is often related to the undrained shearstrength, cu, asqB=NccuQB=ABNccuwhereNc= 9 when the location of the pile base below ground surface exceeds fourstimes the pile diameter
Bored pile in Clays The ultimate shaft friction (qs) for soils in stiffover-consolidated clays may be estimated onthe semi-empirical method as:qs=αCuα is the adhesion factor (range from 0.4 to 0.9)
Driven Pile in ClaysThe design concepts are similar to thosepresented for bored piles in granular soils.However, based on the availableinstrumented pile test results, a designcurve is put forward by Nowacki et al(1992)
Prediction of Ultimate Capacity of PilePile Driving Formula Pile driving formula relate the ultimate bearing capacity ofdriven piles to final set (i.e. penetration per blow). In HongKong, the Hiley formula has been widely used for the design ofdriven piles as:Rd=(ηhWhdh)/(s+c/2)WhereRd is driving resistance, ηh is efficiency of hammer, Wh is theweight of hammer, dh is the height of fall of hammer, s ispermanent set of pile and c is elastic movement of pileNote: Test driving may be considered at the start of a driven pilingcontract to assess the expected driving characteristics.
Prediction of Ultimate Capacity of Pile Pile Load Test Static pile load test is the most reliable means of determining the load capacity of a pile. The test procedure consists of applying static load to the pile in increments up to a designated level of load and recording the vertical deflection of the pile. The load is usually transmitted by means of a hydraulic jack placed between the top of the pile and a beam supported by tow or more reaction piles. The vertical deflection of the top of the pile is usually measured by mechanical gauges attached to a beam, which span over the test pile.