Fundacioon de Pilotes

8/13/2019 Fundacioon de Pilotes

1/23

Pile Foundations

BC3681

2002 Dr. S.K.Au


2/23

BS8004 defines deep foundation with D>B or D>3m.

Pile foundation always more expensive than shallowfoundation 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 isimportant. This is the basis of a simple classification

Pile Foundations


3/23

End Bearing Piles

ROCK

SOFT SOILPILES

End bearing pile rests on a relative

firm soil . The load of the structure istransmitted through the pile into thisfirm soil or rock because the base ofthe pile bears the load of thestructure, this type of pile is calledend bearing pile

Most of the piles usedin Hong Kong are endbearing piles. This isbecause the majority ofnew developments areon reclaimed land


4/23

SOFT SOILPILES

Friction Piles

If the firm soil is at a considerable

depth, 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 the

soil.


5/23

Types of Pile

The pile installation procedure varies considerably,

and has an important influence on the subsequentresponse

Three categories of piles are classified by method ofinstallation 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.


6/23

Combinations of vertical, horizontal and moment loading

may be applied at the soil surface from the overlyingstructure

For the majority of foundations the loads applied to thepiles are primarily vertical

For piles in jetties, foundations for bridge piers, tallchimneys, and offshore piled foundations the lateralresistance is an important consideration

The analysis of piles subjected to lateral and momentloading is more complex than simple vertical loadingbecause of the soil-structure interaction.

Pile installation will always cause change of adjacent soilproperties, sometimes good, sometimes bad.

Loads applied to PilesV

MH


7/23

Modes of failure

The soil is always failure by punchingshear.

The failure mode of pile is always inbuckling failure mode.


8/23

Total and Effective Stress Analysis

To determine drained or undrainedcondition, we may need to consider thefollowing factors: Drainage condition in the various soil strata Permeability of soils

Rate of application of loads

Duration after the application of loadA rough indicator will be the Time Factor

(Tv=cvt/d2)


9/23

Displacement Pile (A/D)

Advantage Disadvantages

Pile material can be inspected for

quality before driving

May break during driving

Construction operation affect byground water

Noise and vibration problems

Can driven in very long lengths Cannot be driven in condition of

low headroom

Construction operation not affected

by ground water

Noise may prove unacceptable.

Noise permit may be required

Soil disposal is not necessary Vibration may prove unacceptable

due to presence of sensitive

structures, utility installation or

machinery


10/23

Replacement Pile (A/D)

Advantage Disadvantages

Less noise or vibration problem Concrete cannot be inspected after

installation

Equipment can break up practically all

kinds of obstructions

Liable to squeezing or necking

Can be installed in conditions of low

headroom

Raking bored pile are difficult to

construct

No ground heave Drilling a number of pile groups maycause ground loss and settlement of

adjacent structures

Depth and diameter can varied easily Cannot be extended above ground

level without special adaptation


11/23

Ultimate capacity of axially load singlepile in soil

Estimated 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 can

be 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.


12/23

W

Qu

Qb

Qs

Qu = Qs + Qb - W

Basic ConceptThe ultimate bearing capacity (Qu )of a pilemay be assessed using soil mechanics

principles. The capacity is assumed to be thesum of skin friction and end-bearingresistance, i.e

Qu=Qb+Qs-W .(1)

whereQutotal pile resistance,Qbis the end bearing resistance andQsis side friction resistance

General behaviourShaft resistance fully mobilized at small pilemovement (


13/23

Loading

Settlement

Behaviour of Frictional Pile

Loading

Settlement

Behaviour of End Bearing Pile

Qu

QS

QB

Qu

QB

QS

Piles founded on strong stratum

Not much benefit in enhancing base

resistance Important to adopt good

construction practice to enhanceshaft friction

Shaft grouting useful in enhancingpile capacity

Piles founded on dense soils

Important to adopt good

construction practice to enhanceshaft friction and base resistance

Shaft and base grouting useful inenhancing pile capacity


14/23


15/23

End Bearing ResistanceAssumptions

1. The weight of the pile is similar to the weight of the soil displaced of

the pile

=> Wp

=Ab

Po

2. The length (L) of the pile is much greater than its width d

=> Wp=AbPo+ Abg dNg/2

3. Similarly for f>0,Nqapproximately equal to Nq-1

Qb=Ab[cbNc+Po(Nq-1)+gd/2Ng+Po] Wp

=> Qb=Ab[cbNc+PoNq]


16/23

End Bearing resistance for Bore pile ingranular soils

Due to the natural of granular soil, the ccan be assumed equation to

zero. The ultimate end bearing resistance for bored pile in granularsoils may be express in terms of vertical effective stress, svand the

bearing capacity factors Nqas :

QB=ABNqsv

Nq is generally related to the angle of shearing resistance f. For

general design purposed, it is suggested that the Nqvalue proposedby Berezantze et al (1961) as presented in Figure ?? are used.However, the calculated ultimate base stress should conservatively belimited to 10Mpa, unless higher values have been justified by loadtests.


17/23

Shaft Friction ResistanceThe ultimate shaft friction stress qsfor piles may be expressed in

terms of mean vertical effective stress as :qs=c+Kssvtands

qs=bsv (when c=0)Where

Ks= coefficient of horizontal pressure which depends on the relative density

and 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-sinfas shown in Table 1.

Qv = mean vertical effective stress

ss = angle of friction along pile/soil interface (see table2)

b= shafte friction coefficient (see Table 3)

Qs = pLqs

Where p is the perimeter of the pile and L is the total length of the pile


18/23

Driven pile in Granular soils

The concepts of the calculation of end-bearingcapacity and skin friction for bored piles in

granular 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 qb be rangefrom 16 to 21Mpa.


19/23

Bored pile in Clays

The ultimate end bearing resistance for piles inclays is often related to the undrained shear

strength, cu, asqB=Nccu

QB=ABNccu

where

Nc= 9 when the location of the pile base below ground surface exceedsfours times the pile diameter


20/23

Bored pile in Clays

The ultimate shaft friction (qs) for soils in stiff

over-consolidated clays may be estimated on the

semi-empirical method as:qs=aCu

ais the adhesion factor (range from 0.4 to 0.9)


21/23


22/23


23/23

Prediction of Ultimate Capacity of Pile

Pile Load Test

Static pile load test is the most reliable means of determining theload capacity of a pile. The test procedure consists of applyingstatic load to the pile in increments up to a designated level of

load and recording the vertical deflection of the pile. The load isusually transmitted by means of a hydraulic jack placed betweenthe top of the pile and a beam supported by tow or morereaction piles. The vertical deflection of the top of the pile isusually measured by mechanical gauges attached to a beam,which span over the test pile.

Fundacioon de Pilotes

Documents

Transcript of Fundacioon de Pilotes