Design of Pile Caps Final

7/25/2019 Design of Pile Caps Final

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Design of Pile Caps

10.1 Introduction

A pile cap is defined as a concrete block cast on the head of a group of piles, totransmit the load from the structure to the group of piles. Generally, pile captransfers the load form the structures to a pile group, then the load furthertransfers to firm soil.

External pressures on a pile are likely to be greatest near the ground surface.Ground stability increases with depth and pressure. The top of the piletherefore, is more vulnerable to movement and stress than the base of the pile.Pile caps are thus incorporated in order to tie the pile heads together so thatindividual pile movement and settlement is greatly reduced. Thus stability ofthe pile group is greatly increased.

The functions of a pile cap are:

. To distribute a single load e!ually over the pile group and thus over agreater area of bearing potential,

". To laterally stabilise individual piles thus increasing overall stability ofthe group. And

#. To provide the necessary combined resistance to stresses set up by thesuperstructure and$or ground movement.


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Pile capsare thick slabs used to tie

a group of piles together to support

and transmit column loads to the piles.

10.2 Pile Cap Arrangement

Spacing of the piles in the pile group

The following should be considered when determining the spacing of the piles%


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. &verall cost of the foundation

". 'ature of the ground

#. Pile behaviour in the group

(. )esulting possible heave or compaction of ground causing damage toad*acent structures

+. ost of pile cap

-. i/e and effective length of ground beam

0. Type and si/e of pile

Piles should be placed in a suitable arrangement so that the spacingbetween piles ranges from 1"2#3D (pile diameter in case of isolated pilecaps and 1"2-3Din case of rafts supported on piles.

The .G. of piles should be placed as far as possible in the .G. ofloads transmitted from the structure to the group of piles.

4n the case of presence of neighbors, piles should be away from theproperty line by a distance not less than Dor as the pile installationmethod re!uires.

The pro*ection of the pile cap should be 52+ cm.

Initial !a"out:The simplest pile layout is one without batter piles. uch a layout should beused if the magnitude of lateral forces is small. ince all piles do not carry ane!ual portion of the load, axial pile capacity can be reduced to 05 percent ofthe computed value to provide a good starting point to determine an initiallayout.


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4n this case, the designer begins by dividing the largest vertical load on thestructure by the reduced pile capacity to obtain the approximate number ofpile. 4f there are large applied lateral forces, then batter piles are usuallyre!uired. Piles with flat batters ".+ 163 to 173, provide greater resistance tolateral loads and the less resistance to vertical loads. Piles with steep batters +163 to 173 provide greater vertical resistance and less lateral resistance.

#inal !a"out:

After the preliminary layout was developed remaining load cases should beinvestigated and the layout revised to provide an efficient layout. The goalshould be to produce a pile layout in which most piles are loaded as near to

capacity as practical for the critical loading cases with tips located at the sameelevation for the various pile groups within a given monolith. Ad*ustments tothe initial layout by the addition deletion, or relocation of piles within thelayout grid system may be re!uired. Generally, revisions to the pile batterswill not be re!uired because they were optimi/ed during the initial pile layout.The designer is cautioned that the founding of piles at various elevations or indifferent strata may result in monolith instability and differential settlement.

T"pical Arrangement of Piles


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$e%uirements for Pile Caps

ame as spread footings with the following additions%

. 8esign must satisfy the punching shear in the vicinity of the individualpiles or shafts

". The effective depth dmust be at least #5 cm. This implies a minimumthickness Tof (5 cm.

#. The bearing force between the individual piles or shafts and the capsmust not exceed the capacity of either element.

Pile Cap $einforcement

The amount of pile cap reinforcement is governed by%

. The loading on the pile cap,

". The spacing of the piles, and

#. The depth of the pile cap.

10.& !oad Distri'ution

To a great extent the design and calculation 1load analysis3 of pile foundationsis carried out using computer software. 9or some special cases, calculationscan be carried out using the following methods

9or a simple understanding of the method, let us assume that the followingconditions are satisfied%


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. The pile is rigid

". The pile is pinned at the top and at the bottom

#. Each pile receives the load only vertically 1i.e. axially applied 3:

(. The force P acting on the pile is proportional to the displacement ; dueto compression.

?x, ?y@BCDF HC> IJKLFx ,y

x , yBCDF @=M NOQFyBCDFJxNRCSF O UJVW XY F

x", y"% N=FZF NF[LF O \ NMCQD] NRCS^F BC_`F IJKR

4x 4o A . "

HM4oDF >J jZ= _FA3 3A1Y S NF[LF @] 1 UJVF q`] N>Q]% NF[LF O N=F N=`F ^R N`F IJKLF @R NSF UJVF NFC>

Where Mx, My moments about the axes x, yThe distance between the y axis and x-axis to any hazing in the group

x", y"moments palaces of the group , as calculated in the followingequation:

4x 4o A . "

4oneglecting the small value, and delete limit ! where !" pile section "of the equation space, we find that the pile load resulting from moments

applied to the value is shown in the equation :


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ccentricit" of load

( Single

ccentricit" of load

( Dou'le


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Graphical ?ethod

Installation error:;ntil now we have been calculating theoretical force distribution on piles.7owever during installation of piles slight changes in position do occur andpiles may miss their designed locations.


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o the designer must compare theoretical and the actual load distribution as aresult of misalignment after pile installation.

De)iation of the piles

?ost piling specifications permit a deviation in pile position of not exceeding0+ mm in any direction from the intended position. Additional deviations of%0+ from the vertical piles and %"+ from the designed rake for raking pilesare also permitted.

Thus, the pile cap should be large enough to accommodate those piles whichhave deviated from the intended position. The pile cap should extend for adistance of 55 to +5 mm outside the outer face of the piles in the group.

!ocation and Alignment Tolerance:

The pile head at cutoff elevation shall be within +5 mm of plan locations forbent caps supported by piles, and shall be within +5mm of plan locations forall piles capped below final grade. The as driven centroid of load of any pilegroup at cutoff elevation shall be within + of the plan location of thedesigned centroid of load.

'o pile shall be nearer than 55mm from any edge of the cap. Any increase insi/e of cap to meet this edge distance re!uirement shall be at the ontractors

expense.Piles shall be installed so that the axial alignment of the top #m of the pile iswithin " of the specified alignment. 9or piles that cannot be inspectedinternally after installation, an alignment check shall be made beforeinstalling the last .+m of pile, or after installation is completed provided theexposed portion of the pile is not less than .+m in length. The Engineer mayre!uire that driving be stopped in order to check the pile alignment. Alignedsection on a misaligned section shall not be permitted.

4f the location and$ or alignment tolerances specified in the precedingparagraphs are exceeded, the extent of overloading shall be evaluated by theEngineer. 4f in the *udgment of the Engineer, corrective measures arenecessary, suitable measures shall be designed and constructed by theontractor. The ontractor shall bear all costs, including delays, associatedwith the corrective action.


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10.* Design of Pile Cap

4f the pile group is analy/ed with a flexible base, then the forces re!uiredto design the base are obtained directly from the structure model.

4f the pile group is analy/ed with a rigid base, then a separate analysis isneeded to determine the stresses in the pile cap.

An appropriate finite element model 1frame, plate and plane stress orplane strain3 should be used and should include all external loads 1water,concrete, soil, etc. 3 and pile reactions.

There are many methods for designing pile caps from which we couldmention the following%

2 irculage ?ethod"2 eam ?ethod#2 9E? methods

10.*.1 Circulage +ethod

irculage method can only be used when the column is loaded with anaxial force and piles are arranged on the circumference of a circle. Piles arenot allowed to carry hori/ontal forces in this case.

As it is shown in the following figure, the force T for which thereinforcement is calculated is calculated using the shown force diagram.

9orce Transmission in irculage ?ethod


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Strut,and,tie model

The strut2and2tie model should be considered for the design of deep footingsand pile caps or other situations in which the distance between the centres ofapplied load and the supporting reactions is less than about twice the memberthickness.


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Struts and ties in a pile cap

The main reinforcement 1As3 can then be calculated from the followingrelation%


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10.*.2 -eam +ethod

The eam ?ethod is the most widely used method as it suitable for anytype of loading and any shape of the pile cap.

Design Procedure:

A, $e%uired Data:

Pile 8ata% 2 Pile diameter and length,

"2 Pile allowable bearing capacity

olumn 8ata% 2 olumn load 1' ? 73,

"2 olumn dimensions

-, Design Steps:

1, Determine re%uired num'er of piles:


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otes:

4n case of 1'3 only multiply by .

4n case of 1?'3 multiply by ."

'umber of piles used is rounded to the upper integer

2, Pile Cap Arrangement and Plane Dimension:

Piles should be placed in a suitable arrangement so that the spacingbetween piles ranges from 1"2#3Din case of isolated pile caps and 1"2-3

Din case of rafts supported on piles, whereDis the pile diameter.

The .G. of piles should be placed as far as possible in the .G. ofloads.

4n the case of presence of neighbors, piles should be away from theproperty line by a distance not less than Dor as the pile installationmethod re!uires.

The pro*ection of the pile cap should be about 52+ cm.

&, Pile Cap Preliminar" Depth:

The depth of the pile cap could be preliminary estimated assuming anallowable punching stress of 5 kg$cm"on the column face.


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*, Chec/ #orces in Piles:

, Chec/ for punching shear:

, Chec/ for shear:


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, Design for moment:

The critical section for moment is taken at the column face.

3, Chec/ for -ond:

The reinforcement used in resisting flexural moment should be checked forbond stress acting on it.

hear at the same section of the bending moment is calculated.

4, Details of reinforcement:


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Plane


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10.*.& #+ +ethod

Grid used for 9zA #8 analysis of pile groups 1After Poulos, "553


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10.

5rade -eams

8eep foundations are sometimes connected with grade beams.

Grade beams are re!uired for all deep foundations sub*ect to seismic loads.9or seismic design, they must resist a hori/ontal load e!ual to 5 of the

column vertical load.

Grade beams must be designed without the support of the underlying soil.

4n the ritish tandard ode of Practice {55(, a ground beam is definedas a beam in a substructure transmitting load1s3 to a pile, pad or otherfoundation. The ground beam connects the two pile caps.

Ground beams should not be confused with capping beams. apping beamsperform the same function as pile caps. 7owever, the function of a groundbeam is to connect ad*acent pile caps to ensure stability of the foundationand to ensure stability against lateral forces.

Ground beams are designed to connect a group of pile caps in a continuousmanner.


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The top and bottom reinforcement of a ground beam are usually made e!ualto overcome lateral forces or settlement of one pile cap relative to thead*acent one.

Ground beams may also re!uire shear reinforcement in the form of binders.

The depth of the ground beam is usually more than $+ of the span. Thewidth of the beam depends on design re!uirements.

Ground beams can also be designed to transmit loads from walls to pile caps.

A pile cap is required to transfer the load from a 400 mm 400 mm column to four 600 mmdiameter piles, as shown in 9ig. (.#5.

Pile caps can be designed either by the truss analogy or by bending theory see !" #$$0% Part $%

&.$$.4.$'((. )n this e*ample bending theory will be used.

+or a pile cap with closely spaced piles, in addition to bending and bond stress checs, a checshould be made on the local shear stress at the face of the column, and a beam shear chec for

shear across the width of the pile cap. +or more widely spaced piles spacing - & diameter(, a

punching shear chec should also be carried out.

zocal shear checkhe ultimate column load is Pu/ 6400 .

1ength of column perimeter is u / 2400 3 400( / $600 mm.

he shear stress at the face of the column is


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ending shear check)n accordance with !" #$$0% Part $% &.$$.4.&, shear is checedacross a section 20 of the

diameter of the pile i.e. 5'( inside the face of the pile. his is section A7A in 9ig. (.#5.

he shear force across this section 7 ignoring the self8weight of the pile cap, which is small in

comparison 7 is gi9en by

he corresponding shear stress is gi9en by 9u / :ub9d, where b9 is the breadth of section for

reinforcement design.

)n accordance with !" #$$0% Part $% &.$$.4.4, this must not e*ceed 2da9(9c where a9 is de;ned

in 9ig. (.#5and 9c is the design concrete shear stress from !" #$$0% Part $% able &.#. hus


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+or grade 46 mm use h / >'0 mm


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8esign Example% Piled ground beams with suspended slab

Pre9iously design e*ample is to be rewored on the assumption that the building is now to be

relocated in an area where the ' m depth of ;ll is of a much poorer quality, and is con8 sideredunsuitable for supporting a ?oating ground ?oor slab. he ground ?oor slab is therefore to be

replaced by wide plan precast concrete ?oors, spanning # m parallel to grid lines A7@.

he additional loads due to this suspended ?oor are shown in 9ig. (.#", and the increased pileloads are indicated.

he increased loads could be catered for by increasing the number of piles along each load8

bearing internal wall parallel to grid lines $7'(. )n this case howe9er, it has been decided to

maintain the same pile and ground beam layout as in 5esign @*ample &.

Pile capacitiesAs pre9iously, the pile capacities gi9en in Table (.|are deri9ed from pre9iously design@*ample 9ig. (."|(.

Piles of 4'0 mm diameter will again be used.


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he calculations will be found to indicate that the 600 mm 62' mm deep ground beams in

pre9iously design e*ample will need to be deepened by appro*imately 200 mm to accommodate

this additional loading.

9ig. (."|Pile safe woring loads for design e*amples.


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9ig. (.#"Piled ground beam and suspended slab design e*ample.Pile Caps.

4ntroductionhe design of pile caps had at one time become a math8ematicians delight 7 and a designers

nightmare. Bighly comple* formulae with numerous empirical 9ariants could result in e*pensi9edesign and construction to sa9e a couple of reinforcing bars. As in all design and construction theaim must be Cto eep it simple.

" The need for pile caps capping beams)t is frequently not possible to sit a superstructure column direct on to a pile because%

13)t is practically impossible to dri9e piles in the e*act position and truly 9ertical. Piles wanderin dri9ing and de9iate from their true position. A normal speci;cation tolerance for position is

D=' mm and for 9erticality not more than $ in =' for a 9ertical pile or $ in 2' for a raing pile. A

column sitting directly on a pile withan eccentricity of =' mm will e*ert bending as well as direct stresses in the pile.

1"3A single, hea9ily loaded column supported by a pile group will need a load spread pile cap(to transmit the load to all the piles.1#3A line of piles supporting a load8bearing wall will need a capping beam to allow both fortolerance of pile positioning and load spreading of the piles concentrated load to the wall.

# i/e and depth
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Pile caps are usually of concrete but can be large slabs of roc or mats of treated timber. his

discussion is limited to the more common use of concrete.

o allow for the pile de9iation the pile cap should e*tend $007$'0 mm beyond the outer face of

the piles. he pile group centroid should ideally coincide with the columns position see 9ig.

(.-(.

9ig. (.- Plan on triple pile cap.

he depth must be adequate to resist the high shear force and punching shear and to transmit the

9ertical load see 9ig. (.0(. he shaded area of the pile cap plan in 9ig. (.0is the area where

the column load is directly transferred to the piles. +or such a condition the shear stresses aregenerally small but bending moments need to be catered for.


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9ig. (.01oad transfer from column to piles.

Alternati9ely, peripheral steel as a ring tension around a cone shaped compression bloc may be

considered to be a suitable equilibrium of forces see 9ig. (.{(, howe9er, full tension laps mustbe pro9ided for the peripheral steel.


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9ig. (.{ Eing tension pile cap.

"ingle column loads supported on larger pile groups can create signi;cant shear and bending in

the cap which will need top and bottom reinforcement as well as shear lins see 9ig. (.|(.


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9ig. (.|Pile cap, typical reinforcement.

he heads of r.c. piles should be stripped and the e*posed reinforcement bonded into the pile cap

for the necessary bond length. Pile caps to steel piles can be reduced in depth if punching shear is

reduced by capping andor reinforcing the head of the pile, as shown in 9ig. (."5.


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9ig. (."5Eeinforced pile head.

Piles for continuous capping beams supporting load8bearing walls can be alternately staggered to

compensate for the eccentricity of loading due to the =' mm out8of8line tolerance see 9ig.

(."$(.

9ig. (."


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)t is sometimes necessary to dri9e a group of piles to support hea9y loadings and it is important

to notice two effects%

13 he pressure bulb of the group affects deeper layers of soils than a single pile of the samedepth see 9ig. (.#( in a similar manner to a wide foundation.

1"3 he load8bearing capacity of a group is not necessarily the product of the capacity of thesingle pile times the number of piles. here can be a pressure Co9erlap see 9ig. (.(( and thecapacity of the group could decrease as the difference between a pad and strip foundation.

A single pile, in dri9ing, displaces soil which can result in hea9e at ground le9el and a group can

cause greater hea9e and displacementF this fact should be checed and considered. 5ri9ing a

single pile, too, in loose sand and ;lls will compact the soil around the pile to a diameter of

appro*imately '.' times the pile diameter and mae it denser. )f a group of piles is dri9en itcould create such a compact bloc of soil as to pre9ent dri9ing of all the piles in the group. he

central piles should be dri9en ;rst and then, woring out to the perimeter of the group, the

remaining piles should be dri9en.

9ig. (.# "ection through pile pressure bulbs.


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9ig. (.( Plan on pile pressure bulbs.

pacing of piles within a group

Appro*imate 9alues for centre8to8centre spacing are as follows%

13+riction piles 7 not less than the perimeter of the pile.1"3@nd8bearing piles 7 not less than twice the diameter of the pile.1#3 "crew piles 7 not less than $.' times the diameter of the blades.1(3Piles with enlarged bases 7 at least one pile diameter between enlarged bases.

hese 9alues are affected by the soil conditions, the group beha9iour of the piles, the possible

hea9e and compaction, and the need to pro9ide suf;cient space to install the piles to the designedpenetration without damage to the pile or group.

Spacing of piles within a group

Approximate values for centre-to-centre spacing are as follows:


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(1)Friction piles not less than the perimeter of the pile.

(2)En-!earing piles not less than twice the iameter of the pile.

(3) "crew piles not less than #.$ times the iameter of the !laes.

(4)Piles with enlarge !ases at least one pile iameter !etween enlarge !ases.

%hese values are a&ecte !' the soil conitions( the group !ehaviour of the piles(

the possi!le heave an compaction( an the nee to provie suf)cient space to

install the piles to the esigne penetration without amage to the pile or group.

Piles - Factor of safet'.

!" #004 recommends a factor of safety of between 2 and & for a single pile. he factor of safety

is not a ;*ed constant and depends on the allowable settlement of the pile which is dependent onthe piles surface and cross8sectional area, the compressibility of the soil, and the reliability of

the ground conditions. he factor should be increased when%

13he soil is 9ariable, little is nown of its beha9iour or its resistance is liely to deterioratewith time.

1"3"mall amounts of differential settlement are critical.1#3he piles are installed in groups.

he factor may be decreased when%

13As a result of e*tensi9e loading tests, the resistance can be con;dently predicted.1"3As a result of e*tensi9e local e*perience, the soil properties are fully nown.

A common factor of safety taen in design is 2.'. A properly designed single '00 mm diameterpile dri9en into noncohesi9e soil is unliely to settle more than about $' mm.

)n a load test the settlement is noted for increasing increments of load and a settlementload

graph is plotted. he graph resembles that of the stressstrain graph for many structural materials

see 9ig. (."(. Gp to woring load there tends to be practically full reco9ery of settlement onremo9al of load but beyond that loading there is liely to be a permanent set as in steel loaded

beyond the elastic limit( and at ultimate load there is liely to be no reco9ery at all.
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9ig. (."1oadsettlement graph.Choice of Pile.

*aving foun a satisfactor' pile an a relia!le an cooperative piling contractor for

a particular site an conitions( there is a temptation for a !us' esigner( with

inae+uate time to investigate the wie choice of piles an s'stems( to use the

same piling contractor for all future pro,ects. %his unerstana!le reaction oes not

mae for cost e&ectiveness nor structural ef)cienc'. A guie to the choices availa!le

is given !elow:

(1)%he piling s'stem must provie a safe founation with an ae+uate factor of

safet' against failure of the founation on supporting soil

(2)%he total settlement an i&erential settlement must !e limite to that which

the structure can tolerate.

(3)%he pile shoul !e the right t'pe of pile for the groun conitions an structure.

(4)%he riving of the piles an the loa the' impose on the soil must not amage

neigh!ouring structures.

(5)%he piles must !e economic an ura!le( an where spee of construction isimportant( +uic to place.

1 Ground conditions and structure

(1)hen invite to tener for the contract the piling contractors shoul !e

provie with a soil report( the position an magnitue of structural loas an the

location of the structure together with information on a,oining properties. %he'
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shoul also !e ase to visit the site to inspect the access for piling plant

movements.

(2)/riven an cast-in-place piles( where the shell is left in( are use on sites over

water 0,etties( piers( etc.1( on sites nown to contain large vois( an on sites

su!,ect to high water pressure. /riven piles shoul not !e chosen where the grounis liel' to contain large !oulers !ut the' are one of the !est piles for loose-to-

compact wet sans an gravels.

%hese t'pes of piles are fre+uentl' the cheapest to use on !uiling sites with light-

to-moerate pile loaings

an where the charges for moving onto site are sprea over a large num!er of piles.

(3)2ore piles are fre+uentl' the lowest cost piles when piling into )rm cla's or

sanstone an when vi!ration an groun heave woul cause pro!lems to existing

a,acent !uilings.

(4)3ace piles nee something to ,ac against an ten to !e expensive. %heir

main use is therefore in unerpinning when the' can prove to !e cost-e&ective.

(5)"teel * piles are often chosen when long length piles with eep penetration into

sans an gravels are re+uire.

2 Durability%he groun conitions can a&ect the choice an metho of protection of piling

material. "ulfates an acis will attac poor-+ualit' concrete( some acis will cause

pro!lems with steel piles an alternate wetting an r'ing can cause tim!ers to rot.

3 CostPiles are( or shoul !e( chosen as the economic an safe alternative to strip an raft

founations !ut there is more to cost anal'sis than comparing the cost per metre

run of piles4 there are on-costs. 5n comparing piling contractors6 estimates it can !e

unwise to accept the lowest cost per metre run.

Examination of extra over-costs for such items as extening lengths of piles(

conucting chec loaing tests( etc. is pruent. %he esigner shoul examine the

piling contractor6s resources availa!le to complete the pro,ect on time( the length of

notice re+uire to start wor an the contractor6s experience in piling on similarsites. %he contractor6s reputation shoul !e investigate an proof o!taine of

ae+uate insurance to inemnif' !uiling owners for an' claims or amage to

a,oining !uilings or failure of piles ue to esign an construction faults.

%o the cost of the piles must !e ae the cost of excavation for constructing pile

caps an an' necessar' tie !eams. %his increases the cost of construction


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supervision an esign.

/ecisions must !e taen earl' so that esign( etailing( construction an planning

can !e complete well in avance of starting the contract. %oo often the time is

restricte !' ela's in site investigations( change of esign !rief( recent changes in

contractors6 prices( etc.

7ethos of Piling.

%here is a wie variet' of methos use for piling an ever' piling contractor has a

num!er of variations for their s'stem improvements in metho an e+uipment

continues. %he main classes onl' are iscusse !elow.

1 Drien piles%his metho is use for piles of tim!er( precast concrete( prestresse concrete an

the various t'pes of steel piles.

%he pile is hammere into the groun !' pile-riving plant shown in outline in !ig

(.5 1a3. Hethods of protecting the head of the pile from shattering are shown in 9ig. (.5 1b3.

9ig. (.55ri9en piling.

5ri9en piles are classi;ed as displacement piles and, where the soil can enter during dri9ing, assmall displacement piles e.g. open ended tubular or other hollow sections often in steel(.

" 8riven cast2in2place piles
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A closed ended hollow steel or concrete casing is dri9en into the ground and then ;lled with

fresh concrete. he casing may be left in position to form part of the whole pile or withdrawn

for reuse as the cast concrete is placed.

he cast concrete is rammed into position by a hammer as the casing is withdrawn ensuring ;rm

contact with the soil and compaction of the concrete.


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he piles are not generally suitable in mining areas where surface mo9ements and lateral strains

may be e*pected to distort or shear the piles.

Concrete Piles.

Concrete piles are the most wiel' use in the evelope countries an ma' !e cast

in situ( precast( reinforce an prestresse.

(a) "recast%his t'pe is commonl' use where:

(i)%he length re+uire can !e realisticall' preicte.

(ii)8ateral pressure from a stratum within the soil pro)le is suf)cient to s+uee9e

0nec1 a cast-in-situ pile.

(iii)here there are large vois in

sections of the soil which woul possi!l' have to !e )lle with an excessive amount

of in situ concrete or coul cause loss of support for wet concrete prior to setting.

(i)For structures such as piers an ,etties a!ove water level on coastal( estuar'

an river sites.

%hough precast piles can !e manufacture on site it is more common to have them

esigne( manufacture an installe !' specialist su!contractors.

%here are isavantages in the use of precast concrete piles as follows:

(i)5t is not eas' to exten their length.(ii)%he' are lia!le to fracture when riven into such o!stacles as large !oulers in

!ouler cla' an the amage can remain out of sight.

(iii)!structions can cause the pile to e;ect from the true vertical line.

(i)%here is an economic limit( restricte !' !ucling( of the unrestraine length of

the pile.

()


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(iii)"tresses ue to riving are usuall' higher than those ue to founation loaing

so that manufacturing faults are more easil' iscovere an( in e&ect( the pile is

preloa teste 0provie the efects can !e etecte1.

%he reinforcement( while aing to the loa-!earing capa- cit'( is mainl' esigne

to cope with hanling( transporting an riving stresses.

(b) Cast in situ%here is an ever increasing variet' of cast in situ piles o&ere !' specialist piling

su!contractors. %he piles are

usuall' circular in cross-section an are regare as small- iameter piles when

their iameters are from =$>?>> mm an larger-iameter piles when their

iameters excee ?>> mm4 large-iameter piles are now possi!le with iameters

up to @.> m.

%he avantages of cast in situ piles are:

(i)%he' can !e constructe immeiatel'( thus cutting out the time re+uire for

casting( maturing an elivering of precast piles.

(ii)%here is no nee to cut o& or exten excessive lengths of the piles as the' can

!e cast in situ to the re+uire level.

(iii)%he' can !e cast to longer lengths than is practical with precast piles.

(i)7ost o!structions can !e hammere an !roen through !' the pile-riving

techni+ues.

()%he placing can cause less noise vi!ration an other istur!ance compare to

riving precast piles.(i) "oil taen from !oring can !e inspecte an compare with the anticipate

conitions.

%he isavantages of cast in situ piles are:

(i)5t can !e if)cult to place an ensure positioning of an' necessar'

reinforcement.

(ii)Concrete +ualit' control is more if)cult.

(iii)%here is a anger of necing from lateral earth pressure.

(i)oung concrete is suscepti!le to attac from some soil chemicals !efore it has

set an harene.

(c) "restressedPrestresse concrete in superstructure esign is mae of higher strength concrete(

re+uires smaller cross-sectional area an can !e mae impact-resistant. %he same

results appl' to prestresse piles relative to comparison with precast reinforce


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piles. %heir avantages compare to precast reinforce are:

(i)*anling stresses can !e resiste !' a smaller cross- section which can result in

a more economical pile.

(ii)5t is easier with the smaller section to achieve longer penetration into loa-

!earing gravels.(iii)%ensile stresses that are generate up from the toe of the pile after the hammer

!low can !e compensate for !' prestress.

(i)%he reuction of tensile cracing of the concrete can lea to greater ura!ilit'.

%he isavantages of prestresse piles are:

(i)%he smaller section provies less en !earing an total peripheral sin friction.

(ii)/eeper penetration into en-!earing strata 0gravel( compact san( etc.1 ma' !e

necessar'.

(iii)5t is more if)cult to exten the length of a precast riven pile.

(i)As in prestresse concrete superstructure elements( stricter +ualit' control in

manufacture is necessar'.

/esign of Founations at Pile *e

A general escription of groun !eams an pile caps is iscusse in previousl' an

restraints an capB!eam etails are !rie;' mentione.

5n aition to proviing restraint( the groun !eam is also use to transfer loas

from the superstructure to the pile an can !e use with or without pile caps. For

example( two alternative la'outs are shown in !ig# 14#22inicating a wie groun

!eam solution an a narrow !eam using pile caps

9ig. (.""Alternati9e beamcap layouts.

Khere the increased width of the beam needed to accommodate the pile diameter, plus the total
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of all necessary tolerance, is only slight and where a reduction in beam depth helps to

compensate for the additional concrete, a wider beam omitting the pile caps can be more

economic.

Lften the ground beam can be designed compositely with the walls abo9e and by using

composite beams a standard nominal siMe ground beam, dictated mainly by the practicalities forconstruction, can be used. his has the ad9antage of standardiMing shuttering, reinforcement and

e*ca9ation, maing site construction simple, economic and quicer than the traditional solution.

Hany different beams designed ignoring the bene;t of the contribution from the structure abo9ecan se9erely complicate the foundations see 9ig. (."#(.

9ig. (."#


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9ig. (."+ Piles and suspended ground slab.


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9ig. (."-Piles and ?at slab construction.

9ig. (."0Piled suspended slab and beamconstruction

Design of Pile Caps Final

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