Pile Foundation

Civil Engineering Practice

Chapter # 4

Pile Foundation

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Pile Foundation

CHAPTER # 04

CHP # 04: Pile Foundation

---(Pile Foundation)---

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Piles are relatively long and slender members used to transmitfoundation loads through soil strata of low bearing capacity to

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deeper soil or rocky strata having a high bearing capacity.

I th d th l d i l t itt d t th il bIn other words, the load is properly transmitted to the soil bydriving long vertical members of either timber or concrete orsteel called “Piles”.

2Civil engineering practice

Depth at which rocky or hard strata is available can normally be


Depth at which rocky or hard strata is available can normally be60 to 80 ft, showing the range of depth for the piles.

Piles usually carry excessive loads as load of traffic on a bridgewhere the underneath strata can be of sand and silt fullysaturated with watersaturated with water.

Bore logs drawn after soil investigation is used to find out depthg g pof piles, diameter of piles and the numbers of piles required.

Pile can also be defined as an element of construction placed inthe ground either vertically or slightly inclined to increase theload carrying capacity of the soil.

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d c y g c p c y s

Civil engineering practice

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Piles are required for setting out of towers for high power


Piles are required for setting out of towers for high powertransmission lines and in some cases are also used for supportingthe multistory buildings.

4.1 Types of Piles (Based Upon Use)

1- Sheet Piles

Sheet piling is used primarily to resist lateral pressures such asSheet piling is used primarily to resist lateral pressures such asflow of water and loose soil. Typical uses include cut-off wallsunder dams, cofferdams, trench sheeting, etc.


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warehouse

ship

sheet pile

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2- Load Bearing Piles


These are used primarily to transmit loads through soilformations with poor supporting properties into or ontoformations that are capable of supporting the loads Dependingformations that are capable of supporting the loads. Dependingupon how load is transferred to the soil, load-bearing piles canbe further classified into:

(a) End Bearing Piles

If l d i i d h il h h h l i h il iIf load is transmitted to the soil through the lower tip, the pile iscalled an “End Bearing Pile”.

These piles transmit the superimposed load to stronger stratasuch as a rock bed. End area of pile times the bearing capacity ofh k i h l l d hi h b li d

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the rock is the total load, which can be applied.


Considering reasonable factor of safety, we can find diameter of


the pile required.


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PILE

weak soil

bed rock

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(b) Friction Piles


These are designed to transmit the loads by the frictional forceexisting between the sides of the pile and the ground such as thestuff clays or dense sandy depositstuff clays or dense sandy deposit.

Friction can be developed for full length as in first figure or it canp g gbe developed in lower portions only depending upon thestratification of soil.

Surface area times the safe friction which can be developed per unitarea determines the load to be carried by the pile. Selectingy p gdiameter or length of pile, the other quantity can be calculated.


(b) Friction Piles



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(c) End Bearing and Friction Piles


These are the piles that dissipate part of the load by friction andtransfer the remaining load to firm strata.

3- Piles as Soil Compactor

Sometimes closely spaced piles are driven to compact theSometimes closely spaced piles are driven to compact thesurrounding soil.


4.2 Types of Piles (Based on material used and method of


construction)

On the basis of the material from which they are made and themethod of constructing and driving them load bearing pilesmethod of constructing and driving them, load-bearing pilesmay be classified as follows:

– TimberU da. Untreated

b. Treated with a preservative– Concrete

a. Pre-castb. Cast-in-place

S l– Steela. I-sectionb. Steel-pipe

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p p– Composite


Driven timber piles, Pacific Highway

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p g y

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4.3 Factors Affecting Selection of Piles


Among the factors that will influence the selection of type of pilesfor a given project are the following:

1. Type, size, and weight of the structure to be supported.1. Type, size, and weight of the structure to be supported.2. Physical properties of the soil at the site.3. Depth to a stratum capable of supporting the piles.4. Possibility of variations in the depth to a supporting stratum.5. Availability of materials for piles.6 Number of piles required6. Number of piles required.7. Facilities for driving piles.8. Comparative costs in place.9. Durability required.10. Types of structures adjacent to the project.11 Depth and kind of water if any above the ground into which

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11. Depth and kind of water, if any, above the ground into whichthe piles will be driven.


4.4 Significance of Pile Foundation


Pile foundation is suitable for works over water such asbridge`-piers, foundation of transmission towers andfoundation of multistory buildingsfoundation of multistory buildings.From head or cap of piles, we can construct the grids withbeams and columns.


4.4 Significance of Pile Foundation


The number of piles depends on the type of pile and the loadcoming from the buildings.


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ooden Pilesooden Piles


4.5 Wooden Piles


These piles, if used below the ground water level, last for avery long period but ordinarily they last no longer than 30years or so even if treated.yea s o so eve t eated.

The circular piles vary in diameter from 12 inch to 16-inch. Ifsquare sizes are used, they also vary in size fro 12 inch to 16-inch square. The length of the piles is about 20 times the topwidth.width.

The timber point may be protected or replaced by a metallicshoe; shoes are used while driving through soils containingboulders, gravels, hard clays etc.


4.5 Wooden Piles


They may be of C.I. and should have sufficient length ofwrought iron straps to connect them with the pile.

The head of the pile is subjected to a heavy impact of thedriving machine; hence, rings of wrought iron are fitted to theg ; , g gpile head. They vary in cross-section from 2 in. to 4 in.

Timber piles should not be loaded above 15 to 20 tons andshould be carefully driven.

Whenever it is necessary to use a longer pile than available inusual lengths, it is necessary to splice the piles together, with

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their ends abutting each other.


4.5 Wooden Piles


Additional strength can be obtained by the use of steelfishplates bolted to the sides of the piles.

Among the advantages of timber piles are the following:

1. The more popular lengths and sizes are available on shortnotice.

2. They are economical in cost.3. They are handled easily, with little danger of breakage.4. They can be cut off to any desired length after they are driven.5. They can be pulled easily in the event removal is necessary.


4.5 Wooden Piles


Among the disadvantages of timber piles are the following:

1. It may be difficult to obtain piles sufficiently long and straightfor some projects.

2. It may be difficult or impossible to drive them into hardformations.

3 I i diffi l li h i h i l h3. It is difficult to splice them to increase their lengths.4. While they are satisfactory when used as friction piles, they are

not suitable for use as end-bearing piles under heavy loads.g p y5. The length of life may be short unless the piles are treated with

a preservative.


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oncrete Pilesoncrete Piles


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4.6 Concrete Piles


(a) Pre-cast Concrete Piles

Square and octagonal piles are cast in horizontal forms, whileround piles are casts in vertical forms.

After the piles are cast, they should be cured for the periodrequired by specifications, frequently 21 days.

With the exception of short lengths, Precast concrete pilest b i f d ith ffi i t t l t t dmust be reinforced with sufficient steel to prevent damage or

breakage while they are being handled from the casting beds tothe driving positions.


4.6 Concrete Piles( ) P C Pil



Among the advantages of Pre cast concrete piles are theAmong the advantages of Pre-cast concrete piles are thefollowing:

1. High resistance to chemical and biological attacks.2. High strength.3. A pipe may be installed along the center of a pile to facilitate

driving.


4.6 Concrete Piles( ) P C Pil



Among the disadvantages of Pre cast concrete piles are theAmong the disadvantages of Pre-cast concrete piles are thefollowing:

1. It is difficult to reduce or increase the length.2. Large sizes require heavy and expensive handling and driving

equipment.3 I bili b i il b h d l h i f3. Inability to obtain piles by purchase may delay the starting of a

project.4. Possible breakage of piles during handling or driving producesg p g g g p

a delay hazard.


4.6 Concrete Piles


(b) Cast-in-Place Concrete Piles

These piles are constructed by depositing the freshly mixedconcrete in place in the ground and letting it cure there. Thetwo principal methods of constructing such piles are:p p g p

1. Driving a metallic shell, leaving it in the ground and filling itwith concrete.

2 D i i t lli h ll d filli it ith t th h ll2. Driving a metallic shell and filling it with concrete as the shellis pulled from the ground.


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4.6 Concrete Piles



Among the advantages of cast in place concrete piles are theAmong the advantages of cast-in-place concrete piles are thefollowing:

1. The lightweight shells may be handled and driven easily.2. Variations in length do not present a serious problem. The

length of a shell may be increased or decreased easily.g y y3. The shells may be slipped in short lengths and assembled at

the job.4 E i f i t i t t d b h dli l4. Excess reinforcing, to resist stresses caused by handling only,

is eliminated.5. The danger of breaking a pile while driving is eliminated.

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6. Additional piles may be provided quickly if they are needed.


4.6 Concrete Piles



Among the disadvantages of cast-in-place concrete piles are thefollowing:

1. A slight movement of the earth around an un-reinforced pilemay break it.

2. An uplifting force, acting on the shaft of an uncased and un-reinforced pile, may cause it to fail in tension.

3 Th b tt f il t b t i l3. The bottom of pile may not be symmetrical.


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teel Pilesteel Piles


4.7 Steel Piles


Steel piles may be of I-section or hollow pipe section 10 inchest 24 i h di t ith 3/8 i h thi kto 24 inches diameter with 3/8 inches thickness.

Because of a small sectional area steel piles are easy to driveBecause of a small sectional area, steel piles are easy to drive.The pipes are driven with open ends. Compressed air with apressure of 100 psi in a 2½ inches or more diameter pipe andh f ili h d i ithus facilitates the driving.

These piles are later on filled with concrete Steel piles areThese piles are later on filled with concrete. Steel piles aremostly used as end bearing piles because of their less availablesurface area to take the loads by frictional forces.


4.7 Steel Piles


In constructing foundations that require piles driven to greatdepths, steel I-section probably are more suitable than anydept s, stee sect o p obab y a e o e su tab e t a a yother type.

Steel piles may be driven through hard materials to a specifieddepth to eliminate the danger of failure due to scouring, suchas under a pier in a river.as under a pier in a river.

Also, steel piles may be driven to great depths through poorsoils to bear on a solid rock stratum.


4.7 Steel Piles


The great strength of steel combined with the smalldisplacement of soil permits a large portion of the energy fromd sp ace e t o so pe ts a a ge po t o o t e e e gy oa pile hammer to be transmitted to the bottom of a pile.

As a result, it is possible to drive steel piles into soils, whichcould not be penetrated by any other type of pile.

By weld splicing sections together, lengths in excess of 200fthave been driven.


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crew Pilescrew Piles


4.8 Screw Piles


Screw piles consist of iron or steel shafth i t it d t i t t lhaving at its end a cast iron or cast steelscrew, with a blade 1.5ft to 5ft in diameter.

These piles can be screwed down to agreat depth in clay, etc., and penetrateh h ll b kthrough small broken stones.

The base area of the screw does most ofThe base area of the screw does most ofthe weight bearing; they can be used forbridges in riverbeds with deep clayf d i

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foundation.


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4.9 Pile Driving


The operation of forcing a pile into ground without anyprevious excavation is called “Pile Driving”.

Following are the various methods of pile driving:

1. Drop hammer.2. Steam-hammer.3 Boring3. Boring.4. Screwing.5. Water jet.


4.9 Pile Driving1 D H M h d


1- Drop Hammer Method

It is the simplest method of pile driving. In this method, at s t e s p est et od o p e d v g. t s et od, ahammer is dropped on the head of the pile and is guidedduring its fall by suitable staging.

The hammer is raised by pulling a rope manually or by steamengine or electric motor. The weight of the hammer forengine or electric motor. The weight of the hammer fordriving concrete or wooden piles is about 225 kg for shortpiles and 900 kg to 2350 kg for long and heavy piles.

Hammer is generally dropped from a height of 15m to 45mand the height is limited to 6 m The pile-driving frame is kept

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and the height is limited to 6 m. The pile driving frame is keptin vertical position by suitable ropes.


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4.9 Pile Driving2 S H M h d


2- Steam Hammer Method

A heavy hammer is dropped on to the pile through a smalleavy a e s d opped o to t e p e t oug a s aheight but in quick succession. Single acting or double actingsteam hammers are available.

These hammers are raised with high-pressure steam and aredropped under gravity when single acting or by steam wheredropped under gravity when single acting or by steam wherethere is double acting hammer. These hammers are speciallyused for driving sheet piles and where small vibrations are

i d t b t i th ilrequired to be set up in the piles.

Special devices are used to protect the heads of the piles from

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Special devices are used to protect the heads of the piles fromdamage due to excessive blows, which they receive.


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4.9 Pile Driving


3- Boring

S ti ili i d b b i h l f it bl di tSometimes, piling is done by boring holes of suitable diameterto the required depth and then dropping piles in them.Generally cast-in-situ piles are laid by this method.

4- Screwing

Screw piles and disc piles are generally driven by this method.They are driven into the ground by turning them round andThey are driven into the ground by turning them round andround in one direction with big levers.


4.9 Pile Driving5 W J


5- Water Jets

Pile driving can also be done by displacing the material at orP e d v g ca a so be do e by d sp ac g t e ate a at onear the foot of the pile by means of one or more water jetsunder pressure.

Sometimes, hammer may be used to force the pile rapidly intothe ground. The pressure of the jet should be sufficient tothe ground. The pressure of the jet should be sufficient todisplace the soil and other removed materials for driving thepiles quickly and satisfactorily.


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End

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