Student / Graduate Introduction to Pile Foundations

Pile Foundations

AbstractThe purpose of this document is to provide a guide for students or graduates to quickly access information on the topic of ground piles. Information will be recorded on the different types of piles available as well as the methods that can be used to install them. Suitable materials, codes of practice, and resources will also be included.

Piles are used to transfer the load of a structure to the bearing ground, at a depth at which there is suitable, stable soil or rock. Piles are often joined together at the surface by pile caps. Piles can help to resist vertical, lateral and uplift loads and are very useful when building structures over water e.g. bridge piers. Piles use 3 main methods to transfer these forces:

End bearing Piles (point bearing piles) Friction Piles (cohesion piles) Combination of Friction and Cohesion Piles

End Bearing PilesThese piles transfer their load on to a firm stratum located at a considerable depth below the base of the structure and derive most of their carrying capacity from the penetration resistance of the soil at the toe of the pile. (See figure 1 below)Design: The pile behaves as an ordinary column and should be designed as such.

+ Even in weak soil a pile will not fail by buckling and this effect need only be considered if part of the pile is unsupported, i.e. if it is in either air or water.

− Sometimes however, the soil surrounding the pile may adhere to the surface of the pile and cause "Negative Skin Friction". This can sometimes have a considerable effect on the load bearing capacity of the pile. Negative skin friction is caused by the drainage of the ground water and consolidation of the soil.

Cohesion Piles

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Figure 1www.fhwa.dot.gov

Figure 2www.fhwa.dot.gov

Student / Graduate Introduction to Pile Foundations

These piles transmit most of their load to the soil through skin friction. The process of driving such piles close to each other, in groups, greatly reduces the porosity and compressibility of the soil within and around the groups. Therefore piles of this category are some times called compaction piles.

+ Advantages: This type of pile can be used when a firm stratum, capable of supporting the super-structure through the use of end-bearing piles cannot be reached.

− Disadvantages: During the process of driving the pile into the ground, the soil becomes moulded and, as a result loses some of its strength. Therefore the pile is not able to transfer the exact amount of load which it is intended to immediately after it has been driven. Usually, the soil regains some of its strength, three to five months after it has been driven.

Friction PilesCarrying capacity is derived mainly from the adhesion or friction of the soil in contact with the shaft of the pile. These piles also transfer their load to the ground through skin friction. The process of driving such piles does not compact the soil appreciably. These types of pile foundations are commonly known as floating pile foundations. The main difference between friction and cohesion piles is the effect of soil compaction. (See figure 2 previous page)

Methods Of Pile Installation

Drilled PilesThere are two common methods of inserting a pile into the ground. The first is to drill a hole in the ground using a rotary auger and then insert a steel casing to seal the hole. A steel reinforcing cage is then inserted into the bore hole and a highly workable concrete mix is poured in through the hollow stem of the auger and allowed to harden. Drilling fluids like bentonite can be added to help this process. Rotary augers are available with diameters of over 2m and can drill to depths of up to 50m.For reinforcing over 9 metres in depth, a vibrator is generally required. The steel casing can either be removed, or let remain in the ground.

+ Drilled piles allow for much wider diameter piles than other methods and can be used in particularly dense or hard strata, provided that it is stable.

+ CFA or continuous flight auger piles are recommended in noise and vibration sensitive sites.s

+ It is effective in sites with interbedded sequences of clays and sands, water bearing sands and gravels and weak rocks.

− In this process, it is important that the rotation of the auger and the flow of concrete are matched, so that collapse of the sides of the hole above concrete on lower flight of auger is avoided. This may lead to voids in the concrete, filled with soil.

− For successful operation of a rotary auger the soil must be reasonably free of tree roots, cobbles, and boulders

− Can only be used if the soil is self-supporting

Underreamed Piles

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Student / Graduate Introduction to Pile Foundations

Underreamed piles have enlarged bases, up to 6m in diameter which are formed mechanically. Underreaming is a special feature of auger bored piles which is sometimes used to exploit the bearing capacity of suitable strata by providing an enlarged base. (See figure 3)

+ Increases the load bearing capacity of the pile.

+ Ideal in stiff to hard clays.

− The soil has to be capable of standing open unsupported to employ this technique.

In its closed position, the underreaming tool is fitted inside the straight section of a pile shaft, and then expanded at the bottom of the pile to produce the underream. Normally, after installation and before concrete is caste, a man carrying cage is lowered and the shaft and the underream of the pile is inspected.

CHDP – Continuous Helical Displacement PilesThe bore hole is created by a short, hollow, tapered steel former complete with a larger diameter helical flange, which is pressed into the ground by a hydraulic ram. (See Figure 5) It is then rotated, pulling itself into the ground like a screw. When the required depth is achieved, concrete is poured into the bore hole, through the hollow centre of the steel former. The former is simultaneously removed, while rotating clockwise. The shape of the concrete pile produced is helical.

+ The thick thread increases both the maximum diameter and the surface area of the pile. (See Figure 4)

+ This technique minimises spoil which can be costly to dispose of, especially in sites where the earth is contaminated.

− If the speed of rotation and rate of concrete input do not match, then the walls of the bore hole will not remain intact and the pile will be contaminated.

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Figure 3http://www.sbe.napier.ac.uk/projects/piledesign/guide/index.htm

Figure 4www.bacsol.co.uk/main.php?s=introduction

Student / Graduate Introduction to Pile Foundations

Driven PilesPrefabricated piles are driven into the ground using a pile driver. They can be made from concrete, wood or steel. Wooden piles are generally made from the trunks of large trees. Concrete piles are usually reinforced and sometimes prestressed. Steel piles are usually either pipe piles, or have a typical beam cross-section e.g. H-section. Piles can be either driven from the top, or bottom driven from within.

+ The main advantage of driven piles, over drilled piles, is that the driving process displaces soil, compacting it. This causes an increased friction between the pile and the soil and therefore a greater load bearing capacity.

+ Self proving on installation.

+ Fast installation

− Driven piles cannot be used in some areas due to noise and vibration.

− Large plant

− Site preparation costs

Methods of pile driving can be categorised as follows:

1. Drop Hammers2. Explosion / Diesel Hammers3. Vibration4. Jacking (restricted to micro-pilling)5. Jetting

Drop HammersA hammer with approximately the weight of the pile is raised to a suitable height in a guide and then released to strike the pile head. This is a simple form of hammer used in conjunction with light frames and test piling, where it may be uneconomical to bring a steam boiler or compressor on to a site to drive a very limited number of piles.There are two main types of drop hammers: Single-acting steam or compressed-air hammers

Double-acting pile hammers

Diesel Hammers

Also classified as single and double-acting, in operation, the diesel hammer employs a ram which is raised by explosion at the base of a cylinder. Alternatively, in the case of double-acting diesel hammer,

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Figure 5www.bacsol.co.uk/main.php?s=introduction

Student / Graduate Introduction to Pile Foundations

a vacuum is created in a separate annular chamber as the ram moves upward, and assists in the return of the ram, almost doubling the output of the hammer over the single-acting type.

+ In favourable ground conditions, the diesel hammer provides an efficient pile driving capacity.

− Diesel hammers are not effective for all types of ground.

Pile driving by Vibration

Vibratory hammers are usually electrically powered or hydraulically powered and consists of contra-rotating eccentric masses within a housing attaching to the pile head.

+ The amplitude of the vibration is sufficient to break down the skin friction on the sides of the pile.

+ Vibratory methods are best suited to sandy or gravelly soil.

− In many areas, including city centres and close to existing buildings, there are limits on vibrations due to construction, therefore other techniques may need to be considered.

Jetting: to aid the penetration of piles in to sand or sandy gravel, water jetting may be employed. However, the method has very limited effect in firm to stiff clays or any soil containing much coarse gravel, cobbles, or boulders.

Types of Piles

Sheet PilesSheet piles are thin interlocking sheets of steel that are driven into the ground to form a wall. They are often used for temporary retaining structures.

Soldier Piles(A.k.a. King Piles or Berlin Walls) Consist of steel H-sections with wide flanges, with 2 or 3m spaces between them. The soldier piles are connected by horizontal timber sheeting. (See Figure 6). The pressure of earth held behind the lagging is concentrated on the piles due to their relative stability. Soldier piles area also used for retaining structures. Soil movement and subsidence is minimized by maintaining the lagging in firm contact with the soil.

+ Soldier piles are most suitable in conditions where well constructed walls will not result in subsidence such as over-consolidated clays, soils above the water table if they have some cohesion, and free draining soils which can be effectively dewatered, like sands.

− Unsuitable soils include soft clays and weak running soils that allow large movements such as loose sands. It is also not possible to extend the wall beyond the bottom of the excavation and dewatering is often required.

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Student / Graduate Introduction to Pile Foundations

MicropilesMicropiles, also called mini piles, are used for underpinning. Micropiles are normally made of steel with diameters of 60 to 200 mm. Installation of micropiles can be achieved using drilling, impact driving, jacking, vibrating or screwing machinery.

+ Where the demands of the job require piles in low headroom or otherwise restricted areas and for specialty or smaller scale projects, micropiles can be ideal. Micropiles are often grouted as shaft bearing piles but non-grouted micropiles are also common as end-bearing piles.

+ Small plant

+ Little site preparation

− Low pile capacities

− Relatively expensive

− Relatively Slow

Pipe pilesPipe piles are a type of steel driven pile foundation and are a good candidate for battered piles.Pipe piles can be driven either open end or closed end. When driven open end, soil is allowed to enter the bottom of the pipe or tube. If an empty pipe is required, a jet of water or an auger can be used to remove the soil inside following driving. Closed end pipe piles are constructed by covering the bottom of the pile with a steel plate or cast steel shoe.In some cases, pipe piles are filled with concrete to provide additional moment capacity or corrosion resistance. In the United Kingdom, this is generally not done in order to reduce the cost. In these cases, corrosion protection is provided by allowing for a sacrificial thickness of steel or by adopting a higher grade of steel. If a concrete filled pipe pile is corroded, most of the load carrying capacity of the pile will remain intact due to the concrete, while it will be lost in an empty pipe pile.The structural capacity of pipe piles is primarily calculated based on steel strength and concrete strength if filled. The thickness of the steel should be increased to account for corrosion, typically by 1/16 in.The amount of corrosion for a steel pipe pile can be categorized; for a pile embedded in a non aggressive and natural soil, 0.015 mm per side per year can be assumed from the British Steel Piling Handbook. Eurocode 3 now specifies various corrosion rates based on the nature or soil conditions and pipe pile exposure.

Prestressed concrete piles

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Figure 6

http://www.pilinguk.com/technicalpiling.asp

Student / Graduate Introduction to Pile Foundations

Concrete piles are typically made with steel reinforcing and prestressing tendons to obtain greater tensile strength (See Figure 7). Pile joints can be used to join two or more short piles to form one long pile. Pile joints can be used with both precast and prestressed concrete piles

+ Have the tensile strength required to survive handling and driving.

+ Can provide greater bending resistance than RC piles.

− More Expensive than RC piles.

− Long piles can be difficult to handle and transport.

Materials

WoodTimber is most suitable for long cohesion piling and piling beneath embankments.The timber should be in a good condition and should not have been attacked by insects.For timber piles of length less than 14 meters, the diameter of the tip should be greater than 150 mm. If the length is greater than 18 meters a tip with a diameter of 125 mm is acceptable. Keeping the timber below the ground water level will protect the timber against decay and putrefaction. To protect and strengthen the tip of the pile, timber piles can be provided with toe cover. Pressure creosoting is the usual method of protecting timber piles.

+ Wood piles are easy to handle

+ Can be relatively inexpensive in regions where timber supplies are plentiful.

+ Section can be joined together easily and any excess length can be removed.

− However piles will rot above the water table and have a limited bearing capacity.

− They can be easily damaged by stones and boulders.

− Wooden piles are not suitable where there is considerable risk of attack from marine borers.

Pre cast concrete Piles or Pre fabricated concrete piles

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Figure 7

http://www.pilinguk.com/technicalpiling.asp

Student / Graduate Introduction to Pile Foundations

Usually of square, triangle, circle or octagonal section, they are produced in short lengths, in one metre intervals between 3 and 13 meters. . Pre-cast piles are available between 250mm to 355mm square and with design loads of up to 1500kN, and have been successfully driven to depths of up to 70m

+ They are pre-cast so that they can be easily connected together in order to reach to the required length. This will not decrease the design load capacity.

+ Pre-cast concrete piles do not corrode or rot.

+ Pre-cast concrete remains stable in squeezing ground and can be driven into the ground again if affected by ground heave.

+ They are unaffected by ground water and can be driven in long lengths.

− Reinforcement is necessary within the pile to help withstand both handling and driving stresses. Pre stressed concrete piles are also used and are becoming more popular than the ordinary pre cast as less reinforcement is required.

− Cost of cutting piles.

Driven and cast in place Concrete pilesTwo of the main types used in the UK are: West’s shell pile : Pre cast, reinforced concrete tubes, about 1 m long, are threaded on to a steel mandrel and driven into the ground after a concrete shoe has been placed at the front of the shells. Once the shells have been driven to specified depth the mandrel is withdrawn and reinforced concrete inserted in the core. Diameters vary from 325 to 600 mm.

Franki Pile: A steel tube is erected vertically over the place where the pile is to be driven, and about a metre depth of gravel is placed at the end of the tube. A drop hammer, 1500 to 4000kg mass, compacts the aggregate into a solid plug which then penetrates the soil and takes the steel tube down with it. When the required depth has been achieved the tube is raised slightly and the aggregate broken out. Dry concrete is now added and hammered until a bulb is formed. Reinforcement is placed in position and more dry concrete is placed and rammed until the pile top comes up to ground level.

Steel piles

+ Their relatively small cross-sectional area combined with their high strength makes penetration easier in firm soil. They can be easily cut off or joined by welding.

+ Steel piles are suitable for handling and driving in long lengths

− If the pile is driven into a soil with low pH value, then there is a risk of corrosion, although tar coating or cathodic protection can be employed in permanent works.

− The small Cross-sectional area of steel piles means that there is little compaction.

It is common to allow for an amount of corrosion in design by simply over dimensioning the cross-sectional area of the steel pile. In this way the corrosion process can be prolonged up to 50 years. Normally the speed of corrosion is 0.2-0.5 mm/year and, in design, this value can be taken as 1mm/year.

Composite pilesA combination of different materials, used in the same of pile. (E.g. Figure 8). As indicated earlier, part of a timber pile which is installed above ground water could be vulnerable to insect attack and decay. To avoid this, concrete or steel pile is used above the ground water level, whilst wood pile is installed under the ground water level.

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Student / Graduate Introduction to Pile Foundations

City Centre LocationsIt is important to be aware of the location of underground services before beginning, as well as adjacent buildings that may be affected by vibrations etc. Restricted access and traffic congestion can also cause difficulties. Where vibration needs to be eliminated, CFA piles are often the best option.

Contractors

Branlow Piling Solutions, www.restricted-access-piling.co.uk/piled_foundation_home.htmBachy Soletanche, www.bacsol.co.uk/main.php?s=introductionFK Lowry Piling (Lisburn) www.skillsware.info/fklowryDawson-Wam, www.dawsonwam.co.uk/Skanska, www.skanska.co.uk/Roger Bullivant, http://www.rb-ireland.com/Keller Ground Engineering, www.keller-ge.co.ukHamilton Bogie (Mini-piling) www.mini-piling.comBilfinger Berger www.bilfingerberger.de

Codes of Practice

BS 8004:1986 Foundations (Section 7, Pile Foundations)BS 7385:1993 Evaluation and Measurement for Vibration in BuildingsBS EN 12699:2001 Execution of Special Geotechnical Work, Displacement PilesBS EN 1536:2000 Execution of Special Geotechnical Work, Bored PilesBS EN 12063:1999 Execution of Special Geotechnical Work, Sheet Pile WallsBS EN 14199:2005 Execution of Special Geotechnical Work, MicropilesBS EN 12794:2005 Precast Concrete Products, Foundation Piles

Resources

http://www.restricted-access-piling.co.uk/piled_foundation_home.htmhttp://www.sbe.napier.ac.uk/projects/piledesign/guide/index.htm (Contains methods of pile design)Foundation Design and Construction, 7th Edition, MJ Tomlinson

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Figure 8

http://www.pilinguk.com/technicalpiling.asp

Student / Graduate Introduction to Pile Foundations

The Foundation Engineering Handbook, Manjriker GunaratneH-pile Design Guide, Publication 335 SCIDesign of laterally loaded piles, Report R103 CIRIASteel Bearing Piles Guide, Publication 156 SCIDesign of Reinforcement in Piles, Report 144 TRLCIRIA Technical note 142, Ground-borne vibrations arising from piling

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