Type of Pile

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3. TYPES OF PILE

3.1 CLASSIFICATION OF PILESPiles can be classified according to the type of material forming the piles, the modeof load transfer, the degree of ground displacement during pile installation or the method ofinstallation.Pile classification in accordance with material type (e.g. steel and concrete) hasdrawbacks because composite piles are available. A classification system based on the modeof load transfer will be difficult to set up because the proportion of skin friction and end-bearing that occurs in practice usually cannot be reliably predicted.In the installation of piles, either displacement or replacement of the ground willpredominate. A classification system based on the degree of ground displacement during pileinstallation, such as that recommended in BS 8004 (BSI, 1986) encompasses all types of pilesand reflects the fundamental effect of pile construction on the ground which in tum will havea pronounced influence on pile performance. Such a classification system is thereforeconsidered to be the most appropriate.In this document, piles are classified into the following four types :

(a) Large-displacement piles, which include all solid drivenpiles, including precast concrete piles, and steel or concretetubes closed at the lower end by a driving shoe or a plug,i.e. driven cast-ill-place piles.

(b) Small-displacement piles, which include rolled steel sectionssuch as H-piles and open-ended tubular piles. However.these piles will effectively become large-displacement pilesif a soil plug forms.(c) Replacement piles, which are formed by machine boring,grabbing or hand-digging. The excavation may need to besupported by bentonite slurry, or lined with a casing that iseither left in place or extracted during concreting for re-use.(d) Special piles, which are particular pile types or variants ofexisting pile types introduced from time to time to improveefficiency or overcome problems related to special groundconditions.

This Chapter describes the types of piles commonly used in Hong Kong together withtheir advantages and disadvantages.


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3.2 LARGE-DISPLACEMENT PILES3.2.1 General

The advantages and disadvantages of large-displacement piles are summarised inTable 2.

3.2.2 Precast Reinforced Concrete PilesThese piles are commonly in square sections ranging from about 250 mm to about450 mm with a maximum section length of up to about 20 m. Other pile sections mayinclude hexagonal, circular, triangular and H shapes, but these are not common in HongKong. Maximum allowable axial loads can be up to about 1 000 kN. The lengths of pile

sections are often dictated by practical considerations including transportability, handlingproblems in sites of restricted area and facilities of the casting yard.These piles can be lengthened by coupling together on site. Splicing methodscommonly adopted in Hong Kong include welding of steel end plates or the use of epoxymortar with dowels. Specially fabricated joints have been successfully used in othercountries, e.g. Scandinavia.This type of pile is not suitable for driving into ground which contains a significantamount of boulders or corestones,

3.2.3 Prestressed Concrete Tubular PilesPrecast prestressed concrete piles commonly used in Hong Kong are closed-endedtubular sections of 400 mm to 600 mm diameter with maximum allowable axial loads up toabout 3 000 kN. Pile sections are normally 12m long and are usually welded together usingsteel end plates. Pile sections up to 20 m can also be specially made.Prestressed concrete piles require high-strength concrete and careful control duringmanufacture. Casting is usually carried out in a factory where the curing conditions can bestrictly regulated. Special manufacturing processes such as compaction by spinning orautoclave curing can be adopted to produce high strength concrete up to about 75 MPa.A variant of the conventional precast prestressed concrete piles is described by Choy(1993). This involves a revised design of the pile shoe details aimed at improving thedriveability characteristics. Experience with this type of pile is limited.Precast prestressed concrete piles may be handled more easily than precast reinforcedconcrete piles without damage. Spalling, cracking and breaking can occur if careful controlis not undertaken and good driving practice is not followed (see Section 7.2.5 for more

details). This type of pile is generally less permeable than reinforced concrete piles and maybe expected to exhibit superior performance in a marine environment. Such piles have beensuccessfully employed in Hong Kong for many port works projects.


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3.2.4 Closed-ended Steel Tubular PilesThe use of box-section steel piles is not common in Hong Kong but steel tubular pilesare becoming increasingly popular, particularly for marine structures.Steel tubular piles have high bending and buckling resistance, and have favourableenergy-absorbing characteristics for impact loading. Steel piles are generally not susceptibleto damage caused by tensile stresses during driving and can withstand hard driving. Drivingshoes can be provided to aid penetration.The tubular piles may be infilled with concrete after driving, as appropriate.

3.2.5 Driven Cast-in-place Concrete PilesDriven cast-in-place concrete piles are formed by driving a steel tube into the groundto the required set or depth and withdrawing the tube after concrete placement. The use ofprecast concrete rings instead of a steel tube is not common in Hong Kong. The tube maybe driven either at the top or at the bottom with a hammer acting on an internal concrete orcompacted gravel plug. A range of pile sizes is available, up to 600 rum in diameter. Themaximum allowable axial load is about 1 400 kN. The maximum length of such pilesconstructed in Hong Kong is about 30 m.Proprietary systems of top-driven, cast-in-place piles have been used in Hong Kong.

In this method, the steel tube is provided with a loose conical or flat cast-iron shoe whichkeeps the tube closed during driving. Light blows are usually imparted to the tube duringextraction, thus assisting concrete compaction.For bottom-driven, cast-in-place piles with an expanded base, the tube does not haveto withstand direct impact and can be of a smaller thickness. Also, the piling rig does notneed to be as tall as rigs for other driven cast-in-place piling systems. When pile driving iscompleted, the tube is held against further penetration and the bottom plug is driven out bythe hammer within the tube. An enlarged pile base is formed using 'dry' mix concrete, witha water/cement ratio of approximately 0.2, which is rammed heavily with the internalhammer.

3.3 SMALL-DISPLACEMENT PILES3.3.1 General

Small-displacement piles are either solid (e.g. steel H piles) or hollow (open-endedtubular piles) with a relatively low cross-sectional area. This type of pile is usually installedby percussion methods; however a soil plug may be formed during driving, particularly withtubular piles, and periodic drilling out may be necessary to reduce the driving resistance. Asoil plug can create a greater driving resistance than a closed end, because of damping on theinner-side of the pile.

The advantages and disadvantages of small-displacement piles are summarised in


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Table 2.

3.3.2 Steel H-pilesSteel H-piles have been widely used in Hong Kong because of their ease of handlingand driving. Compared with concrete piles, they generally have better driveabilitycharacteristics and can generally be driven to greater depths. H-piles can be susceptible todeflection upon striking boulders, obstructions or an inclined rock surface. In areas underlainby marble, heavy-section H-piles with appropriate tip strengthening is commonly used topenetrate the karst surface and to withstand hard driving.A range of pile sizes is available, with different grades of steel. The maximumallowable axial load is typically about 3 000 kN. Very large H sections (283 kg/m) with a

working load of about 3 600 kN have been used in some projects.3.3.3 Open-ended Steel Tubular Piles

Driven open-ended tubular steel piles have been used in marine structures and inbuildings on reclaimed land. This type of pile has been driven to over 50 m. A plug willform when the internal skin friction exceeds the end-bearing resistance of the entire cross-sectional area the pile. Driving resistance can be reduced by pre-boring or by reaming outthe plug formed within the pile. Typical diameters range from 275 mm to about 2 m witha maximum allowable axial load of about 7 000 kN. Maximum pile diameter is oftengoverned by the capacity of the driving machine available, which is generally not greater thanDelonag 100 diesel hammer in Hong Kong.

3.4 REPLACEMENT PILES3.4.1 General

Replacement, or bored, piles are formed by excavation or boring techniques. Whenconstructed in water-bearing soils which are not self-supporting, the pile bore will need to besupported using steel casing, concrete rings or drilling fluids such as bentonite slurry,polymer mud, etc. Excavation of the pile bore may also be carried out by hand-digging inthe dry; and the technique developed in Hong Kong involving manual excavation is knownlocally as hand-dug caissons.The advantages and disadvantages of replacement piles are summarised in Table 3.

3.4.2 Machine-dug Piles(1) General. Machine-dug piles are formed by rotary boring, or percussive methodsof boring, and subsequently filling the hole with concrete. Piles with 600 rum or less indiameter are commonly known as small-diameter piles. Piles greater than 600 mm diameterare referred to as large-diameter piles.


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(2) Small-diameter bored piles. This type of pile has sizes ranging from about300 mm to 600 mm with working loads up to about 1 500 kN.

One proprietary form of small-diameter bored pile involves the use of drop tools forexcavation and compressed air to compact the concrete in the pile shaft. The common sizesof this type of piles used in Hong Kong range from 325 mm to 508 mm, with working loadsup to about 1 000 kN. These piles can be installed in sites where the headroom is limited.These piles are sometimes constructed without reinforcement and the integrity of such un-reinforced piles when subject to ground movements arising from adjacent constructionactivities should be considered.

Another proprietary piling system is the continuous-flight auger (cfa) type piles suchas the 'Pakt-in-Place (PIP) Pile' used in Hong Kong. In this system, the bore is formed usinga flight auger and concrete or grout is pumped in through the hollow stem. Sizes of PIP pilesrange from 300 rom to 700 rom diameter and lengths are generally less than 30 m. The cfapiles have considerable advantages over conventional bored piles in water-bearing andunstable soils by eliminating the need of casing and the problems of concreting underwater.The piles can be installed with little noise and vibration and are therefore suited for sites inurban areas. However, this type of pile cannot cope with boulders. The lack of penetrationunder continuous rotation due to a hard layer or an obstruction can lead to soil flighting upthe auger causing ground loss and settlement.

(3 ) Large-diameter boredpiles. Large-diameter machine-dug piles are used in HongKong to support heavy column loads of tall buildings (e.g. Holt et al, 1982; Fraser, 1985)and highways structures such as viaducts (Fraser &Kwok; 1986). Typical sizes of these pilesrange from 1 m to 2.5 m, with lengths up to about 80 m and working loads up to about36 000 kN. Special mechanical tools are readily available for belling out the base. Pile oflarger diameters have been constructed in Hong Kong, but only to fairly shallow depth.

Traditionally in Hong Kong, large-diameter machine-dug piles are required to be'founded on rock' with a prescribed safe bearing pressure. In reality, for many such boredpiles constructed in saprolite, the load is resisted primarily by skin friction.

3.4.3 Hand-dug Caissons(1 ) General. Hand-dug caissons have been used widely in Hong Kong as foundations

or earth retaining structures with a diameter typically ranging from 1.5 m to 2.5 m, and anallowable load of up to about 25 000 kN. Hand-dug caissons of a much larger size, ofbetween 7 m and 10 m in diameter, have also been constructed successfully (e.g. Humphesonet al, 1986; Bareham & Gillespie, 1988). The advantages and disadvantages of hand-dugcaissons are .summarised in Table 3.

Hand-dug caisson shafts are excavated using hand tools in stages with depths of upto about 1 m, depending on the competence of the ground. Dewatering is facilitated bypumping from sumps on the excavation floor or from deep wells. Advance grouting may becarried out to provide support in potentially unstable ground. Each stage of excavation islined with insitu concrete rings (minimum 75 mm thick) using tapered steel forms whichprovide a key to the previously constructed rings. When the diameter is large, the rings may


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be suitably reinforced against stresses arising from eccentricity and non-uniformity in hoopcompression. Near the bottom of the pile, the shaft may be belled out to enhance thetheoretical load-carrying capacity.The isolation of the upper part of hand-dug caissons by sleeving is sometimesprovided for structures built on sloping ground to prevent the transmission of lateral loads tothe slope or conversely the build-up of lateral loads on caissons by slope movement (GCO,1984). However, there is a lack of instrumented data on the long-term performance of thesleeving.(2) Technical guidance. Given the disturbingly high accident rate and the healthhazard in installation and construction (Ng et al, 1987) of caissons, their use should bediscouraged. Examples of situations where the use of caissons should be avoided include:

(a) coastal reclamation sites with high groundwater table,(b) sites underlain by cavernous marble,(c) deep foundation works (e.g. in excess of say 50 m),(d) landfill or chemically-contaminated sites,(e) sites with a history of deep-seated ground movement,(f) sites in close proximity to water or sewerage tunnels,(g) sites in close proximity to shallow foundations, and(h) sites with loose fill having depths in excess of say 10 m.

Hand-dug caissons should be constructed only when they are the only practicablesolution and there is no safe engineered alternative. All reasonable and appropriateprecautionary measures must be taken to ensure adequate protection of workers.Examples of situations where hand-dug caissons may be considered include:

(a) steeply-sloping sites with hand-dug caissons of less than25 m in depth in soil, and(b) sites with difficult access or insufficient working roomwhere it may be impracticable or unsafe to use mechanicalplant.

In all cases, the desirable minimum internal diameter of hand-dug caissons is 1.8 m.Before opting for hand-dug caissons, a risk assessment should be carried out coveringgeneral safety, the cost of damage arising from dewatering, and the possibility of unforeseenground conditions. The design of caisson linings should also be examined for suitability asfor any other structural temporary works.


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A guide to good practice for the design and construction of hand-dug caissons has beenproduced by the Hong Kong Institution of Engineers (1981). Further discussion on thepotential problems during construction of hand-dug caissons is given in Section 7.4.3.Where hand-dug caissons are employed, consideration should be given to the followingprecautionary measures and preventive works, as appropriate:

(a) carrying out additional ground investigation to obtain bestpossible information about the ground conditions,(b) pre-grouting around each hand-dug caisson to reduce therisk of collapse and limit the groundwater drawdown,(c) installation of cut-off walls or curtain grouting around the

site boundary or around groups of caissons to limit inflowof water,(d) installation of dewatering wells within the site, possiblysupplemented by recharge wells around the periphery ofthe site to limit the groundwater drawdown in adjacentground,(e) construction of the caissons in a suitable sequence,(f) reduction in the depth of each caisson digging stage,(g) provision of immediate temporary support for theexcavated face prior to the casting of the concrete liner,(h) provision of steel reinforcement to the concrete liner,(i) driving dowels radially into the surrounding soil asreinforcement at the bottom of excavation to reduce thechance of heaving,(j) provision of a drainage or relief well at the position ofeach caisson in advance of manual excavation,(k) avoidance of the introduction of new caisson gangs intopartly-completed excavations,(1) completion of proper grouting of ground investigationboreholes and old wells in the vicinity of hand-dugcaissons,(m) provision of good ventilation,(n) use of well-maintained and checked equipment,


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(0) safety inspections,(p) provision of safety equipment,(q) an assessment of the risks by a safety professional to thehealth and safety of the workers whilst at work in caissonsand implementing, monitoring and reviewing the measuresto comply with the requirements under all existing safetylegislation,(r) monitoring and control of the potential health hazards,e.g. poisonous gases, oxygen deficiency, radon and silicadust, and(s) monitoring the ground water table and possibly the groundand sub-soil movement by piezometers and inclinometersinstalled around the site boundary.

For general guidance on the practicable safety and health measures in the constructionof hand-dug caissons, reference may be made to the 'Code of Safe Working Practices forHand-dug Caissons' published by the Occupational Safety&Health Council (1993).(3) Engagement of suitable staff One of the most important elements in the successof a hand-dug caisson project is the engagement of suitably qualified and experienced

professionals in the geotechnical assessment and investigation of the site to identify potentiallyunfavourable geological and hydro-geological conditions that may give rise to engineering andconstruction problems, and to implement the necessary precautionary and preventivemeasures. Likewise, the employment of suitably trained and experienced constructionworkers, together with adequate supervision to promote strict adherence to stringent safetyand health requirements, is also a pre-requisite.

3.5 SPECIAL PILE TYPES3.5.1 General

Three special pile types, viz. barrettes, mini-piles and composite piles, are discussedbelow.

3.5.2 BarrettesA barrette of rectangular section is a variant of the traditional bored pile. Therectangular holes are excavated with the use of grabs. In Hong Kong, common barrette sizesare 0.8 m x 2.2 m and 1.2 m x 2.8 m, although the panels can be up to about 5 m to 6 m,

with depths to about 80 m. Because of their rectangular shape, barrettes can be oriented togive maximum resistance to moments and horizontal forces.Load tests on barrettes founded in saprolite have demonstrated that significant skin


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friction can also be mobilised (e.g. Pratt & Sims, 1990).

3.5.3 Mini-pilesMini-piles generally have a diameter of between 100 mm and 250 mm. Reinforcement

bars are provided in the piles.Construction can be carried out typically to about 60 m depth or more, although

verticality control will become more difficult at greater depths. Mini-piles are usually formedby small drilling rigs with the use of down-the-hole hammers or rotary percussive drills.They can be used for sites with difficult access or limited headroom and for underpinning.In general, they can overcome large or numerous obstructions in the ground.

Given the small diameter and high slenderness ratio of mini-piles, the load is resistedlargely by skin friction. Where mini-piles are installed in soil, the working load is usuallyless than 700 kN but can be in excess of 1 000 kN if post grouting is undertaken using tube-a-manchette. In the case of mini-piles forming rock sockets, the capacity may be limited bythe prescribed permissible structural stresses, which may be up to about 1 350 kN.

The structural design of mini-piles is discussed in Sections 5.12.4 & 5.12.5. Wheremini-piles are employed, the combined resistance of the pile cap and the piles may be usedto take the horizontal loading. Comments on this design approach are given inSection 6.5.2(3).

Corrosion aspects of mini-piles are discussed in Section 5. 14.

3.5.4 Composite PilesSome systems of composite piles have been developed to deal with special site

conditions. Three types of composite piles that have been used in Hong Kong are discussedbelow.

The first type is essentially a combination of driven cast-in-place techniques with pre-formed pile sections in reclamations. In this system, a driven cast-in-place piling tube isinstalled and the expanded base is concreted. A steel H-section is then inserted and beddedusing light hammer blows. Further concrete is introduced to provide a bond length sufficientto transfer the load from the steel section. The concrete is terminated below the soft depositsand the remainder of the piling tube is filled with sand before it is extracted.

Similar composite construction has also been tried with other driven cast-in-place pilingsystems in combination with precast concrete sections, which may be sleeved with bitumen,in order to avoid the risk of damage to the coating during driving. A variant of this type ofcomposite pile involves grouting steel H-sections into sockets drilled in rock, instead of usingrock anchors or dowels, to resist uplift forces (Construction & Contract News, 1993).

The second type of composite pile is the Steel-Concrete Composite (SC) Pile. Thiscomprises a structural steel casing with a hollow spun concrete core and a solid driving shoe.


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By combining the advantages of good quality concrete and high strength external steel pipecasing, SC pipe piles can provide better driveability and lateral load resistance but moreemphasis has to be placed on corrosion protection. Pile sizes are similar to precastprestressed piles with maximum working loads of about 2 800 kN. The piles can be installedwith the centre-augering system (Fan, 1990) which is a non-percussive system with minimalnoise and vibrations. The augering and drilling can be carried out in the centre hole of thepile which is jacked into the predrilled hole by a counter weight and hydraulic jack mountedon the machine. The final set can be obtained using a pile driving hammer.

A third type of composite pile is the drill-and-drive system whereby the open-endedtubular pile is drilled out when it cannot be driven further. Such a system may, in principle,be used to facilitate penetration of cavernous marble in Hong Kong. However, it is importantto exercise stringent control on the drilling procedure to avoid excessive loss of ground.

If concrete is cast into a steel tube after it has been driven, the allowable capacity ofthe composite pile will be influenced by strain compatibility requirements. Considerationshould be given to the possible effect of radial shrinkage of the concrete which can affect thebond with the steel tube. Shear keys may be used to ensure adequate shear transfer in thecase where the upper part of an open-ended steel tube is concreted (Troughton, 1992).

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