Pile foundations are used extensively for the support of buildings, bridges, and other structures to safely transfer structural loads to the ground and to avoid excess settlement or lateral movement. They are very effective in transferring structural loads through weak or compressible soil layers into the more competent soils and rocks below. A “driven pile foundation” is a specific type of pile foundation where structural elements are driven into the ground using a large hammer. They are commonly constructed of timber, precast prestressed concrete (PPC), and steel (H-sections and piles.)
A thorough understanding on the ground conditions of a site is a pre-requisite to the success of a foundation project. The overall objective of a site investigation for foundation design is to determine the site constraints, geological profile and the properties of the various strata. The geological sequence can be established by sinking boreholes from which soil and rock samples are retrieved for identification and testing. In situ tests may also be carried out determine the mass properties of the ground. These investigation methods may be Supplemented by regional geological studies and geophysical tests where justified by the and importance of the project, or the complexity of the ground conditions. The importance of a properly planned and executed ground investigation cannot be over-emphasised. The information obtained from the investigation will allow an appropriate geological model to be constructed. This determines the selection of the optimum foundation system for the proposed structure. It is important that the engineer planning the site and designing the foundations liaises closely with the designer of the superstructure and the project coordinator so that specific requirements and site constraints are fully understood by the project team.
CLASSIFICATION OF PILES
Piles can be classified according to the type of material forming the piles, the mode of load transfer, the degree of ground displacement during pile installation and the method of Installation. Pile classification in accordance with material type (e.g. steel and concrete) has drawbacks because composite piles are available. A classification system based on the mode of load transfer will be difficult to set up because the proportion of shaft resistance and endearing resistance that occurs in practice usually cannot be reliably predicted.
Piles are classified into the following four type.
(a) Large-displacement piles, which include all solid piles, including precast concrete piles, and steel or concrete tubes closed at the lower end by a driving shoe or a plug,i.e. cast-in-place piles.
(b) Small-displacement piles, which include rolled steel sections such as H-piles and open ended tubular piles. However, these piles will effectively become large displacement piles if a soil plug forms.
(c) Replacement piles, which are formed by machine boring, grabbing or hand-digging. The excavation may need to be supported by bentonite slurry, or lined with a casing that is either left in place or extracted during concreting for re-use.
(d) Special piles, which are particular pile types or variants of existing pile types introduced from time to time to improve efficiency or overcome problems related to special ground conditions.
Pile foundation systems
Foundations relying on driven piles often have groups of piles connected by a pile cap (a large concrete block into which the heads of the piles are embedded) to distribute loads which are larger than one pile can bear. Pile caps and isolated piles are typically connected with grade beams to tie the foundation elements together; lighter structural elements bear on the grade beams while heavier elements bear directly on the pile cap.
Also called drilled piers or Cast-in-drilled-hole piles (CIDH piles) or Cast-in-Situ piles. Rotary boring techniques offer larger diameter piles than any other piling method and permit pile construction through particularly dense or hard strata. Construction methods depend on the geology of the site. In particular, whether boring is to be undertaken in 'dry' ground conditions or through water-logged but stable strata - i.e. 'wet boring'.
'Dry' boring methods employ the use of a temporary casing to seal the pile bore through water-bearing or unstable strata overlying suitable stable material. Upon reaching the design depth, a reinforcing cage is introduced, concrete is poured in the bore and brought up to the required level. The casing can be withdrawn or left in situ.
'Wet' boring also employs a temporary casing through unstable ground and is used when the pile bore cannot be sealed against water ingress. Boring is then undertaken using a digging bucket to drill through the underlying soils to design depth. The reinforcing cage is lowered into the bore and concrete is placed by tremie pipe, following which, extraction of the temporary casing takes place.
In some cases there may be a need to employ drilling fluids (such as bentonite suspension) in order to maintain a stable shaft. Rotary auger piles are available in diameters from 350 mm to 2400 mm or even larger and using these techniques, pile lengths of beyond 50 metres can be achieved.
Under ream piles have mechanically formed enlarged bases that have been as much as 6 m in diameter. The form is that of an inverted cone and can only be formed in stable soils. In such conditions they allow very high load bearing capacities.
An auger cast pile, often known as a CFA pile, is formed by drilling into the ground with a hollow stemmed continuous flight auger to the required depth or degree of resistance. No casing is required. A high slump concrete mix is then pumped down the stem of the auger. While the concrete is pumped, the auger is slowly withdrawn, lifting the spoil on the flights. A shaft of fluid concrete is formed to ground level. Reinforcement placed by hand is normally limited to 6 metres in depth. Longer reinforcement cages can be installed by a vibrator, or placed prior to pouring concrete if appropriate specialized drilling equipment is used.
Auger cast piles cause minimal disturbance, and are often used for noise and environmentally sensitive sites. Auger cast piles are not generally suited for use in contaminated soils, due to expensive waste disposal costs. In ground containing obstructions or cobbles and boulders, auger-cast piles are less suitable as damage can occur to the auger.
In most drilled pier foundations, the piers are connected with grade beams - concrete beams at grade (also referred to as 'ground' beams) - and the structure is constructed to bear on the grade beams, sometimes with heavy column loads bearing directly on the piers. In some residential construction, the piers are extended above the ground level and wood beams bearing on the piers are used to support the structure. This type of foundation results in a crawl space underneath the building in which wiring and duct work can be laid during construction or remodelling.
A micro pile installation.
Micropiles, 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.[1]
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.
The use of a tripod rig to install piles is one of the more traditional ways of forming piles, and although unit costs are generally higher than with most other forms of piling, it has several advantages which have ensured its continued use through to the present day. The tripod system is easy and inexpensive to bring to site, making it ideal for jobs with a small number of piles. It can work in restricted sites (particularly where height limits exist), it is reliable, and it is usable in almost all ground conditions.
Sheet piling is a form of driven piling using thin interlocking sheets of steel to obtain a continuous barrier in the ground. The main application of steel sheet piles is in retaining walls and cofferdams erected to enable permanent works to proceed.
A soldier pile wall using reclaimed railway sleepers as lagging.
Soldier piles, also known as king piles or Berlin walls, are constructed of wide flange steel H sections spaced about 2 to 3 m apart and are driven prior to excavation. As the excavation proceeds, horizontal timber sheeting (lagging) is inserted behind the H pile flanges.
The horizontal earth pressures are concentrated on the soldier piles because of their relative rigidity compared to the lagging. 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.
Suction piles are used underwater to secure floating platforms. Tubular piles are driven into the seabed (or more commonly dropped a few metres into a soft seabed) and then a pump sucks water out the top of the tubular, pulling the pile further down.
The proportions of the pile (diameter to height) are dependent upon the soil type: Sand is difficult to penetrate but provides good holding capacity, so the height may be as short as half the diameter; Clays and muds are easy to penetrate but provide poor holding capacity, so the height may be as much as eight times the diameter. The open nature of gravel means that water would flow through the ground during installation, causing 'piping' flow (where water boils up through weaker paths through the soil). Therefore suction piles cannot be used in gravel seabeds.
Once the pile is positioned using suction, the holding capacity is simply a function of the friction between the pile skin and the soil, along with the self-weight and weight of soil held within the pile. The suction plays no part in holding capacity because it relieves over time. The wall friction may increase slightly as pore pressure is relieved. One notable failure occurred (pullout) because there was poor contact between steel and soil, due to a combination of interal ring stiffeners and protective painting of the steel walls.
In extreme latitudes where the ground is continuously frozen, adfreeze piles are used as the primary structural foundation method.
Adfreeze piles derive their strength from the bond of the frozen ground around them to the surface of the pile. Typically the pile is installed in a pre-drilled hole 6"-12" larger then the diameter of the pile. A slurry mixture of sand and water is then pumped into the hole to fill the space between the pile and the frozen ground. Once this slurry mixture freezes it is the shear strength between the frozen ground and the pile, or the adfreeze strength, which support the applied loads.
Adfreeze pile foundations are particularly sensitive in conditions which cause the permafrost to melt. If a building is constructed improperly, it will heat the ground below resulting in a failure of the foundation system.
Another ongoing concern for adfreeze pile foundations is climate change. As the climate warms, these foundations lose their strength and will eventually fail.
Piled walls
Sheet piling, by a bridge, was used to block a canal in New Orleans after Hurricane Katrina damaged it.
These methods of retaining wall construction employ bored piling techniques - normally CFA or rotary. They provide special advantages where available working space dictates that basement excavation faces be vertical. Both methods offer technically effective and cost efficient temporary or permanent means of retaining the sides of bulk excavations even in water bearing strata.
When used in permanent works, these walls can be designed to accommodate vertical loads in addition to moments and horizontal forces.Construction of both methods is the same as for foundation bearing piles. Contiguous walls are constructed with small gaps between adjacent piles. The size of this space is determined by the nature of the soils.
Secant piled walls are constructed such that space is left between alternate 'female' piles for the subsequent construction of 'male' piles. Construction of 'male' piles involves boring through the concrete in the 'female' piles in order to key 'male' piles between them. The male pile is the one where steel reinforcement cages are installed, though in some cases the female piles are also reinforced.
Secant piled walls can either be true hard/hard, hard/intermediate (firm), or hard/soft, depending on design requirements. Hard refers to structural concrete and firm or soft is usually a weaker grout mix containing bentonite.
All types of wall can be constructed as free standing cantilevers, or may be propped if space and sub-structure design permit. Where party wall agreements allow, ground anchors can be used as tie backs.
Materials
As the name implies, timber piles are piles made of timber. Historically, timber has been a plentiful, locally-available resource in many areas of the globe. Today, timber piles are still more affordable than concrete or steel. Compared to other types of piles (steel or concrete), and depending on the source/type of timber, timber piles may not be suitable for heavier loads (Although for instance 350 toe diameter piles sourced from Australian hardwoods can take upward of 3500kN for some species). A main consideration regarding timber piles is that they should be protected from deterioration above groundwater level. Timber will last for a long time below the groundwater level. For timber to deteriorate, two elements are needed: water and oxygen. Below the groundwater level, oxygen is lacking even though there is ample water. Hence, timber tends to last for a long time below groundwater level. It has been reported that some timber piles used during 16th century in Venice still survive since they were below groundwater level. Timber that is to be used above the water table can be protected from decay and insects by numerous forms of preservative treatment (ACQ, CCA, Creosote, PEC, Copper Napthenate, etc.). Splicing timber piles is still quite common and is the easiest of all the piling materials to splice. The normal method for splicing is by driving the leader pile first, driving a steel tube (normally 600-1000mm long, with an internal diameter no smaller than the minimum toe diameter) half its length onto the end of the leader pile. The follower pile is then simply slotted into the other end of the tube and driving continues. The steel tube is simply there to ensure that the two pieces follow each other during driving. If uplift capacity is required, the splice can incorporate bolts, coach screws, spikes or the like to give it the necessary capacity.
Pipe 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.
FACTORS TO BE CONSIDERED IN CHOICE OF PILE TYPE
The determination of the need to use piles and the identification of the range of feasible pile types for a project form part of the design process. In choosing the most appropriate pile type, the factors to be considered include ground conditions, nature of loading, effects on surrounding structures and environs, site constraints, plant availability, safety, cost and programme, taking into account the design life of the piles. Normally, more than one pile type will be technically feasible for a given project. The selection process is in essence a balancing exercise between various, and sometimes conflicting, requirements. The choice of the most suitable type of pile is usually reached by first eliminating any technically unsuitable pile types followed by careful consideration of the advantages and disadvantages of the feasible options identified. Due regard has to be paid to technical, economical, operational, environmental and safety aspects.
SELECTION OF DESIGN PARAMETERS
The selection of parameters for foundation design should take into account the extent, quality and adequacy of the ground investigation, reliability of the geological and geotechnical analysis model, the appropriateness of the test methods, the representativeness of soil parameters for the likely field conditions, the method of analysis adopted for the , and the likely effects of foundation construction on material properties. In principle, sophisticated analyses, where justified, should only be based on high quality test results. The
reliability of the output is, of course, critically dependent on the and accuracy of the input parameters.
PLATE LOADING TEST
The test should mainly be used to derive geotechnical parameters for predicting the settlement of a shallow foundation, such as the deformation modulus of soil. It may be necessary to carry out a series of tests at different levels. The plate loading test may also be used to determine the bearing capacity of the foundation in fine-grained soils, which is independent of the footing size. The elastic soil modulus can be determined using the following equation (BSI, 2000b) :
Es = qnet b {(1-νs2)/ δp} Is
Where qnet = net ground bearing pressure
δp = settlement of the test plate
Is = shape factor
b = width of the test plate
νs = Poisson’s ratio of the soil
Es = Young's modulus of soil
The method for extrapolating plate loading test results to estimate the settlement of a full-size footing on granular soils is not standardised. The method proposed by Terzaghi & Peck (1967) suggested the following approximate relationship in estimating the settlement for a full-size footing :
Δf = δp{ (2Bf) / Bf + b}2
where δp = settlement of a 300 mm square test plate
δf = settlement of foundation carrying the same bearing pressure
Bf = width of the the shallow foundation b = width of the test plate
However, the method implies that the ratio of settlement of a shallow foundation to that of a test plate will not be greater than 4 for any size of shallow foundation and this could under-estimate the foundation settlement. Bjerrum & Eggestad (1963) compared the results of plate loading tests with settlement observed in shallow foundations. They noted that the observed foundation settlement was much larger than that estimated from the method of Terzaghi & Pack (1967). Terzaghi et al (1996) also commented that the method is unreliable and is now recognised to be an unacceptable simplification of the complex phenomena.
Pile Driving Formulae
Pile driving formulae relate the ultimate bearing capacity of driven piles to the final set (i.e. penetration per blow). Various driving formulae have been proposed, such as the Hiley Formula or Dutch Formula, which are based on the principle of conservation of energy. The inherent assumptions made in some formulae pay little regard to the actual forces, which develop during driving, or the nature of the ground and its behaviour. Chellis (1961) observed that some of these formulae were based on the assumptions that the stress wave due to pile driving travels very fast down the pile and the associated strains in the pile are considerably less than those in the soil. As a result, the action of the blow is to create an impulse in the pile, which then proceeds to travel into the ground as a rigid body. Where these conditions are fulfilled, pile driving formulae give good predictions. As noted by Chellis, if the set becomes small such that the second condition is not met, then the formulae may become unreliable. Hiley Formula has been widely used for the design of driven piles.
The formula is as follow:
Rp = (ηh αhWh dh)/{s + 0.5(cp + cq + cc)}
Where Rp = driving resistance
αh = efficiency of hammer
ηh = efficiency of hammer blow (allowing for energy loss on impact)
={Wh + e2 (Wp + Wr)} / { Wh + Wp+ Wr }
e = coefficient of restitution
Wp = weight of pile
Wr = weight of pile helmet
Wh = weight of hammer
dh = height of fall of hammer
s = permanent set of pile
cp = temporary compression of pile
cq = temporary compression of ground at pile toe
cc = temporary compression of pile cushion
The driving hammer should be large enough to overcome the inertia of the pile. the allowable maximum final set limit for driven piles in soils is often designed
to be not less than 25 mm per 10 blows, unless rock is reached. A heavy hammer or a higher stroke may be used, but this would increase the risk of damaging the piles . Alternatively, a lower final set value (e.g. 10 mm per 10 blows) can be adopted, provided that adequate driving energy has been delivered to the piles. This can be done by measuring the driving stress by Pile Driving Analyzer (PDA), which can also be used to confirm the integrity of the piles under hard driving condition. Hiley Formula suffers from the following fundamental deficiencies:
(a) During pile driving, the energy delivered by a hammer blow propagates along the pile. Only the compressive waves that reach the pile toe are responsible for advancing the pile.
(b) The rate at which the soil is sheared is not accounted for during pile driving. The high-strain rates in cohesive soils during pile penetration can cause the viscous resistance of the soil to be considerably greater than the static capacity of the pile. Poskitt (1991) shows that without considering soil damping, the driving resistance can be overestimated by several times.
(c) It only considers the hammer ram and the pile as concentrated masses in the transfer of energy. In fact, the driving system includes many other elements such as the anvil, helmet, and hammer cushion. Their presence also
influences the magnitude and duration of peak force being delivered to the pile.
Nature of Loading
Pile selection should take into account the nature and magnitude of the imposed loads. In circumstances where individual spacing between driven piles could result in the problem
of 'pile saturation', i.e. piles are arranged in minimum spacing, the use of large-diameter replacement piles may need to be considered. For structures subject to cyclic and/or impact lateral loading such as in jetties and quay structures, driven steel piles may be suitable as they have good energy-absorbing characteristics. In the case of large lateral loads (e.g. tall buildings), piles with a high moment of resistance may have to be adopted.
In soft ground, such as marine mud or organic soils, cast-in-place piles can suffer necking care is taken when extracting the temporary casing. Construction of hand-dug caissons can be particularly hazardous because of possible piping or heaving at the base. Machine-dug piles with permanent casings can be used to alleviate problems of squeezing. In these ground conditions, driven piles offer benefits as their performance is relatively of the presence of soft ground. However, soft ground conditions may exhibit consolidation settlement which will induce negative skin friction along the shafts of the driven piles. In case the settling strata are of substantial thickness, a large proportion of the structural capacity of the driven piles will be taken up by negative skin friction. The depth of the founding stratum can dictate the feasibility of certain pile types. Advance estimates of the depth at which a driven pile is likely to reach a satisfactory 'set' are usually made from a rule-of-thumb which relies on SPT results. The SPT N value at which large-displacement piles are expected to reach 'set' is quoted by different practitioners in Hong Kong in the range of 50 to 100, whilst the corresponding N value for steel H-piles to reach 'set' is quoted as two to three times greater.
Barrettes and large-diameter machine-dug piles are generally limited to depths of 60 m to 80 m although equipment capable of drilling to depths in excess of 90 m is readily available.
Site and Plant Constraints
In selecting pile types, due consideration should be given to the constraints posed by operation of the equipment and site access. Apart from mini-piles, all other piles require the use of large piling rigs. The machine for jacking piles carries heavy weights. These may require substantial temporary works for sloping ground and sites with difficult access.
Headroom may be restricted by legislation (e.g. sites near airports) or physical obstructions such as overhead services. In such case, large crane-mounted equipment may not be appropriate. Special piling equipment, such as cranes with short booms and short grab, are available to construct barrette piles in area with restricted headroom. Alternatively, mini-piles will be a feasible option. The construction of replacement piles may involve the use of drilling fluid. The plant may require considerable working space. On the other hand, prefabricated piles similarly will require space for storage and stockpiling. These two types of piles may cause operational problems on relatively small sites.
Safety
Safety considerations form an integral part in the assessment of method of construction. Problems with hand-dug caissons include inhalation of poisonous gas and silica dust by workers, insufficient ventilation, base heave, piping, failure of concrete linings and falling objects (Chan, 1987). Their use is strongly discouraged in general. Accidents involving collapse or overturning of the piling rigs, which can be caused by overloading, swinging loads, incorrect operation, wind gusts or working on soft or steeply sloping ground, can result in casualties. Serious accidents may also occur when loads swing over personnel as a result of failure of chain or rope slings due to overloading, corrosion or excessive wear. Notwithstanding the safety risks and hazards involved in pile construction, it should be noted that most of these can be minimised provided that they are fully recognised at the design stage and reasonable precautions are taken and adequate supervision provided. Vetting of contractor's method statements provides an opportunity for safety measures to be included in the contract at an early stage.
Programme and Cost
The design engineer frequently has a choice between a number of technically feasible piling options for a given site. The overall cost of the respective options will be a significant consideration. The scale of the works is a pertinent factor in that high mobilisation costs of large equipment may not be cost effective for small-scale jobs. The availability of plant can also affect the cost of the works. Contractors may opt for a certain piling method, which may not be the most appropriate from a technical point of view, in order to optimise the material, and plant available to them amongst the ongoing projects.
The cost of piling in itself constitutes only part of the total cost of foundation works.
For instance, the cost of a large cap for a group of piles may sometimes offset the higher cost
of a single large-diameter pile capable of carrying the same load. It is necessary to consider the cost of the associated works in order to compare feasible piling options on an equal basis. A most serious financial risk in many piling projects is that of delay to project completion and consequential increase in financing charges combined with revenue slippage. Such costs can be much greater than the value of the piling contract. The relative vulnerability to delay due to ground conditions, therefore, ought to be a factor in the choice of pile type.
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