A deep foundation is a type of foundation distinguished from shallow foundations by the depth they are embedded into the ground. There are many reasons a geotechnical engineer would recommend a deep foundation over a shallow foundation, but some of the common reasons are very large design loads, a poor soil at shallow depth, or site constraints (like property lines). There are different terms used to describe different types of deep foundations including piles, drilled shafts, caissons and piers. The naming conventions may vary between engineering disciplines and firms. Deep foundations can be made out of timber, steel, reinforced concrete and pre-tensioned concrete. Deep foundations can be installed by either driving them into the ground or drilling a shaft and filling it with concrete, mass or reinforced.
Driven foundations Edit
Prefabricated piles are driven into the ground using a pile driver. Driven piles are either wood, reinforced concrete, or steel. Wooden piles are made from trunks of tall trees. Concrete piles are available in square, octagonal, and round cross-sections. They are reinforced with rebar and are often prestressed. Steel piles are either pipe piles or some sort of beam section (like an H-pile). Historically, wood piles were spliced together when the design length was too large for a single pile; today, splicing is common with steel piles, though concrete piles can be spliced with difficulty. Driving piles, as opposed to drilling shafts, is advantageous because the soil displaced by driving the piles compresses the surrounding soil, causing greater friction against the sides of the piles, thus increasing their load-bearing capacity.
Pile foundation systems Edit
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.
Drilled piles Edit
Also called caissons, drilled shafts, drilled piers, 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'.
Boring is done until the hard rock or soft rock layer is reached in the case of end bearing piles. If the boring machine is not equipped with a rock auger, then socketing of the hard rock layer is done with the help of a heavy chisel which is dropped from a height of about 1.5 metres (depends on the weight of the chisel and design requirements) by suspending it from a tripod stand attached to a winch crane. The socketing is carried out until the desired depth within the rock layer has been attained. Usually, the required depth within the rock layer is considered to be equal to the diameter of the pile in hard rock layers and is taken to be equal to 2.5 times the diameter of the pile in soft rock layers.
'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 tremmie pipe, following which, extraction of the temporary casing takes place.
The reinforcement cage may need to be lapped with another cage if the depth of the pile exceeds 12 metres as that is the standard length of reinforcement bars of diameter 16mm and above.
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.
Such piles commonly fail due to the collapse of the walls of the shaft resulting in the formation of a reduced section which may not be able to bear the loads for which it had been designed. Hence at least a third of piles in projects with a large number of piles are tested for uniformity using a "Pile Integrity Tester". This test relies on the manner in which low intensity shock waves are affected as they pass through the pile and are reflected to judge the uniformity and integrity of the pile. A pile failing the integrity test is then subjected to a pile load test
Under reamed piles Edit
Underreamed 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 augercast 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.
Augercast piles cause minimal disturbance, and are often used for noise and environmentally sensitive sites. Augercast piles are not generally suited for use in contaminated soils, due to expensive waste disposal costs. In ground containing obstructions or cobbles and boulders, augercast piles are less suitable as damage can occur to the auger. An alternative to augercast piles in contaminated soils areas would be CMC ground improvement (Controlled Modulus Column- Developed by Menard ) in which a hollow stemmed displacement auger is used to drill the elements to the required depth. This process minimizes spoils and is usually used for warehouses, residential buildings, Bridge abutments, highway embankments and petrochemical plants.
Pier and grade beam foundationEdit
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 remodeling.
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.
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.
Tripod piles Edit
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 piles Edit
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. Normally, vibrating hammer, t-crane and crawle drilling are used to establish sheet piles.
Soldier piles Edit
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 Edit
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 occurredTemplate:Where (pullout) because there was poor contact between steel and soil, due to a combination of internal ring stiffeners and protective painting of the steel walls.
Adfreeze piles Edit
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 1.5 - 3.0 dm (6"-12") larger than 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.
Vibro Stone Columns Edit
Vibro stone column is a ground improvement technique where columns of coarse aggregates are placed in soils with poor bearing capacity to improve it.
Piled walls Edit
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.
Deep Mixing/Mass Stabilization TechniquesEdit
These are essentially variations of in-Situ reinforcements in the form of Piles (as mentioned above) Blocks or larger Volumes.
Cement, Lime/Quick Lime, Flyash, Sludge and/or other Binders (sometimes called Stabilizer) are mixed into the soil to increase bearing capacity. The result is not solid as concrete, but should be seen as an improvement of the bearing capacity of the original soil.
The technique is most often applied on Clays or organic soils like peat. The mixing can be carried out by pumping the Binder into the soil whilst mixing it with a device normally mounted on an excavator or by excavating the masses, mixing them separately with the Binders and refilling them in the desired area.
The technique can be used on lightly contaminated masses as a means of binding contaminants, as opposed to excavating them and transporting to landfill or processing.
As the name implies, timber piles are piles made of wood. 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 3500 kN 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.
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 considered for determining capacity is typically reduced by 1/16 in. compared to the actual pipe to account for corrosion.
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.
Steel pipe piles can either be new steel manufactured specifically for the piling industry or reclaimed steel tubular casing previously used for other purposes such as oil and gas exploration.
Prestressed concrete pilesEdit
Concrete piles are typically made with steel reinforcing and prestressing tendons to obtain the tensile strength required, to survive handling and driving, and to provide sufficient bending resistance.
Long piles can be difficult to handle and transport. 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.
See also Edit
- Fleming, W. G. K. et al., 1985, Piling Engineering, Surrey University Press; Hunt, R. E., Geotechnical Engineering Analysis and Evaluation, 1986, McGraw-Hill.
- Coduto, Donald P. Foundation Design: Principles and Practices 2nd ed., Prentice-Hall Inc., 2001.
- NAVFAC DM 7.02 Foundations and Earth Structures U.S. Naval Facilities Engineering Command, 1986.
- Rajapakse, Ruwan., Pile Design and Construction Guide, 2003
- Tomlinson, P.J., Pile Design and Construction Practice, 1984
- Stabilization of Organic Soils
- ADSC: The International Association of Foundation Drilling
- Deep Foundations Institute
- International Society for Micropiles
- Federation of Piling Specialists (UK)
- Procedure of Installing Drilled Shaft (Bored Pile)
- Information on the analysis and design of sheeting piles
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