Deep foundation

A deep foundation installation for a bridge in Napa, California.

A deep foundation is a type of foundation. Deep foundations are distinguished from shallow foundations by the depth they are embedded into the ground. While deep foundations are usually more expensive than shallow foundations, they are often used for situations such as very large design loads, low-strength 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 vary between engineers. Deep foundations can be made out of timber, steel, reinforced concrete, and prestressed concrete. Deep foundations can be installed by either driving or drilling the foundation into the ground or drilling a shaft and filling it with reinforced concrete.

Driven foundations

Prefabricated piles are driven into the ground using a pile driver. Driven piles are either wood, concrete, or steel. Wooden piles are made from trunks of tall trees. Concrete piles are cast prior to driving, and 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 only common with steel piles, though concrete piles can be spliced with difficulty. Driving piles, as opposed to drilling shafts, is advantageous because soil is displaced as the pile is driven which causes greater side friction on the pile (thus increasing its capacity).

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

Drilled piles are also called drilled piers or Cast-in-drilled-hole piles (CIDH piles). They are constructed by drilling a shaft into the ground, and placing reinforcing material and concrete into the hole. 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 tremmie 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 and using these techniques, pile lengths of beyond 50 metres can be achieved.

Underreamed piles or belled piers have mechanically formed enlarged bases as much as 6m 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 is formed by drilling into the ground with a hollow stem 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 is placed in the shaft in several ways. 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.

Pier and grade beam foundation

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.

Specialty piles

Micropiles

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.

Sheet piles

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 in dry conditions.

Soldier piles

Soldier piles, also know 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. 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.

Piled walls

Secant piled walls are constructed by drilling female piles at a spacing of slightly less than 2 pile diameters, then constructing male piles by boring through the concrete in the female piles in order to key male piles between them. Steel reinforcement is installed in the male piles, 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, used for construction of the female piles.

Materials

Timber

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), timber piles are not suitable for heavier loads. 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 can be treated with paints and various other techniques to protect from boring insects. One of the main disadvantages of timber piles is the difficulty in splicing. Splicing is the process of joining two piles to make a longer pile. Unlike steel and concrete piles, splicing is a difficult process with timber piles.

Pipe piles

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 should be reduced 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.

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 piles

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.

See also

• Geotechnical engineering

• Foundation