Pile
From Free net encyclopedia
- For other uses, see Pile (disambiguation).
A pile is one type of building foundation. Piles are used when the soil near the ground surface is not strong and the weight of the building must be carried by deeper soil layers. The strength or bearing capacity of the soil can be measured using a cone penetration test.
Historically, piles were wood, later reinforced concrete and pre tensioned concrete.
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Driven piling
Piles can be prefabricated and driven into the ground using a hydraulic hammer, a vibrator or a diesel pile driving rig supported on a drag line excavator, or can be formed in situ in a bore hole.
Piles are of different types. Piles basically could be divided into Driven piles and bored piles. Pile materials could be timber, steel, fiber glass and plastic.
Continuous flight auger
Continuous flight auger (CFA) piles are suitable for most construction projects. Causing minimum disturbance, they are ideal for noise and environmentally sensitive sites for both load bearing piles and excavation support. Also known as auger cast pile.
A CFA pile is formed by a hollow stemmed continuous flight auger which is rotated into the ground 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 While the concrete is pumped, the auger is slowly withdrawn, lifting the spoil on the flights.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.
Mini piling, micropiling
Where the demands of the job require piles in low headroom or otherwise restricted areas, and for specialist and or smaller scale projects, many companies offer a range of mini piling services using hydraulic pressure or percussive and rotary. The micropiles can be installed using either drilling [1], impact driving, jacking, vibrating or screwing machinery. The micropiles are often grouted as shaft bearing piles but non-grouted micropiles are also common in Nordic Countries as end-bearing piles.
Rotary bored piling
Rotary bored techniques offer larger diameter piles than any other piling method and permit pile construction through particularly stiff 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.
Rotary bored piles are constructed using either self erecting hydraulic rigs or crane mounted boring rigs.
Underreamed piles
Underream piles have mechanically formed enlarged bases that may be up to 6300 mm in diameter. The form is that of an inverted cone with the sloping surface at 55 degrees to the horizontal and can only be formed in stable soils. In such conditions they allow very high load bearing capacities.
Tripod piling
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 system is based upon forming the bore by means of a percussion tool which will penetrate most strata. Special tools are used to penetrate obstructions.
As boring proceeds, the bore is lined with (temporary) steel casings, screwed together in sections. These steel lining tubes are used to support unstable ground and exclude water from the bore. Where the bore has penetrated into clay it is often unnecessary to use lining tubes and the bore is advanced 'open hole'. On reaching the depth necessary to develop the required bearing capacity of the pile, a steel reinforcement cage is placed centrally in the pile. Self-compacting concrete of high workability is then poured and at the same time, the steel lining is steadily withdrawn or can be left in situ. The method ensures that all weaknesses in the stratum are adequately filled with concrete and cover to the steel reinforcement is maintained.
The advantages that this system offers over others are, it is easy and inexpensive to bring to site, making it ideal for a very small number of piles. It can work in restricted sites (particularly where height limits exist), and it is a very reliable, tried and tested system, useable in almost all ground conditions.
Piled walls
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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.
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.
See also slurry wall.
Sheet piling
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The main application of steel sheet piles is in retaining walls and cofferdams erected to enable permanent works to proceed.
See also
References
- Ruwan Rajapakse, Pile Design and Construction Guide, 2002.