Piling
A deep foundation is one type of building foundation. Deep foundations are used for structures or heavy loads when shallow foundations can not provide adequate capacity, due to size and structural limitations. There are different types of deep foundations including piles, drilled shafts, caissons, piers, and earth stabilized columns. The naming conventions for different types of foundations vary between different engineers. Historically, piles were wood, later steel, reinforced concrete and pre tensioned concrete.
Driven piling
Prefabricated piles are driven into the ground using a hydraulic hammer, a vibrator or a diesel pile driving rig supported on a drag line excavator.
Driven piles are typically either wood, concrete, or steel. Wooden piles are typically made from trunks of tall trees. Concrete piles are available in square, octagonal, and round cross-section, are reinforced, and are often prestressed. Steel piles are either pipe piles or some sort of beam section. Historically, wood piles were spliced together when the bearing layer was too deep for use with a single pile; today splicing is only common with steel piles, though concrete piles can be spliced with difficulty.
Drilled piles
Also called drilled piers.
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 The casing can be withdrawn or left in situ.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
Underream piles have mechanically formed enlarged bases that have been 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.
Auger Cast Pile
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.
Specialty piling
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, impact driving, jacking, vibrating or screwing machinery. Micropiles are often grouted as shaft bearing piles but non-grouted micropiles are also common as end-bearing piles.
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 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), and it is a very reliable, tried and tested system, useable in almost all ground conditions.
Sheet piling
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.
Soldier Pile
Also know as King Pile or Berlin Wall
Soldier piles are constructed of wide flange steel H sections spaced about 2-3 m apart, 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 minimised 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 overconsolidated clays, soils above the water table if they have some cohesion, and free draining soils which can be effectively de-watered, e.g. 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 de-watering is often required.
Piled walls
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.
Timber
As the name implies, timber piles are architectural piles made of timber.
Cost
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.
Strength
Compared to other types of pile (ie, steel or concrete), timber piles are not suitable for heavier loads.
Longevity
The main point to remember 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.
Reasons for deterioration
For timber to deteriorate, two elements are needed: water and oxygen. Below groundwater level, oxygen is lacking even though there is ample water. Hence timber tends to last for a long time below groundwater level. It is been reported that some timber piles used during 16th century in Venice still survive since they were below groundwater level.
Treatments to improve longevity
Timber can be treated with paints and various other techniques to protect from boring insects.
Difficulty of splicing
One of the main dis-advantages 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 for buildings and are a good candidate for batter 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 wateror 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 bending capacity or corrosion resistance. Generally in the United Kingdom, this is 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 1/16th of an inch.
The amount of corrosion for a steel pipe pile can be categorised, for a pile embedded in non aggressive and natural soils as 0.015mm per side per year British Steel Piling Handbook.
Eurocode 3 now specifies various corrosion rates based on the nature or subsoil 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.
See also
- Crosshole Sonic Logging
- Drilling rig
- Dynamic Load Testing
- Dynamic Pile Monitoring
- Geotechnical engineering
- High Strain Dynamic Testing
- Wave Equation Analysis
- Foundations
- Slurry wall
References
- Ruwan Rajapakse, Pile Design and Construction Guide, 2002.
- Fleming, W. G. K. et al, 1985, Piling Engineering, Surrey University Press; Hunt, R. E., Geotechnical Engineering Analysis and Evaluation, 1986, McGraw-Hill.
- Caduto, 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