Kamis, 30 Desember 2010

Foundation_Engineering


Foundation

Shallow foundations of a house versus the deep foundations of a Skyscraper.

A foundation is a structure that transfers loads to the ground. Foundations are generally broken into two categories: shallow foundations and deep foundations.Contents [hide]
1 Shallow foundations
2 Deep foundations
3 Design
4 See also


Shallow foundations
Main article: Shallow foundation

Shallow foundations are usually embedded a meter or so into soil. One common type is the spread footing which consists of strips or pads of concrete (or other materials) which extend below the frost line and transfer the weight from walls and columns to the soil or bedrock. Another common type is the slab-on-grade foundation where the weight of the building is transferred to the soil through a concrete slab placed at the surface.

Deep foundations
Main article: Deep foundation

Deep foundations are used to transfer a load from a structure through an upper weak layer of soil to a stronger deeper layer of soil. 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. Sometimes these foundations penetrate bedrock.

Design

Foundations are designed to have an adequate load capacity with limited settlement by a geotechnical engineer, and the foundation itself is designed structurally by a structural engineer.

The primary design concerns are settlement and bearing capacity. When considering settlement, total settlement and differential settlement is normally considered. Differential settlement is when one part of a foundation settles more than another part. This can cause problems to the structure the foundation is supporting. It is necessary that a foundation is not loaded beyond its bearing capacity or the foundation will "fail".

Other design considerations include scour and frost heave. Scour is when flowing water removes supporting soil from around a foundation (like a pier supporting a bridge over a river). Frost heave occurs when water in the ground freezes to form ice lenses.

Changes in soil moisture can cause expansive clay to swell and shrink. This swelling can vary across the footing due to seasonal changes or the effects of vegetation removing moisture. The variation in swell can cause the soil to distort, cracking the structure over it. This is a particular problem for house footings in semi-arid climates such as South Australia, Southwestern US, Turkey, Israel, Iran and South Africa where wet winters are followed by hot dry summers. Raft slabs with inherent stiffness have been developed in Australia with capabilities to resist this movement.

When structures are built in areas of permafrost, special consideration must be given to the thermal effect the structure will have on the permafrost. Generally, the structure is designed in a way that tries to prevent the permafrost from melting.

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See also

Shallow foundation
From Wikipedia, the free encyclopedia

A shallow foundation is a type of foundation which transfers building loads to the earth very near the surface, rather than to a subsurface layer or a range of depths as does a deep foundation. Shallow foundations include spread footing foundations, mat-slab foundations, and slab-on-grade foundationsContents [hide]
1 Spread footing foundation
2 Mat-slab foundations
3 Slab-on-grade foundation
4 See also

Spread footing foundation

In ground reinforced concrete foundation in cyclonic area, Northern Australia.

Spread footing foundations consists of strips or pads of concrete (or other materials) which transfer the loads from walls and columns to the soil or bedrock. Embedment of spread footings is controlled by several factors, including development of lateral capacity, penetration of soft near-surface layers, and penetration through near-surface layers likely to change volume due to frost heave or shrink-swell.

These foundations are common in residential construction that includes a basement, and in many commercial structures.

Mat-slab foundations

Mat-slab foundations are used to distribute heavy column and wall loads across the entire building area, to lower the contact pressure compared to conventional spread footings. Mat-slab foundations can be constructed near the ground surface, or at the bottom of basements. In high-rise buildings, mat-slab foundations can be several meters thick, with extensive reinforcing to ensure relatively uniform load transfer.

Slab-on-grade foundation

Example of slab on grade foundation

Raft slab house foundation in cyclonic area, Northern Australia.

Raft slab house foundation in cyclonic area, Northern Australia.

Slab-on-grade foundations are a structural engineering practice whereby the concrete slab that is to serve as the foundation for the structure is formed from a mold set into the ground. The concrete is then poured into the mold, leaving no space between the ground and the structure. This type of construction is most often seen in warmer climates, where ground freezing and thawing is less of a concern and where there is no need for heat ducting underneath the floor.

The advantages of the slab technique are that it is relatively cheap and sturdy, and is considered less vulnerable to termite infestation because there are no hollow spaces or wood channels leading from the ground to the structure (assuming wood siding, etc., is not carried all the way to the ground on the outer walls).

The disadvantages are the lack of access from below for utility lines, the potential for large heat losses where ground temperatures fall significantly below the interior temperature, and a very low elevation that may expose the building to flood damage in even moderate rains. Remodeling or extending such a structure may also be more difficult. Over the long term, ground settling (or subsidence) may be a problem, as a slab foundation cannot be readily jacked up to compensate; proper soil compaction prior to pour can minimize this. The slab can be decoupled from ground temperatures by insulation, with the concrete poured directly over insulation (for example, Styrofoam panels), or heating provisions (such as hydronic heating) can be built into the slab (an expensive installation, with associated running expenses).

Slab-on-grade foundations are commonly used in areas with expansive clay soil, particularly in California and Texas. While elevated structural slabs actually perform better on expansive clays, it is generally accepted by the engineering community that slab-on-grade foundations offer the greatest cost-to-performance ratio for tract and semi-custom homes. Elevated structural slabs are generally only found on large custom homes or homes with basements.

Care must be taken with the provision of services through the slab. Copper piping, commonly used to carry natural gas and water, reacts with concrete over a long period, slowly degrading until the pipe fails. Copper pipes must be lagged, run through a conduit, or plumbed into the building above the slab. Electrical conduits through the slab need to be water-tight, as they extend below ground level and can potentially expose the wiring to groundwater.

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. 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.

Pile driving operations in the Port of Tampa, FloridaContents [hide]
1 Driven foundations
1.1 Pile foundation systems
2 Drilled piles
2.1 Underreamed piles
2.2 Auger cast pile
2.3 Pier and grade beam foundation
3 Specialty piles
3.1 Micropiles
3.2 Tripod piles
3.3 Sheet piles
3.4 Soldier piles
4 Piled walls
5 Materials
5.1 Timber
5.2 Pipe piles
5.3 Prestressed concrete piles
6 See also
7 Notes
8 References
9 External links

Driven foundations

Pipe piles being driven into the ground.

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 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 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

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

A pile machine in Amsterdam

Also called drilled piers or Cast-in-drilled-hole piles (CIDH 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 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 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.


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.


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

A micropile installation

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.[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.

Tripod 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 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.

Soldier piles

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.

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

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.

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.

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