Underpinning

Underpinning

In construction, underpinning is the process of strengthening and stabilizing the foundation of an existing building or other structure. Underpinning may be necessary for a variety of reasons:

  • The original foundation is simply not strong or stable enough.
  • The usage of the structure has changed.
  • The properties of the soil supporting the foundation may have changed (possibly through subsidence) or were mischaracterized during design.
  • The construction of nearby structures necessitates the excavation of soil supporting existing foundations.
  • It is more economical, due to land price or otherwise, to work on the present structure's foundation than to build a new one.

Underpinning is accomplished by extending the foundation in depth or in breadth so it either rests on a more supportive soil stratum or distributes its load across a greater area. Use of micropiles [1] and jet grouting are common methods in underpinning. An alternative to underpinning is the strengthening of the soil by the introduction of a grout. All of these processes are generally expensive and elaborate.

Underpinning may be necessary where P class (problem) soils in certain areas of the site are encountered.

Through semantic change the word underpinning has become to encompass all abstract concepts that serve as a foundation.

underpinning

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Mass Concrete Underpinning

Also known as 'traditional underpinning,' the mass concrete underpinning method is nearly 100 years in age, and the protocol has not changed since. This underpinning method strengthens an existing structure's foundation by digging boxes by hand underneath and sequentially pouring concrete in a strategic order. The final result is basically a foundation built underneath the existing foundation. This underpinning method is generally applied when the existing foundation is at a shallow depth, however, the method still works very well even at fifty feet deep. The method has not changed since its inception with its use of utilitarian tools such as shovels and post hole diggers. Heavy machinery is not called for in this method due to the tight nature of the boxes being dug. There are several advantages to using this method of underpinning including the simplicity of the engineering, the low cost of labor to produce the result, and the continuity of the structure's uses during construction.

Beam and base underpinning

The beam and base method of underpinning is a more technically advanced adaptation of traditional mass concrete underpinning. A reinforced concrete beam is constructed below, above or in replacement of the existing footing. The beam then transfers the load of the building to mass concrete bases, which are constructed at designed strategic locations. Base sizes and depths are dependent upon the prevailing ground conditions. Beam design is dependent upon the configuration of the building and the applied loads. Anti-heave precautions are often incorporated in schemes where potential expansion of clay soils may occur.[1]

Mini-piled underpinning

Mini-piles have the greatest value where ground conditions are very variable, where access is restrictive, where environmental pollution aspects are significant, and where structural movements in service must be minimal.[2] Mini-piled underpinning is generally used when the loads from the foundations need to be transferred to stable soils at considerable depths - usually in excess of 5.0 metres. Mini-piles may either be augured or driven steel cased, and are normally between 150mm and 300mm in diameter. Structural engineers will use rigs which are specifically designed to operate in environments with restricted headroom and limited space, and can gain access through a regular domestic doorway. They are capable of constructing piles to depths of up to 15 metres. The technique of minipiling was first applied in Italy in 1952, and has gone through a plethora of different names, reflecting worldwide acceptance and expiration of the original patents.[3]
The relatively small diameter of mini-piles is extremely distinctive of this type of underpinning and generally uses anchoring or tie backs into an existing structure or rock. Conventional drilling and grouting methods are used for this method of underpinning. These mini-piles have a high slenderness ratio, feature substantial steel reinforcing elements and can sustain axial loading in both senses.[4] The working loads of mini-piles can sustain up to 1,000kN loads.
In comparison to Mass Concrete Underpinning, the engineering aspect of mini-piles is a bit more involved, including rudimentary engineering mechanics such as statics and strength of materials. These mini-piles must be designed to work in tension and compression, depending on the orientation and application of the design. In detail, attention with design must be paid analytically to settlement, bursting, buckling, cracking, and interface consideration, whereas, from a practical viewpoint, corrosion resistance, and compatibility with the existing ground and structure must be regarded. [5]

Mini-piled underpinning schemes

Mini-piled underpinning schemes include pile and beam, cantilever pile-caps and piled raft systems. Cantilevered pile-caps are usually used to try and avoid disturbing the inside of a building and require the construction of tension and compression piles to each cap. These are normally linked by a beam. The pile and beam system usually involves constructing pairs of piles on either side of the wall and linking them with a pile cap to support the wall. Again, the pile caps are usually linked by reinforced concrete beams to support the entire length of the wall. Piled raft underpinning systems are commonly used when an entire building needs to be underpinned. The internal floors are completely removed, a grid of piles is installed and a reinforced concrete raft is then constructed over the complete floor level, picking up and fully supporting all external and internal walls.

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Look at other dictionaries:

  • underpinning — UK US /ˈʌndəˌpɪnɪŋ/ noun [C or U] ► something that provides support for something, or that is the starting point from which something can develop: »A strong opening on Wall Street provided a firm underpinning for the FTSE 100 …   Financial and business terms

  • Underpinning — Un der*pin ning, n. 1. The act of one who underpins; the act of supporting by stones, masonry, or the like. [1913 Webster] 2. (Arch.) (a) That by which a building is underpinned; the material and construction used for support, introduced beneath… …   The Collaborative International Dictionary of English

  • underpinning — index foundation (basis) Burton s Legal Thesaurus. William C. Burton. 2006 …   Law dictionary

  • underpinning — (n.) late 15c., action of supporting or strengthening from beneath, from UNDER (Cf. under) + prp. of PIN (Cf. pin) (v.). Figurative sense of prop, support is recorded from 1580s …   Etymology dictionary

  • underpinning — [un′dər pin΄iŋ] n. 1. a supporting structure or foundation, esp. one placed beneath a wall 2. a support or prop ☆ 3. [pl.] Informal the legs …   English World dictionary

  • underpinning — UK [ˈʌndə(r)ˌpɪnɪŋ] / US [ˈʌndərˌpɪnɪŋ] noun [countable/uncountable] Word forms underpinning : singular underpinning plural underpinnings 1) an important basic part of something that allows it to succeed or continue to exist 2) a strong piece of… …   English dictionary

  • Underpinning — Underpin Un der*pin , v. t. [imp. & p. p. {Underpinned}; p. pr. & vb. n. {Underpinning}.] 1. To lay stones, masonry, etc., under, as the sills of a building, on which it is to rest. [1913 Webster] 2. To support by some solid foundation; to place… …   The Collaborative International Dictionary of English

  • underpinning — noun A support or foundation, especially as a structure of masonry that supports a wall or a metaphorical basis for something. This houses underpinning is not up to standard …   Wiktionary

  • underpinning — ˈ ̷ ̷ ̷ ̷ˌ ̷ ̷ ̷ ̷ noun Etymology: under (III) + pinning 1. a. : the material and construction used for support and introduced beneath a wall already constructed b. : the foundation of a frame house …   Useful english dictionary

  • underpinning — noun Date: 15th century 1. the material and construction (as a foundation) used for support of a structure 2. something that serves as a foundation ; basis, support often used in plural < the philosophical underpinnings of educational methods > 3 …   New Collegiate Dictionary

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