underpinning-120412004141-phpapp02.pptx

8/11/2019 underpinning-120412004141-phpapp02.pptx

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UNDERPINNING


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DEFINITON

In construction, underpinningis the process

of strengthening and stabilizing the

foundation of an existing building or other

structure.


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Reasons for Underpinning

The original foundation is simply not strong orstable enough.

The usage of the structure has changed.

The properties of the soil supporting the

foundation may have changed (possibly throughsubsidence) or were mischaracterized duringdesign.


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The construction of nearby structures

necessitates the excavationof 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.
http://en.wikipedia.org/wiki/Earthworks_(engineering)http://en.wikipedia.org/wiki/Earthworks_(engineering)


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Process Of Underpinning

Underpinning is accomplished by extendingthe foundation in depth or in breadth so iteither rests on a more supportive soil stratum

or distributes its load across a greater area.Use of micropiles and jet grouting arecommon methods in underpinning. Analternative to underpinning is the

strengthening of the soil by the introductionof a grout. All of these processes are generallyexpensive and elaborate.


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Types of Underpinning

Mass Concrete Underpinning

Beam and base underpinning

Mini-piled underpinning


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Mass Concrete Underpinning Also known as 'traditional underpinning, 'This

underpinning method strengthens an existingstructure's foundation by digging boxesby 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.


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Beam and base underpinning

The beam and base method of underpinning

is a more technically advanced adaptation. Areinforced concrete beam is constructedbelow, above or in replacement of the existingfooting. The beam then transfers the load of

the building to mass concrete bases, whichare constructed at designed strategiclocations. Base sizes and depths are

dependent upon the prevailing groundconditions. Beam design is dependent uponthe configuration of the building and theapplied loads


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Mini-piled underpinning

Mini-piles have the greatest value whereground conditions are very variable, whereaccess is restrictive.Mini-piled underpinning is

generally used when the loads from thefoundations need to be transferred to stablesoils at considerable depths - usually in excessof 5.0 metres. Mini-piles may either be

augured or driven steel cased, and arenormally between 150mm and 300mm indiameter.


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Underpinning the foundations of a railway bridge using a

timber box crib to support the bridge. A completed

concrete pad underpinning can be seen at the bottom

right.]


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Mass Concrete Underpinning


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Left: Mini Piling Auger Rig

Right:Beam and Base construction of beam

under existing wall


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THANK YOU

underpinning-120412004141-phpapp02.pptx

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