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