Underpinning

DEFINITON
• In construction, underpinning is the process
of strengthening and stabilizing the
foundation of an existing building or other
structure.

Reasons for Underpinning
• 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.

Process Of Underpinning
• 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 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.

Types of Underpinning
• Mass Concrete Underpinning

• Beam and base underpinning

• Mini-piled underpinning

Mass Concrete Underpinning
• Also known as 'traditional underpinning, '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.

Beam and base underpinning
• The beam and base method of underpinning
is a more technically advanced adaptation. 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

Mini-piled underpinning

• Mini-piles have the greatest value where
ground conditions are very variable, where
access is restrictive.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.

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.]

Left: Mini Piling Auger Rig
Right: Beam and Base construction of beam
under existing wall

Underpinning

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