In construction or renovation, underpinning is the process of
strengthening 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 or were mischaracterized during design.
The construction of nearby structures necessitates the excavation of soil
supporting existing foundations. To increase the depth or load capacity
of existing foundations to support the addition of another storey to the
building. It is more economical, due to land price
or otherwise, to work on the present structure's foundation than to build a
new one. Earthquake, flood, drought or other
natural causes have caused the structure to move, thereby requiring stabilisation
of foundation soils and/or footings. Underpinning may be 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,
including expanding urethane-based engineered structural resins.
Underpinning may be necessary where P class soils in certain areas of the site
are encountered. Through semantic change the word
underpinning has evolved to encompass all abstract concepts that serve as a
foundation. Mass Concrete Underpinning
'Traditional underpinning,' the mass concrete underpinning method is nearly
100 years in age, and the protocol has not changed. 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.
Mini-piled underpinning Mini-piles have the greatest use 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. 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 m. Mini-piles may either be
augured or driven steel cased, and are normally between 150 mm and 300 mm 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 m. 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.
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. The working loads of mini-piles can sustain
up to 1,000 kN 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.
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. Underpinning by expanding resin
injection A mix of structural resins and hardener
is injected into foundation ground beneath footings. On entering the ground
the resin and hardener mix and expansion occurs due to a chemical reaction. The
expanding structural resin mix fills any voids and crevices, compacts any weak
soil and then, if the injection is continued, the structure above may be
raised and re-levelled. This relatively new method of underpinning, has been in
existence for approximately 30 years, and because it does not involve any
construction or excavation set-up, is known to be a clean, fast and
non-disruptive underpinning method. Gallery
References External links
An illustrated glossary of the terms used in temporary types of construction
work. Formwork, scaffolding etc. An explanation of the Underpinning
process with pictures.