THE KITALE NATIONAL POLYTECHNIC
CLASS: CIVIL ENGINEERING, BUILDING TECHNOLOGY
CLASS TUTOR: HUMPHREY KOSGEI
SUB MODULE: UNDERPINNING
UNDERPINING
Underpinning is a process of placing a new foundation under an existing one or
strengthening an existing foundation.
Purpose of Underpinning
The underpinning is done to serve the following purposes.
To strengthen a shallow foundation when a building with deep foundation is to
be constructed adjoining it.
To strengthen an existing foundation, which has suffered from cracks and
settlements
To deepen an existing foundation.
To construct a basement under an existing building.
General precautions before underpinning is done;
Notify the adjoining owners of the proposed works giving full details and
temporary shoring and tying
Carry out a detailed survey of the site- a careful record of any defect found
should be made and where possible agreed with the owners before being
lodged in a safe place.
Indicators / tell tales should be fixed over existing cracks so that any
subsequent movement can be noted and monitored.
If settlement is the reason for underpinning works a thorough investigation
should be carried out to establiosh the cause and any necessary remedial work
put in hand before any underpinning work is started
Before any underpinning works started the loads of the buildingto be
underpinned should be reduced as much as possible before removing the
imposed loads from the floors before installing any prop /shoring which is
required
Any service which are in the vicinity of the proposed underpinning work
should be identified .traced carefully ,exposed supported and protected as
necessary.
Methods of Underpinning
There are two methods of underpinning. They are:
Pit Method
Pile Method
Pit Method – In this method the length of the foundation to be underpinned is
divided into sections of 1.2 to 1.5 m lengths as shown in Fig.1 below. For each
section a hole is made on the wall above the plinth level and a needle is inserted in
the hole. Bearing plates are placed above the needle to support the masonry above
it. Needle is supported on either side of the wall on wooden supports and screw
jacks. The foundation pit is excavated up to the desired level and new foundation is
laid. In the round, the work is undertaken in alternate sections. Once the alternate
sections are undertaken, the remaining sections are worked out. If the wall for
which underpinning is to be carried out is weak, raking shores is provided to
support them.
If an interior strong column exists, or if the foundation is to be extended only to
one side, cantilever needle beams may be used in place of central needle beam as
shown in Fig.2 below. A jack is placed between the column and wall.
Precautions to be taken in pit method - In pit method of underpinning, the
following points are noteworthy.
Alternate sections are taken up in the round. Remaining sections should be
taken next.
For long wall the work should be started from the middle and worked
outwards.
If the foundation is deeper, proper timbering of the foundation trench may be
done.
The needle beam should be removed only when the new foundation has
gained strength.
It is desirable to do the new foundation work in concrete.
The needle should be closed in masonry using cement mortar.
Pile Method – In this method, piles are driven at regular interval along both sides
of the wall. The piles are connected by concrete or steel needles, which penetrate
through the walls. These beams also act as pile caps. This method is effective in
clayey soil and in waterlogged areas. The existing foundation is very much
relieved of the load. Fig 3 illustrates the pile method of underpinning.
Underpinning
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
(possibly through subsidence) 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 (above or below grade).
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.
