All concepts related Underpinning.

1. UNDERPINNING
S.N.Chaudhari

2. UNDERPINNING
OBJECTIVES
• Define the term underpinning.
• Describe underpinning,when it is needed and
how it is done.
• State the reasons for underpinning.
• State the methods of underpinning a structure.

3. UNDERPINNING
Learning Outcome
• Students should be able to define the term
underpinning along with its function

4. UNDERPINNING OF BUILDINGS.

5. INTRODUCTION
❖ Whenever a new building is to be built especially in urban
areas,it is quite common to have the foundations lower
than the foundations of adjacent buildings.
❖ It is therefore essential that the stability of the existing
building(s) is safeguarded by performing underpinning as
well as shoring.`that care and forethought should be
undertaken if these operations are to be successfully
carried out.
❖ Only highly skillied and experienced personnel or companies
should perform these operations.

6. DEFINITIONS
UNDERPINNING
• It is a broad term to describe the process of
modifying an existing foundation by adding support.
• It can also be described as the installation of
temporary or permanent support to an existing
foundation to provide either additional depth or an
increase in bearing capacity.(Nemati 2007).

7. DEFINITIONS CONT
• Underpinning can also be reffered to as a process of
strengthening the foundation of an existing building
or other structure(wikipedia,the free encyclopedia).
REASONS FOR UNDERPINNING
Construction of a new project with a
deeper foundation adjacent to an existing
building.
To enable the foundations to be deepened for
structural reasons e.g to construct a basement or
addition of another storey to the building.

8. REASONS CONT
To support a structure that is sinking or tilting due
to poor soil or instability of the superstructure.
Change in use of a structure.
As a safeguard against possible settlement of a
structure when excavating close to or below its
foundation level.
To support a structure while making alteration to its
foundations or main supporting members.
To increase the width of a foundation to permit
heavier loads to be carried e.g when increasing the
height of a building with new levels.
To enable a building to be moved bodily to a new

9. REASONS CONT
The original foundation is simply not strong or
stable enough.
The properties of the soil supporting the
foundation may have changed possibly through
subsidence or were mischaracterized during
design stages.
Settlement of an existing structure.

10. UNDERPINNED FOUNDATIONS

11. SOME CAUSES OF SETTLEMENT
OF A BUILDING
▪ Lowering of the water table due to tidal fluctuations,wells for
a water district etc.
▪ Lowering of the water table can then cause tops of timber
piles to decay overtime and will require remedial
underpinning.
▪ Rising of the water table in some soils can effect a decrease in
bearing capacity of the soil causing settlement on the building
which will then require underpinning.
▪ Construction of buildings on unsuitable bearing material or
over compressible layer e.g peat,organic silts or poorly
compacted backfill may also cause settlement.
▪ Earthquakes,floods or other natural causes can cause the
structure to move thereby requiring stabilisation of
foundation soils or footings.

12. DETERMINING THE NEED
❖ Underpinning as a direct support of an existing building
foundation provides the opportunity to preload i.e jacking to
limit settlement and improve poor foundations.
❖ When a structure starts showing signs of settlement or distress, it is
of utmost importance to precisely monitor the settlement or
movement by a professional on a daily,weekly or monthly basis.
❖ Data collected or readings will indicate if the movements are
decreasing or increasing.
❖ Analyzing the results will also tell whether underpinning is
required or not to safeguard the structure e.g cracks or any
other weaknesses
❖ Before the beginning of any excavation on a new structure a
professional must examine all the structures in close proximity to
the construction site so as to determine whether underpinning is
also necessary.

13. PROCEDURES INVOLVED
WHEN USING PILES FOR
UNDERPINNING
❑ The piles are formed of precast concrete sections.
❑ Each section has a steel-linked hole running through it.
❑ The steel lining helps the locating of the sections and ensures
that they are all in line.
THE PROCESS
❑ A hole is dug below the existing foundation of the structure.
❑ The first section which has a pointed steel toe-piece to make
penetration into the ground easier is placed into position below
the foundation .
❑ A bearing plate is then positioned between the jack and the
foundation.

14. PROCESS
CONT
❑ The jack is then put between the first section and the bearing
plate and the first section is forced downward by the hydraulic
jack powered by a pump outside the excavation.
❑ When the top of the first section is almost flush with the
ground,the jack is removed and the process is repeated with the
second and subsequent sections.
❑ As each section is added,a length of steel tube is inserted into
the hole and grouted into position to make an effective joint
between the sections.
❑ The operation is continued until the pressure gauge indicates
sufficient penetration resistance to ensure adquate bearing
capacity.

15. THE PROCEDURES INVOLVED WHEN
UNDERPINNING BUILDINGS Cont.
❖ The wall to be underpinned must be carefully inspected for any
cracks or weaknesses and these carefully noted.
❖ The wall should then be divided into a convenient number of
working bays,generally not exceeding about 1.5m,depending on
the quality and stability of the wall.These working bays should
have sufficient room to enable a person to work inside them.
❖ A batten should now be fixed along the face of the wall
throughout its length to act as a datum.
❖ A sequence of working which will often be determined by the
conditions on the site and the accessibility of the working area
adjacent to the wall should now be carefully outlined.

16. PROCEDURE CONT
❖ No two adjacent bays must be excavated at the same time. If
consecutive bays have to be worked in,it is essential that the first
bay is fully completed and allowed to harden before work is
begun on the second bay.
❖ Each bay excavated in turn,usually by hand.The earth is taken
out at the front of the bay which allows support to be given to
the foundations for as long as possible,while this excavation is
being dug.
❖ The earth is then dug out from underneath the foundations
until new level is reached.

17. PROCEDURE CONT.
❖ The ground is now levelled and thoroughly rammed.
❖ The projection of the foundations is cut off so that it is
flush with the face of the wall.
❖ The foundation concrete is laid to the required thickness,
provided there is adequate projection on both side of the
wall. Any water in the excavation should be pumped out
first.
❖ The brickwork is then built up in the working bay with good
quality bricks and cement mortar usually 1:3 or 1:4.
❖ The work should be thoroughly pinned up to the concrete.
❖ The gap behind the new wall can be filled in with hardcore or
weak concrete as the new wall is built.Nash 1988:146

18. UNDERPINNING OF
FOUNDATIONS
UNDERPINNED STRUCTURE CONVENTIONAL PIT METHOD

19. UNDERPINNING METHODS
o The means and methods of supporting a
structure foundation depends on some of the
following factors:
✔ Foundation Loads: static and dynamic,permanent and
temporary.
✔ Type and magnitude of allowable structural movement i.e
deformations.
✔ Subsurface soil conditions.
✔ Subsurface ground water conditions.
✔ Access and mobility to the foundations.
✔ Potential for environmental hazards.
✔ Seismic loading.

20. WAYS OF ACHIEVING
UNDERPINNING
It can be done by :
i. Load transfer
ii. Soil treatment or
iii. A combination of the above two mechanisms
LOAD TRANSFER
This literally take structural loads and transfer them to an
underlying stratum that is more suitable for support.
SOIL TREATMENT
This changes the physical properties of the ground to make it
stronger and more supportive ,often without any change to existing
foundations.
In some cases, ground treatment can be utilized to strengthen the
ground while also acting as a load transfer.

21. UNDERPINNING METHODS
▪ There are numerous methods that can be used
when underpinning.
▪ Each methods having its own specific soil and
loading conditions where its application is most
effective.
▪ In some cases constructability can also determine
the system to be used.
▪ Each method have also its own advantages and
disadvantages depending on the specific
characteristics of the project.

22. 1. CONVENTIONAL PIT METHOD
o It consists of enlarging and or deepening existing foundations by
removing soil from beneath the foundations and replacing it with
concrete,reinforcements and a grout material.
o The structure can also be temporary shored to prevent
settlement.
DISADVANTAGES OF THE PIT METHOD
i. It often results in moderate deformation of the structure.
ii. Unsafe working conditions.
ADVANTAGES OF THE PIT METHOD
i. Simplicity of the engineering.
ii. Low cost of labour to produce the result.
iii. There is continuity of the structure’s uses during construction.

23. 2. JET GROUTING
▪ It is a load transferring system for the underpinning,often
also serving as an excavation support and ground water
control system.
▪ It is an in-situ method of construction undertaken
beneath foundations.
▪ High velocity injection of fluids,often enclosed in air erodes
the soils and replaces the soil with and engineered
grout,forming a cementations product known as
SOILCRETE that is capable of attaining unconfined
compressive strengths in excess of 70.3 kg/cm2.
▪ Groups of 1 to 1.2m diameter soilcrete columns are
constructed to transfer foundation loads to
underlying suitable bearing material.

24. Use Of Jet Grouting Method
It Is a Load Transferring System for The
Underpinning.
Serving as an Excavation Support and Ground
Water Control System.

25. 3.MICROPILES
It is used to stabilize or upgrade existing foundation
by installing micropiles through pre-drilled holes
determined by load characteristics.
Micropiles are described as small diameter piles that
can be installed in almost any type of soil and that can
carry loads up to 500 tonnes depending on the type
of it.
These micropiles are steel reinforced placed into a
small diameter hole and scaled to the ground by
grout injections under relatively high pressure.
Micropiles are widely recognized as a common
remedial option for underpinning structures
having foundations problems after completion or
during service period.

26. ADVANTAGES OF MICROPILES
i. They have high carrying capacity.
ii. Less site constraint problems.
iii. Low noise and vibration.
iv. It is a self –sustained operation.
v. It can be designed to have very low settlement.
DISADVANTAGES OF MICROPILES
i. Higher cost as compared to other pilling systems.

27. TYPES OF MICROPILES
▪ Pile and Beam.
▪ Pile and Cantilever Beam.
▪ Knuckle pile.
▪ Piled raft.
▪ Angle pile.
▪ Cantilever Ring Beam.
▪ Twin pile.
▪ Jack Down piles.

28. METHODS OF UNDERPINNING
1.NEEDLE BEAM 2.NEEDLE
WALL

29. UNDERPINNING METHODS
FOUNDATION UNDERPINNING FOUNDATION UNDERPINNING

30. CONCLUSION
• Finally it is important to note that underpinning
works require experts in the design and execution
levels.
• Because of its complexity and specially the
dangerousness for the renovation building and
the buildings around it,safe working practices
should be undertaken.

31. USEFUL LINKS
• https://www.youtube.com/watch?v=J3ofx2ZgqEA
• https://www.youtube.com/watch?v=wab6idg-iJ4