lec 2 - underpinning
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BSS452BUILDING CONSTRUCTION AND
MATERIALS II
Wan Zuriea Wan Ismail
Centre of Studies for Building Surveying
UNDERPINNING
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LEARNING OUTCOMES :
1) Students are able to understand the conceptand type of underpinning works
2) Students are able to identify the needs ofunderpinning works
3) Students are able to understand the
construction methods for underpinning works
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LECTURE CONTENTS :
1.0 Introduction
2.0 Underpinning
3.0 The reasons of underpinning
4.0 By-law requirements
5.0 Methods and inspections
6.0 Methods of underpinning
7.0 Conclusion
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INTRODUCTION :
Underpinning is the process of strengtheningand stabilizing the foundation of an existingbuilding or other structure.
Permanent construction, which directlytransmits existing structure foundation loads
to a lowest bearing elevation or strata. Underpinning is accomplished by extending
the foundation in depth or in breadth so iteither rests on a stronger soil stratum or
distributes its load across a greater area.
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UNDERPINNING :
Underpinning is a method of construction that seesthe depth of the foundations to a building beingincreased.
The soil beneath the existing foundation isexcavated and is replaced with foundation material,normally concrete.
Underpinning requires close attention to design,methodology and safety procedures.
The main objective of most underpinning work is to
transfer the load carried by a foundation from its
existing bearing level to a new level at a lowerdepth.
Also can be used to replace an existing weak
foundation.
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The reasons for Underpinning :
Underpinning may be necessary for a variety ofreasons:
1) The original foundation is simply not strong or stable enough, e.g due to decay of timber piles under the
foundation.
2) The existing foundations of the building havemoved-this is caused by poor soil or changes to thesoil conditions (e.g. subsidence has occurred).
3) The properties of the soil supporting the foundationmay have changed or was mischaracterized duringplanning.
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The reasons for Underpinning :
Underpinning may be necessary for a variety ofreasons:
4) The usage of the structure has changed ie there hasbeen a decision to add another storey to the
building, either above or below ground level, and thedepth of the existing foundations is inadequate tosupport the modified building or load (weight) of it.
(increase in loading)
5) The construction of nearby structures necessitatesthe excavation of soil supporting existingfoundations.
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By Law Requirements :
Underpinning work requires very careful planningand execution. Any proposed to underpin an existingfoundation, approval under the building regulationwill normally be required.
Obtaining approval will usually involve thepreparation of a structural design of theunderpinning, including the process to carried outduring construction
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General precautions :
Notify adjoining owners of proposed works giving full
details and temporary shoring or tying.
Carry out detailed survey of the site, the building to
be underpinned and of any other adjoining or
adjacent building or structures. A careful record ofany defects found should be made and where
possible agreed with the adjoining owner(s) before
being lodged in a safe place.
Indicators or ‘tell tales’ should be fixed over existingcracks so that any subsequent movements can be
noted and monitored.
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General precautions :
If settlement is the reason for the underpinning
works, a thorough study should be carried out to
establish the cause and any necessary remedial
work put in hand before any underpinning works are
started. Before any underpinning work is started, the loads
on the building to be underpinned should be reduced
as much as possible by removing the imposed loads
from the floors and installing any props and/orshoring which is required.
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General precautions :
Any services which are in the vicinity of the
proposed underpinning works should be identified,
traced, carefully exposed, supported and protected
as necessary.
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Method and Inspections :
The exact method to be employed for underpinningwill depend on the many circumstances of the case.
To avoid excessively undermining the existingfoundations, causing further damage to thestructure, the excavations for the underpinningshould be carried out to engineer’s instructions anddetails.
The excavation for each section of underpinning willnormally be inspected by a design engineer and abuilding control surveyor before it can be carry out.
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Method and Inspections :
The timing of each stage and the specification of thematerials to be used will vary on a case by basebasis and should normally be subject of a structuralengineer’s design.
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Methods of Underpinning:
Standard Methods of underpinning:
i. Traditional Underpinning
ii. Needle Beams underpinning
iii. Cantilever Beamsiv. Raft Underpinning
v. Piled Underpinning
vi. Jack pile underpinning
vii. Pynford Stool Underpinning
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Methods of Underpinning:
1. Traditional Underpinning:
• This method stabilises existing wall foundations bydigging under the present foundation in sequencedbays to a depth where firm strata exists andreplacing the excavated material with massconcrete.
• The loading capabilities are based upon the strataencountered. In some circumstances when theexisting footing is adequate to span between
supports, intermediates piers can be used toreduced cost, i.e missing out piers 2 and 4 as shownon the diagram.
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Methods of Underpinning:
1. Traditional Underpinning:
To prevent fracture, damage or settlement of the
walls being underpinned the work should always be
carried out in short lengths called legs or bays.
The length of bays will depend upon the following
factors:-
• Total length of wall to be underpinned;
• Wall loading;
• General state of repair and stability of wall and
foundation to be underpinned;
• Nature of subsoil beneath existing foundation;
• Estimated spanning ability of existing foundation.
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Methods of Underpinning:
1. Traditional Underpinning:
Generally suitable bay length are:-
1.000 to 1.500 for mass concrete strip foundations
supporting walls of moderate loading. 1.500 to 3.000 for reinforced concrete strip
foundations supporting walls of traditional
construction.
In all cases, the total sum of the unsupportedlengths of wall should not exceed 25% of the
total wall length.
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Methods of Underpinning:
1. Traditional Underpinning:
The sequence of bays should be arranged so
that working in adjoining bays is avoided until
one leg of underpinning has been completed,pinned and cured sufficiently to support the wall
above.
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Traditional Underpinning :
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Traditional Underpinning :
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Traditional Underpinning :
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Traditional Underpinning :
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Methods of Underpinning:
2. Needle Beams Underpinning:
• This method stabilises wall foundations by the use ofpiles installed either side of an existing wall. A smallpocket is broken out below the ground level and a
reinforced concrete needle beam is cast in-situconnecting the piles and supporting the wall.
• Reducing the span between needle beams canaccommodate very high loads. However, the bearing
capacity of the underlying strata will determine thenumber, diameter and spacing of piles used.
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Needle Beams Underpinning
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Needle Beams Underpinning
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Needle Beams Underpinning
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Methods of Underpinning:
3. Cantilever Beams:
• This method stabilises existing wall foundationseither internally or externally whichever the mostappropriate.
• Two mini-piles are installed, one compression andone tension.
• A pocket is broken out into the existing wall and areinforced concrete beam is cast linking the twopiles.
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Methods of Underpinning:
3. Cantilever Beams:
• This system is used where traditional underpinningis not appropriate due to the existing foundationsbeing deep that it is uneconomical to dig. (depths
greater than 1.5m). Or needle beams cannot beused due to access constraints.
• Reducing the span between cantilevers canaccommodate high loads. However, the bearing
capacity of the underlying strata will determine thenumber, diameter and spacing of piles used.
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Methods of Underpinning:
3. Cantilever Beams:
• Piles are installed at approximately 1.0m - 1.5mcentres and 1.2m apart determined by loadings.Then pockets are broken out and reinforced beams
are placed to pick up the walls. Beams are usuallycast alternate.
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3. Cantilever Beams:
Advantages of this system are:
• Faster than traditional underpinning
• Access from one side only• Occupants can stay in the property duringconstruction
• Economical at depths greater than 1.5m
• Suitable for restricted access
• Cantilever beam constructed at a higher level ifexisting foundation too deep
• High load capability
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Cantilever Beams Underpinning
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Cantilever Beams Underpinning
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Methods of Underpinning:
4. Raft Underpinning:
• This method stabilises existing wall foundations towhole rooms or buildings. Mini piles are installedwithin the property and capped with an integral
reinforced concrete raft. Needle beams project fromthe slab into walls below ground level.
• Reducing the span between projecting needlebeams and increasing pile numbers can
accommodate highly loaded structures. However,the bearing capacity of the underlying strata willdetermine the number, diameter and spacing of pilesused.
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Methods of Underpinning:
4. Raft Underpinning:
• This system is used where whole rooms or wholestructures are to be underpinned and includes theprovision of a new integral floor slab.
• It is appropriate when existing foundations are deep,or that good bearing strata is so deep that it isuneconomical to dig (depths greater than 1.0m)
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Methods of Underpinning:
4. Raft Underpinning:
• Piles are installed at centers determined byloadings. Pockets are then broken out andreinforced needle beams are placed to pick up the
walls.
• A ring beam is constructed and linked to needles,mesh is laid and the whole structure poured.
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Methods of Underpinning:
4. Raft Underpinning:
Advantages of this system are:
Provides lateral traverse ties throughout thestructure
Provides new floor slab upon completion
Economical at depths greater than 1.5m
No need for external access
Minimise disruption to drainage and services
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Raft Underpinning
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Methods of Underpinning:
5. Jack pile underpinning This method can be used when the depth of a
suitable bearing capacity subsoil is too deep to
make traditional underpinning uneconomic.
Jack pile underpinning is quiet, vibration free and
flexible since the pile depth can be adjusted to suit
subsoil conditions encountered.
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Methods of Underpinning:
5. Jack pile underpinning This method can be used when the depth of a
suitable bearing capacity subsoil is too deep to
make traditional underpinning uneconomic.
Jack pile underpinning is quiet, vibration free andflexible since the pile depth can be adjusted to suit
subsoil conditions encountered.
The existing foundations must be in a good condition
since they will have to span over the heads of thepile caps which are cast onto the jack pile heads
after the hydraulic jacks have been removed.
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Jack Pile Underpinning
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Methods of Underpinning:
6. Pynford’ Stool Underpinning This method can be used where the existing
foundations are in a poor condition and it enables
the wall to be underpinned in a continuous run
without the need for needles or shoring. The RC beam formed by this method may well be
adequate to spread the load of the existing wall or it
may be used in conjunction with other forms of
underpinning such as traditional and jack pile.
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ynford’ Stool Underpinning
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It could be that one edge of the building was on much softer soil than
the other, so that part of the building settled down a little bit more," Gavin
explained. "That could easily lead to an instability that would precipitate acollapse."
Another possibility is that weight on the top factory floors—where the
crack was spotted—was unevenly distributed.
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