Lec 2 - Underpinning

8/17/2019 Lec 2 - Underpinning

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BSS452BUILDING CONSTRUCTION AND

MATERIALS II

Wan Zuriea Wan Ismail

Centre of Studies for Building Surveying

UNDERPINNING



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



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



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.



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.



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.



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.



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



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.




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.




Any services which are in the vicinity of the

proposed underpinning works should be identified,

traced, carefully exposed, supported and protected

as necessary.



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.



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.



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




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.





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.





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.





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.



Traditional 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.







Needle Beams 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.





• 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.





• 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.




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



Cantilever Beams Underpinning



Cantilever Beams 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.





• 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)





• 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.





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



Raft 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.




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.



Jack Pile 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.



ynford’ Stool Underpinning







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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Lec 2 - Underpinning