1.0 CONTINOUS TRADITIONAL UNDERPINNING

The traditional technique of underpinning involves excavation in sections

beneath the existing foundations and casting a new concrete foundation. Continuity

bars of steel reinforcement are driven into each end of the excavation to connect

intervening bases into a continuous strip on completion. Either the width of

foundation is ½ B or 1B.

The individual bases are cast in a staggered sequence of alternative bays to

ensure the building is fully supported at all times. Special care and often smaller

bases should be allowed for at corners or projections such as bay windows or

porches.

New bases are normally cast to within 50 - 75mm below the underside of the

existing foundation. Following curing of the concrete, a dry pack mix of strong cement

and sand is driven into this gap to fully support the foundations. This part of the

operation may also be carried out by pressure grouting equipment. The purpose of

this is to allow for any variations in the underside of the existing foundations and for

any shrinkage in the new concrete. Once this work has been done, excavations may

commence on the adjacent base.

1.2 ADVANTAGES OF TRADITIONAL UNDERPINNING

a) Simple engineering and easily understood

b) Suitable for heavy loads and massive structures

c) Occupants can remain in the property work can be undertaken from outside

d) Method can be used in restricted access areas

e) Suitable for formation of new cellars and basements

f) Low cost solution at shallow depths

g) Minimal disruption and noise generated

1.3 DISADVANTAGES OF TRADITIONAL UNDERPINNING

a) Only available for strip foundation only

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2.0 PRETEST METHOD UNDERPINNING

Pretest method is can be used for strip and pad foundation. It’s suitable for

building which increase the load bearing capacity to the building. This is suitable for

increased storey from 5 to 10 storeys. Subsoil in new excavation level will be

compact and compress to give predetermined loads to the soil before underpinning

works done.

Before the load form is applied in large and heavy buildings a system of pre –

compressing the soil on which the new work will be carried out by consolidating the

soil under the new foundation.

2.1 ADVANTAGES OF PRETEST METHOD UNDERPINNING

a) Minimal disruption and noise generated

2.2 DISADVANTAGES OF PRETEST METHOD UNDERPINNING

b) Not suitable for raft foundation

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3.0 MIGA OR JACK UNDERPINNING

Jack  Pile  Underpinning  method  can  be  used  when  the  depth  of  a

suitable bearing  capacity subsoil is too deep to make traditional underpinning

uneconomic. It is consist of short pre cast concrete pile lengths jacked into the

ground until suitable subsoil is reached. When the jack pile has reached the

required depth the space between the top of the pile and underside of the existing

foundation is filled with a pinned concrete cap.

The existing foundations must be in a good condition since they will have to

span over the heads of the pile caps which are cast onto the jack pile heads after the

hydraulic jacks have been removed. 

3.1 ADVANTAGES OF MIGA OR JACK UNDERPINNING

a) This method is suitable for strip and pad foundation

b) Suitable for subsoil with deep bearing capacity which is not practical to use

traditional wall underpinning

c) Using jack in underpinning works can avoid deep excavation.

d) No vibration and noise in using jack which will not disturb nearest building or

environment.

e) Suitable for heavy loads and massive structures

3.2 DISADVANTAGES OF MIGA OR JACK UNDERPINNING

a) Used where a vibration less system capable of supporting high compressive

loads is required

b) The movement of hydraulic jacking system tool is slow, so the moving

process need long time especially for distance of one pile driving point to

other point is relatively far

c) If there is thin rock or thin hard soil layer at the end of pile which is pressed,

then it will result in errors during erection

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4.0 BORED PILE UNDERPINNING

Bore pile is most suitable piling work for underpinning because piling rig can

be use in small and limited space especially under existing foundation. The best

diameter pile for the underpinning work is 300mm-950mm. It is using to replace the

strip foundation or pad foundation.

4.1 ADVANTAGES OF BORED PILE UNDERPINNING

a) Suitable method to replace existing foundation with piling

b) Simple method of construction which include excavation, steel casing,

reinforcement and concrete works

c) Quite and not produce a lot of vibration

d) It will be done stage by stage

e) It can be used in space as low as 1.80 meter

f) Suitable for heavy loads and massive structures

4.2 DISADVANTAGES OF BORED PILE UNDERPINNING

a) Inappropriate for raft foundation

b) Extremely required a high cost construction

c) Time consuming due to the augured process which depending on the weather

condition

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5.0 CANTILEVER BEAM UNDERPINNING

  This method stabilises existing wall foundations either internally or externally

whichever the most appropriate. Two mini-piles are installed, one compression and

one tension. A pocket is broken out into the existing wall and a reinforced concrete

beam is cast linking the two piles. Load Capabilities using mini-piles: 

105mm dia SWL 60KN

150mm dia SWL 90KN

Piles are installed at approximately 1.0m – 1.5m centers and 1.2m apart

determined by loadings. Then pockets are broken out and reinforced beams are

placed to pick up the walls. Beams are usually cast alternate.Piles either driven or

augured are installed and then connected to the reinforced concrete needle

beams, positioned through the walls

5.1 ADVANTAGES OF CANTILEVER BEAM UNDERPINNING

a) Faster than traditional underpinning

b) Access from one side only. Occupants can stay in the property during

construction

c) Economical at depths greater than 1.5m

d) Suitable for restricted access

e) Cantilever beam constructed at a higher level if existing foundation too

deep

f) High load capability

5.2 DISADVANTAGES OF CANTILEVER BEAM UNDERPINNING

a) Uneconomical to dig due to the existing foundations being deep, or that good-

bearing strata is so deep which the depths greater than 1.5m

b) Needle beams cannot be used due to access constraint

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6.0 NEEDLE BEAMS UNDERPINNING

Needle and pile is use if the wall to be underpinned has a weak foundation

that is considered unsuitable for spanning over the heads of jack piles. This method

uses pairs of jacks or usually bored piles in conjunction with an in-situ reinforced

concrete beam or needle place above the existing foundation.

The system works on the same principles as a dead shoring arrangement

relying on the arching effect of bonded brickwork. If water is encountered when using

bored piles a pressure pile can be used as an alternative. There are two ways of

arrangements to enable the work to be carried out. Either from both sides of the wall

or from the external faces only.

6.1 ADVANTAGES OF CANTILEVER BEAM UNDERPINNING

a) Suitable for restricted access

b) Needle beam constructed at a higher level if the existing foundation is too

deep.

c) Faster than tradition underpinning.

d) Reducing the span between needle beams can accommodate very high loads

e) More economical at greater depths

f) Less disruption, less spoil generated and completed quickly

6.2 DISADVANTAGES OF CANTILEVER BEAM UNDERPINNING

a) Like beams, they maintain their shape by the opposition of large tensile and

compressive forces, as well as shear, and are therefore relatively massive.

b) Truss construction is used in the larger examples to reduce the weight.

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7.0 ‘PYNFORD’ STOOL METHOD OF UNDERPINNING

The reinforced concrete 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. The procedure is to cut

away portions of brickwork, above the existing foundation to enable precast concrete

stool to be inserted and pinned.

7.1 ADVANTAGES OF ‘PYNFORD’ STOOL METHOD UNDERPINNING

a) This method is only suitable for strip foundation

b) It’s enable walls to be underpinned in continuous runs without the use of

needles or raking shoring

c) Can applied existing foundation in poor condition

7.2 DISADVANTAGES OF ‘PYNFORD’ STOOL METHOD UNDERPINNING

a) Inappropriate for pad foundation

b) Time consuming due to many work to carried out

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8.0 ‘BULLIVANT’ PATENT ANGLE PILING

‘Bullivant’ patent process is to stabilize existing sub structural walls and

foundation. RC angle piles installed by using air – flushed rotary percussion drill from

inside and outside of the building. The voids are lined with steel casing cut off at the

surface prior to lowering of reinforcement and placing concrete

The pile is installed by using air flushed rotary percussion drill from inside and

outside of the building. The spacing and depth of boring will only depend on site

conditions such as the occurrence of solid bearing strata and extent of structural

damage. The suitable pile diameter to be used in this underpinning works is between

120mm – 280mm.

8.1 ADVANTAGES OF ‘BULLIVANT’ PATENT UNDERPINNING

a) Low cost modern alternative to traditional underpinning for stabilizing

existing sub structural walls and foundations

b) Short duration and minimal excavation involved

c) Simpler alternative to traditional underpinning

d) Suitable for heavy loads and massive structures

e) Less disruptive as large volumes of excavations are avoided

8.2 DISADVANTAGES OF ‘BULLIVANT’ PATENT UNDERPINNING

a) Disruption to the building’s interior is most likely

b) Required a high cost installation

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9.0 HOOPSAVE METHOD

Hoopsave method is the process which involves the new construct of

concrete beam at the external excavation above the existing foundation.A limited

amount of external excavation is needed to expose the sub structural wall to a depth

just above foundation level In situ concrete beam with purpose made longitudinal

voids created with plastic conduits, is cast around the building periphery.

The small diameter voids accommodate steel stressing tendons for post –

tensioning to bind the walls into a solid unit. Continuity provided by the post –

tensioned beams integrates the sub structural wall and compensates for weaker

areas of subsoil

9.1 ADVANTAGES OF HOOPSAVE METHOD

a) Fast method when applied to regular plan shapes

b) Remedial treatment where differential settlement can be identified

9.2 ADVANTAGES OF HOOPSAVE METHOD

a) Time consuming when the plan shape is irregular

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10.0 OTHER METHOD OF UNDERPINNING

Other method of underpinning;

10.1 UNDERPINNING COLUMNS

This system is usually used for lighter structures such as porches, decks, etc.

A concrete pad is poured below the frost line and a concrete column is then poured

on top of the pad to support the structure. It can be carried out on brick /stone

columns by inserting a series of stools, casting a reinforced concrete base and then

underpinning. In addition, underpinning columns also is more difficult than wall.

The column loading is transferred from the collar to cross beams or needles

which in turn transmits the loads to the ground at a safe distance from the proposed

excavations. Cantilever techniques which transfer the loadings to one side of the

structural member are possible providing sufficient kentledge and anchorage can be

obtained.

10.2 MICRO PILE

Involves the drilling the pile shaft to the required depth, placing the steel

reinforcement, initial grouting by tremie placing additional grout under pressure

where applicable. The advantages of using this method is

a) High carrying capacity

b) Less site constraint problems

c) Self sustained operations

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11.0 CONCLUSION (SITUATION CASES)

Underpinning is the installation of temporary or permanent support to an

existing foundation to provide either additional depth or an increase in bearing

capacity. Generally used for remedial or precautionary purposes. For example if

currently the situation is as below:

One storey bungalow house

Nearby ongoing construction which are 10 storeys building.

Using strip foundation to receive load.

Congested area and nearby shop houses

Based on this situation, the suitable method has chosen are needles and pile

underpinning. This method of underpinning can be used where the condition of the existing

foundation is unsuitable for traditional or jack pile underpinning techniques. The brickwork

above the existing foundation must be in a sound condition since this method relies on the

‘arching effect’ of the brick bonding to transmit the wall loads onto the needles and ultimately

to the piles. The piles used with this method are usually small diameter bored piles.

Note:

General maximum length is 1:500m with the provision that at no time should the sum total of

unsupported lengths exceed 25% of the total wall length.

The reason for choosing this method:

a) Suitable for restricted access

The house is need for underpinning work because is close to the shop. Emphasis should

be placed on the working of the existing underpinning likes needle note the use of piles is

the most suitable.

b) High load capability

The house must be supported by a temporary structure to repair cracks in the walls work in

house. Therefore the new structure is able to carry out the loading of the building.

c) Less disruption

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These underpinning works do not interfere with any path where dredging is occurring around

the wall only cracked.

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