UNDERPINNING

1.0 INTRODUCTION

1.1DEFINITION

1.2 PURPOSE

2.0 CAUSES OF FOUNDATION FAILURE

3.0 SYMPTOMS OF FOUNDATION FAILURE

4.0 SEQUENCE OF OPERATION

5.0 TEMPORARY WORKS

6.0 TYPES OF UNDERPINNING

7.0 COST

8.0 CASE STUDIES

9.0 CONCLUSION

CONTENT

1.0 INTRODUCTION • Underpinning is the process of

modifying an existing foundation system by extending into subsurface stratum that is deeper & more stable than the near surface soil that supports the existing foundation system.

• This is done to provide vertical support that is not present in the existing design.

• Also to increase the load bearing capacity of a foundation.

WHY UNDERPINNING?

The main objective of most underpinning

work is to transfer the load carried by a

foundation from it’s existing bearing level to

a new level

1.1 DEFINITION

• Methods of underpinning include:

– the construction of footings

–stem walls

–driven pilings for drilled piers.

• The underpinning, if properly designed &

installed, provides the basis to lift the

structure to a more acceptable elevation &

provides vertical support to prevent the

underpinned area from settling.

1.1 PURPOSE To support a structure that is

sinking or tilting

Act as safeguard against possible settlement

To support a structure while making alteration

Enable the foundation to be deepened

To increase the width of a foundation to permit heavier loads

Others purposes

of

underpinning

Conversion

Remedial

Mining

Protective

General precautions

Notify the Adjoining Owners

Detailed Survey

Technical Expertise's

Indicators

Investigation

2.0 CAUSES OF FOUNDATION FAILURE

Many houses are constructed with foundations that are inadequate for the soil conditions existing on the site because:

– The lack of suitable land,

– Homes are often built on marginal land that has insufficient bearing capacity to support the substantial weight of a structure.

– Near surface soils consist predominately of expansive clays that shrink & swell as their moisture content changes.

– If the bearing soils consist of expansive clays, foundation movement can occur if wetting and drying of the clays does not occur uniformly across the entire slab.

So… it is due to…

Overloading of Foundations

Differential Settlement in Foundation

Undermining Of Foundations

Consolidation of Soil below

Foundations

Movement of Soil below

Foundations

Any of the following

can cause moisture in

the soil to fluctuate:

– Vegetation (roots)

– poor drainage

– plumbing leaks

– Evaporation wet/dry

rain cycles

3.0 SYMPTOMS OF FOUNDATION FAILURE

The signs of foundation failure are:

- doors that do not close properly anymore

- windows that stick

- diagonal cracks in the interior & exterior walls

- Small cracks in the foundation or floor & uneven floors.

4.0 SEQUENCE OF OPERATION

SERVE

NOTICE

DETAILED

SURVEY

‘GLASS

SLIPS’

SERIES OF

CHECK

LEVELS

PERMISSION

FORECAST ANY

FUTURE

MOVEMENT

LOADING ON

STRUCTURE

4.0 SEQUENCE OF OPERATION

Before any underpinning is commenced the following surveying & preliminary work should be carried out:

1.Serve notice of adjoining owners

2.Setting out in detail the intention

3.Proceed in the proposed temporary supports.

4.Detailed survey of the building to be underpinned

5.Records of detected, cracks, supplement by photographs.

6.Glass slips should be fixed cross any vertical and lateral cracks to give visual indicator of any further movement taking place.

4.0 SEQUENCE OF OPERATION

Before any underpinning is commenced the following surveying & preliminary work should be carried out (cont’d)

7.Levels check periodically.

8.Permission from adjoining owners to stop all flues and fireplace to prevent nuisance and damage.

9.Underpinning is required to counteract unacceptable settlement of the existing foundation – forecast any future movement to ensure the design is adequate.

10.Loading on structure should be needed by removing floor loads and installing any shoring that may necessary.

5.0 TEMPORARY WORKS

• “installations required to provide access,

protection, support, or services for workers,

equipment & materials during the construction,

renovation, retrofit, maintenance, or demolition of

permanent works”

• Required to provide temporary service, repair, or

support for any part of permanent works, until the

permanent works have achieved a state of

completion allowing temporary works to be

removed.

• Shoring & temporary bracing for masonry walls and structural frames, and shoring for excavation and trenches;

• Temporary underpinning & guying, & roof and parapet anchors;

• Caissons, cofferdams and tunnels;

• Access scaffolding for construction purposes;

• Cranes & crane foundations;

• Environmental encapsulation during the removal of hazardous materials;

• Protection of vehicular & pedestrian traffic during construction;

• Provisions for loading testing

• Temporary piping, electrical, mechanical and heating services

For the underpinning works, the most important temporary work is shoring.

• Shoring is temporary support applied to a building or structure. The following situations may justify application of shoring:

• To give support to walls which are dangerous or are likely to become unstable due to subsidence, bulging or leaning during the underpinning works.

• To avoid failure of sound walls caused by the removal of a subjacent support such as where the underpinning work is being constructed near to a sound wall.

• During the underpinning works to give support to an adjacent building or structure.

• To support the upper part of a wall during formation of a large opening in the section of the wall during underpinning works.

6.0 TYPES OF UNDERPINNING

6.1 CONTINOUS TRADITIONAL UNDERPINNING

6.2 PRETEST METHOD UNDERPINNING

6.3 MIGA OR JACK UNDERPINNING

6.4 BORED PILE UNDERPINNING

6.5 CANTILEVER BEAMS UNDERPINNING

6.6 NEEDLE BEAMS UNDERPINNING

6.7 ‘PYNFORD’ STOOL METHOD OF

UNDERPINNING

6.8 ‘BULLIVANT’ PATENT ANGLE PILING

6.9 HOOPSAVE METHOD

6.10 OTHER METHODS OF UNDERPINNING

6.1 CONTINOUS TRADITIONAL

UNDERPINNING / WALL UNDERPINNING

• Traditional underpinning to prevent fracture, damage of

settlement of the wall(s) being underpinned the work

should always carried out in short lengths called legs bay.

• The length of these 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.

• Estimate spanning ability of existing foundation.

• Excavating in stages alongside and underneath

the existing foundation

• Casting new foundation

• Building up to the underside of the existing

foundation in brickwork or concrete

• Pinning between the old and new work with a rich

dry mortar.

• To prevent dangers of fracture / settlement

underpinning stages or bays should be kept short

and formed to a definite sequence pattern so that

no two bays are worked consecutively.

• The number and length of bays will depend upon the following factors:

i. Total length of wall to be underpinned

ii. Width of the existing foundation

iii. Condition of substructure

iv. Superimposed loading of existing foundation

v. Estimated spanning stability of existing foundation

vi. Subsoil condition encountered

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.

Process of underpinning:

i. Bays are excavated and timbered

ii. Bottom of excavation prepared to receive new foundation

iii. To give the new foundation strip continuity – dowel bars are inserted at the end of each bay.

iv. Brick and concrete block is toothed at each end to enable continuous bonding

v. Brickwork – common brick 1:3 cement mortar laid in English Bond, concrete block work – compressive strength of 7N/mm2, preferably 10N/mm2. Concrete used in underpinning – 1:2:4/20mm aggregate mix using rapid-hardening cement.

vi. The projection of existing foundation is cut back to the external wall line – so that loads are transmitted to the new foundation.

Bays & Stages Of Excavation

Process Of Underpinning

Installation Procedures

• The procedures for installation traditional underpinning are:

– Traditional Underpinning is usually applied when the existing foundations are at shallow depth. Bays are excavated generally 1.0m – 1.2m in length, 0.6m wide, and up to 2.5m from ground level. However a mini piled solution would be more economical over depths of 1.5m.

– Bays are excavated and timbered as necessary after which the bottom of the excavation is prepared to receive the new foundation. To give the new foundation strip continuity, dowel bar are inserted at the end of each bay.

– Brick and concrete block underpinning is toothed at each end to enable the bonding to be continuous, whereas in-situ concrete underpinning has splice bars or dowels projecting to provide the continuity. Brickwork mould normally is in a dense clay common brick bedded in (1:3) cement mortar laid in English bond for strength.

Installation Procedures (cont’d)

• The procedures for installation traditional underpinning are:

– Concrete block work should meet manufacturer’s specifications and have a compressive strength of 7N/mm2, preferably 10N/mm2. Concrete used in underpinning is usually specified as (1:2:4-20 mm aggregate) mix using rapid-hardening cement.

– The final pinning mix should consists of one part rapid-hardening cement to three parts of well graded fine aggregate from 10mm down to fine sand with a water/cement ratio of 0.35. In both methods the projection of existing foundation is cut back to the external wall line so that the loads are transmitted to the new foundation strip on the backfilled material.

6.2 PRETEST METHOD UNDERPINNING

• The method of underpinning and final pinning will result in negligible movement due to settlement of the new work, but 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 before the load form the underpinning is applied. The perimeter of the wall to be underpinned is excavated in stages as described for wall underpinning.

Installation Procedures

• The procedures for installation pre test underpinning are:

– First, the soil underneath the pile cap was excavated in a volume sufficient to allow the installation of a new pile.

– Then, a pair of hydraulic jacks with a combined capacity of 680 kN (153 kips) was mounted to the underside of the pile cap as illustrated.

– After positioning the first segment, the hydraulic jacks were extended and the segment pushed into the ground far enough to allow installation of the following segment.

– The connection rod was then threaded into the end of the installed segment, epoxy was placed around the bar and the top surface of the pile segment, and the next segment was threaded onto the connection rod projecting from the installed segment.

Installation Procedures (cont’d)

• The procedures for installation pre test underpinning are:

– The pile segments were tightened using a chain wrench to a torque of approximately 125 to 190 N- m (92 to 140 lbf-ft). This process was repeated until the pile achieved the required capacity of 600 KN (135 kips), which was typically achieved at a depth ranging from 11 to 13 m (36 to 43 ft) for this project.

– Once the pile had been advanced to a sufficient depth to carry the required proof load, the installed pile was loaded to 600kN (135 kips) for one hour while the settlement of the pile was monitor

– After completion of the proof test, each pile was locked off under a load typically equivalent to 50% of the proof load. This pre-loading of the new piles was accomplished by means of sizing and installing steel transfer members while the pile was still under the applied load from the installation jacks.

– At this stage, the pressure on the hydraulic jacks was released, and the load was transferred to the permanent steel transfer members.

6.3 MIGA OR JACK UNDERPINNING

• This method can be used when the depth of 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 condition encountered.

• 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 jack have been removed.

Installation Procedures

• This is the method which can be used in the following circumstances:

– Depth of suitable bearing capacity subsoil is too deep to make traditional wall underpinning practical or economic.

– Where the system giving no vibration is required, it is worth noting that this method is also practically noiseless.

– If the system of variable depth is required.

– The existing foundation is structurally sound.

– The perimeter of the wall to be underpinned is excavated in stages as described for wall underpinning. The high of excavation is 1.8m and perimeter 1.5m. ( if using strip foundation)

– The system consists of short precast 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 foundation must be in a good condition since in the final context it will act as a beam spanning over the piles.

– The condition and hence the spanning ability of the existing strip foundation will also determine the spacing of the piles.

6.4 BORED PILE UNDERPINNING

• Bored pile underpinning is using for replace the strip foundation or pad foundation. Formation of the piles is quiet and free of variation and can be carried out with headroom of only 1.80m.

Installation Procedures

• The perimeter of the wall to be underpinned is excavated in stages as described for wall underpinning (if using strips foundation)

• The piles are bored at angle by means of a rotary drilling rig, either directly through the foundation or from a point up the wall.

• When the hole has been drilled reinforcement is introduced, a cement and sand grout is pumped in under pressure to form the pile lining tube is extracted.

6.5 CANTILEVER BEAMS 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.

• This system is used where traditional underpinning is not appropriate due to the existing foundations being deep, or that good-bearing strata is so deep that it is uneconomical (Depth greater than 1.5m). Or needle beams cannot be used due to access constraints.

• Reducing the span between cantilevers can accommodate high loads. However, the bearing capacity of the underlying strata will determine the number, diameter and spacing of piles used.

Installation Procedures

• 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.

6.6 NEEDLE BEAMS UNDERPINNING • This method stabilizes wall foundations by the use of piles installed either

side of an existing wall.

• A small pocket is broken out below the ground level and a reinforced concrete needle beam is cast in-situ connecting the piles and supporting the wall.

• Reducing the span between needle beams can accommodate very high loads.

• However, the bearing capacity of the underlying strata will determine the number, diameter and spacing of piles used.

Installation Procedures

• This system is used where traditional underpinning is not appropriate due to the existing foundations being too deep, or that good bearing strata is so deep that it is uneconomical to dig. (Depths greater than 1.5m)

• Piles are installed in pairs at 1.0m - 1.5m intervals and approximately 1.0m - 1.5m apart.

• A small pocket is broken out below the ground level and a reinforced concrete needle beam is cast in-situ connecting the piles and supporting the wall.

• Beams are usually cast alternate

6.7 ‘PYNFORD’ STOOL METHOD OF

UNDERPINNING

• This method can be used where the existing foundations are in a poor condition and it enable the wall to be underpinned in a continuous run without the need needle or shoring.

• The reinforced concrete beam formed by this method may well be adequate the 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.

Installation Procedures

• Holes is cut in position of beam to receive ‘stool’ which is the concrete blocks with holes so that the reinforcing bars can passed through it.

• Stools is positioned and pinned. Use the steel stools for heavy loads and pre cast concrete stools for light loads. Removed the brick wall between stools.

• Positioned the reinforcement fabrication around the stools. Erect the formwork and cast the concrete beam.

• Removed formwork and cured beam pinned to underside of existing walls stools remain as an integral part of new beam.

6.8 ‘BULLIVANT’ PATENT ANGLE PILING

• This is a much simpler alternative to traditional underpinning

technique applying modern concrete drilling equipment to achieve

cost benefits through time saving.

• The process is also considerably less disruptive as large volumes

of excavation are avoided.

• Where sound bearing strata can be located within a few meters of

the surface, wall stability is achieved through lined reinforced

concrete piles installed in pairs at opposing angles.

• The existing floor, wall and foundation are pre drilled with air

flushed percussion auger, giving access for a steel lining to be

driven through the low grade /clay subsoil until it impacts with firm

strata. The lining is cut to terminate at the underside of the

foundation and the void steel reinforced prior to concreting.

Purpose:

• To stabilize existing substructural walls & foundation RC angle

piles installed by using air – flushed rotary percussion drill from

inside & outside of the building

• The voids are lined with steel casing cut off at the surface prior

to lowering of reinforcement and placing concrete

•Short duration & minimal excavation involved

Installation Procedures:

• Where a sound bearing strata can be located within a few meters of the surface, wall stability is achieved through lined reinforced concrete piles installed in pairs at opposing angles. • The existing floor, wall & foundation are predrilled with air flushed percussion auger, giving access for a steel lining to be driven through the low grade /clay subsoil until it impacts with firm strata. • The lining is cut to terminate at the underside of the foundation & the void steel reinforced prior to concreting

6.9 HOOPSAVE METHOD

• Remedial treatment where differential settlement can be

identified

• 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 substructural wall & compensates for weaker areas of

subsoil

• Relatively this method is fast when applied to regular plan

shapes.

6.10 OTHER METHODS OF UNDERPINNING

• Other forms of underpinning using beams:

• Stressed steel

–Consist of std universal beam sections

–Short length of wall are removed and the steel

beam inserted

• Pre-stressed concrete

–Short pre cast concrete blocks are inserted

over the existing foundations as the brickwork

is removed

– The blocks are formed to allow for post –

tensioning stressing tendons to be inserted,

stressed and anchored to form a continous

beam

6.10.1 Underpinning Columns

• more difficult than wall

• can be carried out on brick /stone columns by inserting a

series of stools, casting a reinforced concrete base and

then underpinning.

• Structural steel & R.C column must be relieved of their

loading before any underpinning can take place.

• A collar of steel or pre cast concrete members is fixed

around the perimeter of the column. Concrete column –

chased to a depth of 25 to 50mm to receive support

collar.

• 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.

Types of underpinning works:

• Traditional Mass Concrete Underpinning

• Pad & Beam Underpinning

• Jacked-in Steel Pipe Piles Underpinning

• Jet Grouting Underpinning

• Timber pile foundation

• Bracket pile underpinning

Pad & Beam Underpinning Jacked-in Steel Pipe Piles

Underpinning

Timber Pile Foundation

Bracket Pile Underpinning

7.0 THE COST OF UNDERPINNING

• Whether the building needs to be underpinned should be determined by engineering on performance of a structural survey. If the underpinning required, the cost of underpinning will depend on: – the extent of the cracks

– the loads are added

– size of the house