underpinning

Underpinning Of Structures

Jamani Mohammedabbas R(SD0510)

Faculty Guide : Prof R.J.ShahSchool Of Building Science And Technology

CEPT University , Ahmedabad

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Contents

Introduction What is underpinning Why is underpinning required Methods of underpinning Advantages and disadvantages Case study conclusion References

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Introduction

The construction industry over the years has faced many problems right from the time of its inception

The problems related to the construction can be classified under two major categories i.e.

i) superstructure. ii) substructure.

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… Continue Several techniques have been developed

in order to deal with these types of problems by the engineers but the feedback obtained from the engineers is ”the problems related to superstructure can be solved and rectified much easily as compared to the substructure”

Underpinning is one of the engineering solutions to deal with the problems related to substructure.

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What is underpinning ???????? The word underpinning is used in

foundation engineering to denote the process of transferring the weight of an existing structure from its original foundation to temporary supports and from them to a new permanent substructure.

It may also denote the construction of new foundation under the structure without the intermediate use of temporary supports.

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…Continue When the work is completed, the

structure is said to be underpinned. This may be necessitated occasionally by

the inadequacy of original foundations or by their harmful settlements.

In most cases, however, it is caused by the fact that the new work in the immediate vicinity of the structure or even in the building itself, requires new foundations at a lower level.

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…Continue Successful underpinning requires a

knowledge of the state of balance of a building, its foundation and of the ground conditions.

Paths of load transfer need to be fully investigated within any structure to be underpinned.

It will help us in knowing the probable concentrations of stress the building is subjected to.

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Why underpinning ????Some of the modifications and

constructionLikely to make underpinning of existingStructures necessary are the following. Provision of a basement in a building

that had none before. Installations of tunnels , sewers under

or near columns or walls. Construction of pits, tanks, wells etc. Lowering of basement floors.

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…Continue

Construction of subways. Lowering of water table by adjacent

excavations that endanger the safety of footings on piles.

Increase of loads on columns or walls that requires replacement of present foundations.

The construction of deep foundations in positions that endanger present ones.

Lowering the ground surface by regarding so that footings have to be set down farther to prevent damage from frost action.

Scour threat caused by flowing water.

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General Principles The process of underpinning involves

great care and patience as conditions are likely to differ for every job and every job is special in its own way.

The load has to taken off the existing footing and held by temporary means while a new support is provided.

The load is then has to be transferred to the new bearing.

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….Continue It is important to do all this in a

manner that does not require or produce raising or lowering of the structure sufficiently to cause cracking or other harm to it.

There are several means to achieve the above mentioned objective but the common ones are use of wedges and screw jacks, hydraulic jacks etc.

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Methods Of Underpinning

Several methods used for underpinning are as

Follows: Conventional concrete underpinning. Micro piles. Angle piles Bracket piles. Needle beams. Cantilever beams.

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…Continue Piled R.C. Raft. Sheet piles. Steel pipe piles. Pressure grouting.

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Conventional Concrete Underpinning Conventional concrete underpinning is

accomplished from small, sheeted and braced pits constructed beneath the foundation to be supported, typically accessed via an approach pit.

Following excavation to the desired level, the pit is filled with concrete to within three inches of the foundation underside.

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Traditional Underpinning 

This method stabilizes existing wall foundations by digging under the present foundation in sequenced bays to a depth where firm strata exists and replacing the excavated material with mass concrete.

The loading capabilities are based upon the strata encountered.

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…Continue In some circumstances when the

existing footing is adequate to span between supports, intermediate piers can be used to reduce cost, i.e miss out piers 2 and 4 as shown on the diagram below.

A suitable bearing stratum is assessed by the Supervising Officer or Building Inspector upon completion of the first sequence of bays.

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…Continue 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.

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…Continue

Traditional underpinning

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Advantages

 simple engineering and easily understood.

Suitable for heavy loads. Occupants can remain in the property as

work is under taken from outside. Method can be used in restricted access

areas. Suitable for formation of new cellars and

basement constructions.  Low cost solution at shallow depths.

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Needle Beam Method

This method stabilises existing wall foundations by the use of piles installed either side of the 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 walls.

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

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…..Continue However, the bearing capacity of the

underlying strata will determine the number, diameter and spacing of piles used.

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 to dig. (Depths greater than 1.5m) .

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Advantages

Suitable for restricted access. Needle beam constructed at a higher

level if the existing foundation is too deep. Faster than tradition underpinning. High load capability. More economical at greater depths. Less disruption, less spoil generated

and completed quickly.

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…Continue

Needle beam method

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Piled R.C. Raft

This method stabilizes existing wall foundations to whole rooms.

Mini-piles are installed within the property and capped with an integral reinforced concrete raft.

Needle beams project from the slab into to the walls below ground level.

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….Continue Reducing the span between projecting

needle beams and increasing pile numbers can accommodate highly loaded structures.

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

Load Capabilities using mini-piles: i) 105mm dia SWL 60KN ii) 150mm dia SWL 90KN

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…Continue This system is used where whole

rooms or whole structures are to be underpinned and includes the provision of a new internal floor slab.

It is appropriate when existing foundations are deep, or that good-bearing strata is so deep that it is uneconomical to dig. (Depths greater than 1.0m).

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……Continue Piles are installed at centers

determined by loadings. Pockets are then broken out and

reinforced needle beams are placed to pick up the walls.

A ring beam is constructed linking the needles, mesh is laid and the whole structure poured.

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…Continue

Piled R.C raft method

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…Continue

Piled R.C raft method

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Advantages Provides lateral and traverse ties

throughout the structure. Provides new floor slab upon

completion. Economical at depths greater than

1.0m. No need for external access. Minimises disruption to drainage and service pipes.

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Cantilever Beam Method

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.

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….Continue 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 to dig. (Depths greater than 1.5m). Or needle beams cannot be used due to access constraints.

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….Continue

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.

 Piles are installed at approximately 1.0m – 1.5m centers and 1.2m apart determined by loadings.

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…Continue Then pockets are broken out and

reinforced beams are placed to pick up the walls. Beams are usually cast alternate.

Load Capabilities using mini-piles:  i) 105mm dia SWL 60KN . ii) 150mm dia SWL 80KN.

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…..Continue

Cantilever method

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…..Continue

Cantilever method

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Advantages Faster than traditional underpinning. Access from one side only. Occupants can stay in the property during construction. Economical at depths greater than 1.5m. Suitable for restricted access. Cantilever beam constructed at a higher level if existing foundation too deep. High load capability.

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Micro-piles

Micro-piles are small diameter (100 to 300 mm in diameter), reinforced load transfer elements capable of supporting design loads in excess of 200 tons.

Micro-piles can be installed through almost any type of subsurface condition, from sands to clay to bouldery soils to rock.

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…Continue Micro-piling is accomplished using low-

vibratory drilling techniques, a critical consideration when working around sensitive structures.

The micro-piles can readily be installed in confined space and low headroom situations.

Often installation can be accomplished without disrupting normal facility operations.

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…. Continue Vertical and/or inclined micro-piles

can be installed to provide additional foundation support needed to withstand increased structural loading, underpin existing shallow foundations during adjacent new construction, arrest ongoing settlement, and for seismic retrofit.

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Case Study

Imperial College. Location : London Considered as a major centre for

teaching and research in science and engineering.

Type of structure :- load bearing brick walls.

Constructed in the beginning of the 19th century

Reason for underpinning:- reconstruction and increase in the building height.

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….Continue Method used :- needle beam with the

help of concrete stools. Soil profile:- soil consisted of gravels

overlyed by clay. Water table was at approx 2m below

the existing basement level.

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….Continue The first stage was to excavate a

very deep basement at the rear of the main building.

The walls of main building comprised of a series of heavily loaded piers each of 1m thick* 2m wide.

The most heavily loaded pier carried at load of 4000 KN and the staircase wall loading was 250 KN/m.

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…Continue The depth of basement excavation

was required to be 9 m below the existing basement floor level and walls were underpinned to a depth of 11m.

Needle beam method was used to underpin the wall.

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….Continue

Intial stage of beam construction

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….Continue

Reinforcement between stools

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….Continue

Placing of stools

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Final stool positions

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Schematic view

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Conclusion

Underpinning of a structure is a complex and a time consuming process.

It is necessary to study in detail the pre-requites before taking the underpinning process.

Work should be allocated to the consultants who have already worked on similar nature of projects and are confident enough to achieve the desired results without further damage.

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References

Underpinning and retention S .Thorburn and G. S. Littlejohn Foundation engineering Dr. K .R. Arora Underpinning and framing J.F.S Pyrke Differential movement of domestic

buildings J.F.S Pyrke

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Experience along with expertise is necessary to deal with any kind of structural problems and the aim of the construction should be to carry out the work with all the strength and quality required along with skill and economy.

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Thank you