UNDERPINNING Text Material

7/31/2019 UNDERPINNING Text Material

http://slidepdf.com/reader/full/underpinning-text-material 1/14

UNDERPINNING

What is underpinning?

underpinning is the process of strengthening and stabilizing the

foundation of an existing building or other structure.

Underpinning was carried out mainly because of settlement of structures

as a result of inadequate foundation

Types

Remedial underpinning:-

It is done to provide additional foundation strength to an

inadequately supported structure which may be settling or in danger of

collapse.

Precautionary underpinning:-

It cannot be avoided as it becomes essential by the

construction of a new foundation, deeper than those of the existing

building

UNDERPINNING IS REQUIRED

To strengthen the shallow footings of the existing buildings.

To safeguard against differential or excessive settlement of existing

structure.

To deepen the existing foundation to increase the bearing capacity so

as to sustain heavier loads .

To provide a basement to an existing building structure.

The original foundation is not strong or stable enough.

The usage of the building has changed.

The type of soil supporting the foundation may have changed or were

misjudged during planning.

The construction of nearby structures causes the excavation of soil

supporting existing foundations.

It is more economical, due to land price or otherwise, to work on the

current structure's foundation than to build a new one.



Causes of Foundation Problems

Water is the main enemy in expansive soils problems

TOO MUCH WATER will cause the soil to swell

LESS WATER will cause the soil to shrink

Evaporation And Transpiration cause the soils to dry and shrink

Uneven moisture causes uneven movement, which can lead to

structural damage to the structure.

If all soils beneath a foundation swells uniformly or shrinks uniformly it

is unlikely to cause a problem.

METHODS OF UNDERPINNING

1. Pit method

2. Pile method

3. Pier method

4. Miscellaneous methods

PIT METHOD

The most common and oldest method of underpinning is the pit

method

It Requires careful and skilled work as loss of ground will cause

building settlement

Columns/ walls above the affected footing should be braced as much

as possible

A pit of 3’ wide, 4’ long and 5’ deep is excavated in front of the footing

to be underpinned



Pit is extended laterally to reach under the foundation to be

underpinned

The foundation is then deepened to the required depth

Vertical formwork is built in the pit and then is concreted up to the

foundation

Dry packing operation is then carried out

Steps:-

1.The wall over the foundation is divided into various sections (1.2 to 1.5

m)

2.Holes are made to adequate height in the existing wall.

3.In these holes Steel Needle Beams with bearing plates are inserted and

supported on either side of the wall by means of crib supports (wooden

blocks)

4.The jacks in turn are provided below the crib supports ,

5.The pit is now excavated up to the level of new foundation directly or by

cutting down the lower part of the old foundation

Points to be noted

The entire wall foundation has to be replaced or deepened in

sections.

For long section of walls , work is started from center and progressed

in sideways.

Instead of central needle beams, cantilever beams may be used .

Soon after the completion of the work each excavated section should

be filled with cement concrete(1 : 2 :4).

Vertical mild steel bars of circular section are used of connection

between old foundation and new foundation



Needle beams and shores have to be removed only after the

completion of the new foundation

Pile method

Pit method is sometimes impracticable or uneconomical under

certain circumstances like,

1.underpinning in water logged structures

2.when heavy loads are to be transferred to greater depths

3. the cost of underpinning by pit method is more compared to pile

method.

4. piles with concrete grouting

In pile method ,underpinning is carried out in two stages

STAGE 1

1.In the first stage a precast Rcc pile is installed by carefully digging a

approach pit under the existing wall foundation to obtain about 2 m of the

head room .

2. A jacking arrangement is set up below the foundation to access theoverload capacity and to drive in the welding beams accordingly

3. All the materials surrounding the piles are removed by means of earth

augers , water jets or air lifts .

4.The gap so formed is filled up with cement concrete to provide a firm

bearing ground.



Stage 2

1. Concrete is filled up to the second level .

2. The jacks are then removed leaving a completed underpinning cylinder .

Methods of installation of piles

Displacement piles

Non-displacement piles

Displacement piles

Dropping weight

Diesel hammer

Vibratory methods of pile driving

Jacking methods of insertion

Jacked piles

Non-displacement piles

Small diameter bored cast-in-place piles

Large diameter bored cast-in-place piles

Large diameter bored cast-in-place piles



Diameter - 750mm up to 3m

Depth - 70m

Constructed by - cranes

Uses bentonite slurry to eliminate some of the difficulties involved in

drilling in soft silts and clays, and loose granular soils

By using an expanding drilling tool the diameter at the base of the pilecan be enlarged, significantly increasing the end bearing capacity of

the pile.

Small diameter bored cast-in-place piles

Diameter - 600mm or less

equipment :- tripod

a winch

a cable operating a variety of tools.

In granular soils, the basic tool consists of

heavy cylindrical shell with a cutting edge and a flap valve

at the bottom.

Water is necessary to assist in this type of excavation.

To prevent sides of the bore in granular soil a temporary casing

should be advanced by driving it into the ground.

In cohesive soils, the borehole is advanced by repeatedly dropping a

cruciform-section tool with a cylindrical cutting edge into the soil and

then winching it to the surface with its burden of soil.



PIERING

Piering a foundation is the process of installing steel piers under the

footing of a foundation and driving the piers through the soil down to

load bearing stratum.

The piers can be used to either support the structure or lift thestructure.

The oldest methods of underpinning is installing piers under

foundation of the structure

This method is generally adopted in dry ground, because of the

difficulty of pits below the water level without loose of ground, which

would increase the tendency for the building to settle.

The pits may be placed continuously or at suitable spacing.

Their number and size are determined by the weight of the structure

to be underpinned and the bearing capacity of the soil.

After filling the pits with concrete ,the pits become supporting piers

beneath the foundation

The depth to which underpinning pits are carried depends upon the

stratum which can support the load ,depth of the adjacent footings

,and whether there was settlement before

In pier underpinning it is essential that proper care has to be taken to

prevent the loss of ground in installing the sheeting, since loss of

ground can cause settlement of the structure during or afterunderpinning.

The least size of underpinning to provide workers proper working

space is 1 m by 1.3 m

Various shapes of pit can be sunk such that the soil under the

foundation is disturbed to a minimum.



The pits can be taken down as much as 10m to 15m until a stratum

strong enough to support the desired load Is met

Types of piers

RESISTANCE PIERS

HELICAL Piers

STEEL PIERS

RESISTANCE PIERS

These are also called as PUSH piers

Typically they have the best lifting potential.

This is because these types of piers are "pushed" down into the

ground, through the soil until they hit a hard surface - load bearing

stratum.

The weight of the structure is used as a counter-force to hydraulically

push these piers into the ground.

Once they hit this bottom, they can be pushed even further so that

they actually lift the structure back to their original level.

Once that is accomplished, the pier is capped off and permanently

attached to the bottom of the foundation.

This should prevent any further settling of the structure

The advantage of this type of pier is that a soils report is not really

necessary to know how deep the piers must be pushed.

Once the "bottom" is reached, that is, stratum that is strong enough

to support the structure, the structure can be lifted.

These types of piers are typically used for heavier structures but are

also perfect for single story residential homes.



Push Pier Installation Steps:

Step 1: The footing is exposed and prepared for the bracket

Step 2: Foundation Bracket is secured to the footing.

Step 3: Steel pier sections are hydraulically driven through the bracket

to competent soil or bedrock

Step 4: The weight of the home is transferred through the piers to

load bearing strata. Home is lifted back to level if possible.

Other types of piers

Hydraulically Advance Push pier

Injection pier

HELICAL piers

These are typically used to support a structure and not to lift it

These types of piers are "turned" into the ground much like a

corkscrew.

Each pier has one or more "flights" that are like blades that do the

pulling into the ground.

These flights keep the pier in place after installation.

The advantage of a helical pier is that is can be used on very light

structures, unlike resistance piers .

However, to install a helical pier properly, a soils test should be

performed by a geotechnical engineer

Helical piers are very useful in other applications such as tie-backs.

Since they are screwed into the ground, and held in place with their

flights.



Helical piers are ideal in pre-construction applications. They are

installed before the foundation is placed to prevent any future

foundation settlement.

Also, helical piers can be used in conjunction with resistance piers on

hillside homes.

Helical Pier Installation Steps

STEP 1:

Footing is exposed and prepared for the bracket.

step 2: Round-shaft

helical piers are mechanically advanced into the soil

Step 3: Foundation Bracket is secured to the footing.

Step 4: The weight of the home is transferred through the piers to

load bearing strata. Home is lifted back tolevel if possible.

Helical Pier Advantages:

Round shaft has a high resistance to bending

Suitable for both heavy and light loads

Long life span – galvanized steel is resistant to corrosion

Doesn't rely on the weight of the structure for depth and

capacity

Steel Piers

steel piers is stronger deep foundation product.

Most importantly, concrete pilings are not as strong as steel

piers.

Also, installing concrete pilings is messy and disruptive.



And finally, concrete pilings are not a permanent solution to

foundation settlement.

If the foundation pilings are installed in clay soil the structure

could begin to settle again.

steel pier systems are designed to withstand the test of time andare a permanent solution to settlement.

The installation is cleaner than concrete pilings

the steel piers are stronger and safer than that of concrete

pilings

LIMITATIONS OF PIER FOUNDATIONS

Foundation Upheaval Caused by Poor Drainage:

Swelling soil can also push the supporting piers upward, if they are not

designed and constructed to adequately resist uplift.

As a result, the pier will lift causing differential movement and

subsequent structural damage

Foundation Settlement Caused by Inadequate Pier Depth:

The piers supporting many older pier and beam foundations may not

extend below the zone of expansive soil that is affected by the

climate.

During periods of dry weather, these shallow piers may not provide

sufficient support to portions of the foundation, which may result in

differential settlement.



Miscellaneous methods

CEMENT GROUTING

CHEMICAL CONSOLIDATION

FREEZING

VIBROFLOTATION

CEMENT GROUTING

Cement grouting, also known as slurry grouting or high mobility

grouting, is a grouting technique that fills pores in granular soil or

voids in rock or soil, with flowable particulate grouts.

Depending on the application, Portland cement or microfine cement

grout is injected under pressure at strategic locations either through

single port or multiple port pipes.

JET GROUTING is an advanced technique used in cenent grouting.

CHEMICAL CONSOLIDATION

Chemical consolidation is a grouting technique that transforms

granular soils into sandstone-like masses, by permeation with a low

viscosity grout

The most common permeation grout for structural applications is

sodium silicate.

The grout is injected under pressure through previously installed

pipes.

The grouted soil has increased strength and stiffness, and reduced

permeability.

The equipment is relatively small, and therefore suitable for use on

sites with restricted access.

Here the soil under the existing footing is consolidated using

chemicals



Procedure:-

1.The perforated pipes are driven in inclined direction

2.When the pipes are driven a solution of sodium silicate in water is

injected through the pipes.

3.The pipes are withdrawn and during withdrawal magnesium or

calcium chloride is injected through the pipes.

4.The chemical reaction takes place between these chemicals and the

soil is consolidated

This method is useful when soil consist of sand or granular materials

The cost of construction depends upon the nature of the soil, depth of

consolidation, site of work, etc,

Freezing

In this method ,freezing pipes are driven below the existing footingand the soil is frozen

This method is rarely adopted for two reasons

1.It is expensive

2.More time is required for installation of freezing pipes

Vibroflotation

Vibro-flotation is a sand compaction method that is used to compact

layers of loose sand which are underground, and is a common

requirement in sand treatment for land reclamation works.

Without vibro-flotation treatment, roads built in these areas will

suffer settlement due to the heavy weight of travelling vehicles.



In this method underpinning is carried out by vibrating the sand and

thereby increasing the density which ultimately results in the increase

of the bearing capacity of the soil

This method is useful for granular soil or sandy soil

Before the process of underpinning starts ,the structure is shored

carefully.

The vibrating equipment is isolated from the building or the shoring

Vibro-flotation involves using high concentric vibrations together with

quick-charging water in the sand.

Quick-charging a high volume of water in the sand creates a quicksand

(boiling) effect which, together with the effect of the vibrations,

causes the sand particles to re-orientate from a loose layer to a tight

and dense layer.

Documents

UNDERPINNING Text Material