unit i - foundation

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Building Construction

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Unit-1 : Foundation


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inside of the house:

1. Doors and windows that

are misaligned

2. Cracks in the sheetrock

3. Doors and windows that

are sticking

4. Sloping of the floor

5. Cracks in the floor or tile

outside of the house:

6. Cracks in the brick

7.Gaps around the doors and

windows

8. Cracks in the foundation

9. Fascia board pulling away Mani Mohan


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10. Separation from door

11. Wall rotating out

12. Cracked brick

13. Walls leaning in or out

14. Cracks in the wall

15. Water intrusion

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Movements of the ground can often result in displacement of housefoundations. This can cause damage or cracking to the house. There are four main

causes of foundation movement:

Subsidence or settlement (due to consolidation or collapse of the soils below the

foundations)

Shrink-swell effects (resulting from the wetting or drying of clayey soils that

causes the clay to shrink when dried or swell when wetted up. Leaking drains or

trees can contribute to this effect)

Ground instability (due to slope movement)

Bearing capacity failure (due to inadequate strength in the foundation soils)

Liquefaction (of loose sands induced by earthquakes)

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http://www.geoconsult.co.nz/liquefaction.htmhttp://www.geoconsult.co.nz/liquefaction.htm


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1.EVAPORATION

2. TRANSPIRATION

3. PLUMBING LEAKS

4. DRAINAGE

5. INFERIOR FOUNDATION CONSTRUCTION

6. INFERIOR GROUND PREPARATION

7. POOR SOIL CONDITIONS Mani Mohan


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1.EVAPORATION: Hot dry wind and

intense heat will often cause the soil to

shrink beneath the foundation. Thissettlement may cause cracks to appear

throughout the structure.

2. TRANSPIRATION: Tree roots may

desiccate the soil beneath a home causingthe soil to shrink and the home to settle.

3. PLUMBING LEAKS: Water from

plumbing leaks is often a cause of

foundation problems.

4. DRAINAGE: Improper drainage is one

of the leading causes of foundation

failure. Excess moisture will erode or

consolidate soils and cause settlement.

5. INFERIOR FOUNDATION

CONSTRUCTION: Insufficient steel andinferior concrete will contribute to

movement in the slab.

6. INFERIOR GROUND PREPARATION:

Soft, low density soils and/or improperlycompacted soil beneath a home is the

leading

cause of foundation failure. Cut and fill

situations should be properly

prepared before the soil is ready to

support a structure.

7. POOR SOIL CONDITIONS: Poor soil and

its expansion and/or contraction

contribute to foundation failure.

CAUSES OF FOUNDATION FAILURE

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Building Parts and Loads

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Introduction

Sub structure : The part of structure that is

constructed underground. The sub structure

consists of foundation and in some building

include the basement area. Super structure : The part of structure that is

constructed above ground level. The super

structure consists of column, beam, floor, wall,roof, etc.

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Foundation

That part of structure which is in direct

contact with ground to which the loads are

transmitted.

The basic function of a foundation is to

transmit the dead loads, superimposed loads

and wind loads from a building to the soil in

such a way that settlements are withinpermissible limits and the soil does not fail.

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Function of Foundation To provide a level surface for construction of the foundation wall.

To provide adequate transfer and distribution of building loads to

the underlying soil.

To prevent differential settlement of the building in weak oruncertain soil conditions.

To place the building foundation at a sufficient depth to avoid frost

heave or thaw weakening in frost-susceptible soils and to avoid

organic surface soil layers

To provide adequate anchorage or mass (when needed in addition

to the foundation wall) to resist potential uplift and overturning

forces resulting from high winds or severe seismic events. Mani Mohan


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Excavation

Some amount of excavation required for

every building - Top soil consisting of organic

matter is removed - Below the region of soil

erosion (by water and wind) & below thelevel of permafrost - To the required depth at

which the bearing capacity necessary for the

building is met

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Machines used for excavation

Backhoe loader:

With an excavator-style digging arm on one

end and a two-armed bucket on the other,

backhoe loaders can tackle a wide variety of

trenching, loading, scooping, filling, and

levelling chores that would otherwise require

multiple machines or considerably more time

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Dredge*

Dredging is the activity of excavating

underwater bottom sediments and disposing

of them at a different location. This technique

is often used to keep waterways navigable orreclaim land. There are several types of

dredgers available.

*Not part of building Construction

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Fatal Facts!

An employee was installing a small

diameter pipe in a trench 3 wide, 12-15

deep and 90 feel long. The trench was not

sloped or shored nor was there a box orshield to protect the employee. Further,

there was evidence of a previous cave-in.

The employee apparently reentered the

trench, and a second cave-in occurred,burying him. He was found face down in

the bottom of the trench.


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Four employees of a mechanicalcontractor were laying a lateral sewerline at a building site. The foreman, aplumber by trade, and a laborer werelaying an eight-inch, 20-foot long plasticsewer pipe in the bottom of a trench 36inches wide, nine feet deep, andapproximately 50 feet long. The trench

was neither sloped nor shored, andthere was water entering it along ashale seam near the bottom. The westside of the trench caved in near thebottom, burying one employee to hischest and completely covering the

other. Rescue operations took two andfive hours - too late to save the men.


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How could this incident must have prevented?


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Protection Against Cave-in

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Site Exploration

What is it?

Why Site Exploration.

What are the different methods. Choice of exploration method.

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Site Investigation


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What?

Soil exploration is a part of site investigation. Site investigation, in general deals with determining in

general, the suitability of the site for the proposedconstruction.

It attempts at understanding the subsurface conditions suchas: Soil and rock profile

Gelogical features of the region

Position and variation of ground water table

Physical properties of soil and rock

Contamination, if any

General data of adjacent structures, hydrological data,topography, soil maps, seismicity, etc.

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Why?

To determine the type of foundation required for

the proposed project at the site, i.e. shallow

foundation or deep foundation.

To make recommendations regarding the safebearing capacity or pile load capacity.

Ultimately, it is the subsoil that provides the

ultimate support for the structures.

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Methods

Open Excavation

Borings

Subsurface Soundings Geo-physical methods

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Open excavation (Open trial pits)

Cheapest method of exploration in shallow deposit.

In this method, pits are excavated at the site, exposingthe sub-soil surface thoroughly.

This method not only affords sampling and testing insitu but also permit visual inspection of the soil and

rock formations in their natural state This method is considered as the only means of

obtaining reliable information in a soil deposit of mixedsand, gravel and boulders where boring may provedifficult.

Disadvantage is that they are slow and becomerelatively expensive with increasing depths of exploration. Hence this method is only suitable forshallow depth.

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Fig: open trial pits


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Borings

The boring methods are used for exploration

at greater depths. These provide both

disturbed as well as undisturbed samples

depending upon the method of boring. Auger Boring

Wash boring

Percussion boring Rotary boring

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Auger Borings

Simplest method of exploration and sampling.

Power driven or hand operated.

Max. depth 10 m

Suitable in all soils above GWT but only incohesive soil below GWT

Hollow stem augers used for sampling or

conducting Standard Penetration Tests. Samples recovered from the soil brought

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Hand operated augers

Power driven augers

W h B i


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The advantage of this is the use of inexpensive

and easily portable handling and drilling

equipments. A casing is driven with a drop hammer. A

hollow drill rod with chopping bit isinserted inside the casing.

Soil is loosened and removed from theborehole using water or a drilling mud

jetted under pressure. The water is jetted in the hole through

the bottom of a wash pipe and leaves thehole along with the loose soil, from theannual space between the hole and washpipe.

The water reaches the ground levelwhere the soil in suspension is allowed tosettle and mud is re-circulated.

Boulders and rock cannot be penetrated by

this method

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The change in soil stratification can be

guessed from the rate of progress and

colour of wash water

Wash Boring

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Percussion Boring

Simple to operate and maintain.

Suitable for a wide variety of rocks.

Operation is possible above and below the

water-table.

It is possible to drill to considerable depths.

Disadvantage of this method is, It is slow and

the soil formation gets disturbed by the

impact.

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Working of percussion boring

The drill involves a heavy steel bit attached to a rope which is lifted, either

by hand or by machine, and then dropped to cut the earth.

As the bit chops the earth, water is added to the well hole so that the bit

makes mud out of the earth it has cut.

After the hole is filled with several feet of mud, the heavy bit is withdrawn

and a tool called a bailer is attached to the rope and lowered into the

hole.

The bailer is a hollow tube with a door at the bottom.

The door, called a flap valve, opens when it hits the mud to allow the mud

to fill the bailer, and then closes to trap the mud inside the tube so thatthe mud can be lifted to the surface.

The tube is emptied at the surface and the procedure is repeated until the

hole is clear.


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Sub surface soundings

These tests are carried our to measure the resistance topenetration of a sampling spoon, a cone or other shapedtools under dynamic or static loading.

Variations in this resistance shows dissimilar soil layers andthe numerical values of the resistance permits an estimateof some of the physical properties of the strata.

These tests are used for exploration of erratic solid profilesfor finding depth to bed rock or stratum and to getapproximate indication of the strength and otherproperties of soil.

Two important widely used test are:

Standard Penetration Test (SPT)

Cone penetration test or Dutch cone test

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Standard Penetration Test (SPT)

The standard penetration test (SPT) is an in-

situ dynamic penetration test

It is a simple and inexpensive test to estimate

the relative density of soils and approximate

shear strength parameters.

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C td


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Contd.. Standard Penetration Test, SPT, involves driving

a standard thick-walled sample tube into the

ground at the bottom of a borehole by blowsfrom a slide hammer with standard weight andfalling distance.

The sample tube is driven 150 mm into theground and then the number of blows neededfor the tube to penetrate each 150 mm

The sum of the number of blows required forthe second and third 150mm of penetration isreported as SPT blow count value, commonlytermed "standard penetration resistance" orthe "N-value".

The N-value provides an indication of therelative density of the subsurface soil, and it isused in empirical geotechnical correlation toestimate the approximate shear strengthproperties of the soils.

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Cone penetration test or Dutch cone test

Assignment for student. Advantages and disadvantages of SPT and

Cone penetration test.

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Geo Physical Method

Geo-Physical methods are used when the depthof exploration is very large,

Also when the speed of investigation is of primaryimportance.

Geo-Physical investigations involve the detectionof significant differences in the physicalproperties of geological formation.

The major methods of geo-physical investigationwidely used in civil Engineering are seismicrefraction method, and electrical resistivity.

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Application of Seismic Refraction

method

Mapping bedrock topography

Determining the depth of gravel, sand or clay

deposits

Delineating perched water tables

Determining the depth to the water table

Detecting subsurface caverns

Estimating rippability

Detecting shallow faults and fracture zones

Detecting large boulders Mani Mohan

Contd


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Contd.. In this method, shock waves are created into the soil at their

ground level or a certain depth below it by exploding small

charge in the soil or by striking a plate on the soil with hammer A number of geophones are arranged along a line. Some of the

waves travel directly from the shock point along the ground

surface and are picked first by the geophones. The other waves

which travel through the soil get refracted at the interface of

two soil strata. The refracted rays are also picked up by the

geophones.

As the distance between the shock point and the geophone

increases, the refracted waves are able to reach the geophone

earlier than the direct waves are able to reach the geophoneearlier than the direct waves.

By knowing the time of travel primary and refracted waves at

various geophones, the depth of various strata can be evaluated,

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Using Seismic Refraction toMap the Subsurface

El i l i i i h d


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Electrical resistivity method The electrical resistivity method is based on the measurement

and recording of changes in the mean resistivity of various soils.

Each soil has its own resistivity depending upon its water

content, compaction and composition; for example, it is low for

saturated silt and high for loose dry gravel or solid rock.

The test is conducted by driving four metal spikes to serve as

electrodes into the ground along a straight line at equaldistance. A direct voltage drop is measured between the inner

electrodes.

The depth of exploration is roughly proportional to the electrode

Spacing. For studying vertical changes in the strata, theelectrode system is expanded, about a fixed central point, by

increasing the spacing gradually from an initial small value to a

distance roughly equal to the depth of exploration required. The

method is known as resistivity sounding. Mani Mohan


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Fig: Electrical resistivity

method

Ch i Of E l ti M th d


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Choice Of Exploration MethodThe choice of a particular exploration method depends on the following factors:

1. Nature of ground

In clayey soils, borings are suitable for deep exploration and pits for shallow

exploration.

In sandy soils, boring is easy but special equipment's are required for taking the

samples below the water table. Such samples can however, be readily taken in trial

pits provided that, where necessary, some form of ground water lowering is used.Borings are suitable in hard rocks while pits are preferred in soft rocks. Core

boring are suitable for the identification of types of rock but they cannot supply

data on joints and fissures which can also be examined in pits and large diameter

boring.

When the depth of exploration is large, and where the area of construction site islarge, geophysical methods can be used with advantage. However, borings at one

or two locations should be carried out, for calibration purposes. In soft soil,

sounding method may also be used to cover large area in relatively shorter

duration.

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Ch i Of E l ti M th d


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Choice Of Exploration Method

2. Topography

In hilly country, the choice between vertical openings and horizontalopenings may depend on the geological structure, since steeply inclinedstrata are most effectively explored by heading and horizontal strata by trialpits or borings. Swamps and areas overlain by water are best explored byboring which may have to be put down from a floating craft.

3. Cost.

For deep exploration, boring are usual, as deep shafts are costly. However,if the area is vast, geophysical methods or soundings methods may be usedin conjunction with borings.

For shallow exploration in soil, the choice between pit and borings willdepend on the nature of the ground and the information required for

shallow exploration in rock; the cost of boring a core drill to the site willonly be justified if several holes are required; otherwise trial pits will bemore economical.

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Bearing Capacity of Soil

Bearing capacity is the ability of soil to safelycarry the pressure placed on the soil from anyengineered structure without undergoing a shearfailure with accompanying large settlements.

Applying a bearing pressure which is safe withrespect to failure does not ensure that settlementof the foundation will be within acceptable limits.

Therefore, settlement analysis should generallybe performed since most structures are sensitiveto excessive settlement.

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Plate Load test

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Plate Load Test is a field test for determining the ultimate bearing capacity of soil

and the likely settlement under a given load

The test basically consists of loading a steel plate placed at the foundation level

and recording the settlements corresponding to each load increment.

The test load is gradually increased till the plate starts to sink at a rapid rate. The

total value of load on the plate in such a stage divided by the area of the steel plate

gives the value of the ultimate bearing capacity of soil

The ultimate bearing capacity of soil is divided by suitable factor of safety (which

varies from 2 to 3) to arrive at the value ofsafe bearing capacity of soil.

For better understanding, this test can be sub-divided into the following

heads,1. Test set-up

2. Testing procedure

3. Interpretation of results

4. Limitations of the test.

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Test set up

A test pit is dug at site upto the depth at which thefoundation is proposedto be laid.

The width of the pit

should be at least 5 timesthe width of the testplate.

A rigid steel plate, roundor square in shape, 300

mm to 750 mm in size,25 mm thick acts asmodel footing.

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Testingprocedure

The load is applied to the testplate through a centrally

placed column. The test load

is transmitted to the column

by one of the following twomethod.

(i) By gravity loading or

reaction loading method

(ii) By loading truss method.

Gravity loading


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Gravity loading

In case of gravity loadingmethod, a loading platform is

constructed over the columnplaced on the test plate andtest load is applied by placingdead weight in the form ofsand bags, pig iron, concreteblocks, lead bars etc. on the

platform. Many a times ahydraulic jack is placedbetween the loading platformand the column top forapplying the load to the testplate the reaction of thehydraulic jack being borne by

the loaded platform. Thisform of loading is termed asreaction loading.

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Reaction Truss Method


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Reaction Truss Method

In case of reaction truss method, instead of constructing a loading

platform, a steel truss of suitable size is provided to bear the

reaction of the hydraulic jack. The truss is firmly anchored to the

ground by means of steel anchors and guy ropes are provided for

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At least two dial gauges are used to account

for differential settlement. The dial gauges are

placed at diametrically opposite ends of theplate and one dial gauge is mounted on

independently supported references beam or

datum rod.

At every applied load, the plate settles

gradually. The dial gauge readings are

recorded after the settlement reduces to least

count of gauge (0.002 mm) & average

settlement of 2 or more gauges is recorded.

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Interpretation of results

The load intensity andsettlement observations of

the plate load test are

plotted in the form of load

settlement curves.

The safe bearing capacity

is obtained by dividing the

ultimate bearing capacityby a factor of safety

varying from 2 to 3.

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Limitation of Plate load test


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Limitation of Plate load test

The test results reflect only the character of the soil

located within a depth of less than twice the width ofbearing plate. Normally the foundations are larger thanthe test plates, the settlement and shear resistance ofsoil against shear failure will depend on the propertiesof much thick stratum. Thus the results of test could be

misleading if the character of the soil changes atshallow depths.

It is essentially a short duration test. Hence, it does notreflect the long term consolidation settlement of clayeysoil.

Size effect is pronounced in granular soil. Correction forsize effect is essential in such soils.

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Nuclear Power Plant Underground

Safety underground: the advantages immune to military attack from the air containment unbreachable

(given proper choice of ground conditions, hydrogeology and rocktypes) and so immune to attack from, say, a suicide bomber. Evenmajor LOCAs would be better contained than anything aboveground

no need ever to remove irradiated fuel assemblies. When thereactor reaches the end of its operating lifetime, the whole facilitycould be sealed, complete with its spent fuel. Monitoring would beneeded but because nothing is above ground, access would only beminimal

planning consent more likely to be straightforward since therewouldn't be much surface infrastructure to object to. Most of theusual public fears and objections wouldn't be serious issues

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Nuclear Power Plant Underground

Disadvantages:

Cost:

Location:

Cooling:

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Types of


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Types of foundation

Shallowfoundation

Spread

footing

Combined

footing

Strap

Footing

Mat

Foundation

DeepFoundation

Deep striprectangular orsquare footing

Pile

Foundation

Pier foundation ordrilled caisson

foundation

Well foundation

or caissons

Foundations must bedesigned to maintain soil

pressures at all depths

within the allowable bearing

capacity of the soil.

It must also limit total and

differential movements to

within levels that can be

tolerated by the structure.

yp

Foundation

Shallow foundation


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Shallow foundation

As per Terzagi,

If depth (D) of footing is width (B) of footing

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Types of Shallow Foundation

Spread Footing Combined Footing

Strap Footing Mat Foundation

These foundations may be used where there is a suitable

bearing stratum near the ground surface and settlement from

compression or consolidation of underlying soil is acceptable.


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Shallow Foundation

1. SPREAD FOOTING:

It is a wide base placed directly beneath the

load bearing wall or column.

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Types of spread footing

1. Isolated footing: for column

Single footing for a

column

Stepped footing Sloped footing

2. Wall footing

Strip footing Stepped footing for wall

3. Grillage Foundation


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Fig: Single footing for a column


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PLAN

SECTION


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Fig: Stepped footing for a column


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PLAN

Fig: Sloped footing for a column


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PLAN

Fig: Strip footing for

Wall

Suitability:

This is the cheapest type of spread footing

foundations and is largely used for walls of

ordinary buildings.


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Fig: Step footing for

Wall


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Fig: Grillage foundation

Suitability: Steel grillage foundations are useful for structures like columns, piers, stanchions

subjected to heavy concentrated loads and hence are employed for foundations of the buildings

such as theaters, factories, town, halls etc.


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Combined Footing

Combined footing support more than onecolumn and are used for reasons such as:

Property line or other obstructions make it

undesirable to use a single column on anindependent single footing.

Allowable soil pressure is so low or column loads

are so large that individual footings would overlap.

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Strap FootingCantilever or strap footings consist of two single

footings connected with a beam or a strap andsupport two single columns. This type replaces acombined footing and is more economical

This is provided when distancebetween column is large.

The strap is assumed to be infinitely

Rigid and serves to transfer the

Column load to soil with uniform

Pressure at both footing

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Mat Foundation

A mat or raft is acombinedfooting thatcovers the entire

area beneath astructure andsupports all thewalls and

column.

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When Mat foundation is required

When the allowable soil pressure is low.

When building loads are very heavy.

When soil mass containing compressible layer

or the soil is sufficiently erratic so that

differential settlement would be difficult to

control.

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Deep Foundation

Deep foundations transfer loads far below thesubstructure.

These foundations penetrate incompetent soil

until a satisfactory bearing stratum is reached.

Used to reach deeper layers with greater

bearing.

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h d i


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When to Use Deep Foundations

Used where Soil is soft and hard strata is far below the ground

Shallow foundation is expensive

Structural loads are high; Required spread footings aretoo large

Upper Soils are subject to scour or undermining

Foundation must penetrate through water

Need large uplift capacity

Need large lateral load capacity

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Types

Pile Foundation

Pier Foundation

Caisson Foundation

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Pile Foundation

What is a Pile Foundation

It is a foundation system that transfers loads to a

deeper and competent soil layer.

When To Use Pile Foundations Inadequate Bearing Capacity of Shallow Foundations

To Prevent Uplift Forces

To Reduce Excessive Settlement

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PILE CLASSIFICATION


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Friction Pile:

Load Bearing Resistancederived mainly from skin

friction.

End Bearing Pile:

Load Bearing Resistance

derived mainly frombase

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PILE INSTALLATION METHODS


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PILE INSTALLATION METHODS

Diesel / Hydraulic / Drop Hammer

Driving

Jacked-In

Prebore Then Drive

Prebore Then Jacked In

Cast-In-Situ Pile

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Diesel Drop Hammer Driving Hydraulic Hammer Driving


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Jacked-In Piling

Diesel Hammer


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Diesel Hammer

Diesel Hammer impart compression, impact

and explosion energy to the pile.

Process:

1. Raise the piston to start.

2. Injection of diesel fuel and compression.

3. Impact and explosion.

4. Exhaust Ports exposed and gases escapes.

5. Draw fresh air through Exhaust.

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TYPES OF PILES


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TYPES OF PILES

Treated Timber Piles

R.C. Square Piles

Pre-Stressed Concrete Spun Piles

Steel Piles

Bored piles

Caisson Piles

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R C Square Piles


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R. C. Square Piles

Size : 150mm to 400mm

Lengths : 3m, 6m, 9m and 12m

Structural Capacity : 25Ton to 185Ton

Material : Grade 40MPa Concrete

Joints: Welded

Installation Method : Drop Hammer, Jack-In

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RC Square Piles


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Pile Lifting

Pile Fitting to Piling Machine


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Pile Positioning Pile Joining

Pre stressed Concrete Spun Piles


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Pre-stressed Concrete Spun Piles

Size : 250mm to 1000mm

Lengths : 6m, 9m and 12m (Typical)

Structural Capacity : 45Ton to 520Ton

Material : Grade 60MPa & 80MPa Concrete

Joints: Welded

Installation Method : Drop Hammer, Jack-In

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Spun Piles


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Spun Piles vs RC Square Piles


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Spun Piles vs RC Square Piles

Spun Piles have Better Bending Resistance

Higher Axial Capacity

Better Manufacturing Quality Able to Sustain Higher Driving Stresses

Higher Tensile Capacity

Easier to Check Integrity of Pile Similar cost as RC Square Piles

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Steel H Piles


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Steel H Piles

Size : 200mm to 400m

Lengths : 6m and 12m

Structural Capacity : 40Ton to 1,000Ton

Material : 250N/mm2to 410N/mm2 Steel

Joints: Welded

Installation Method : Hydraulic Hammer, Jack-In

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Steel H Piles


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Over Driving of Steel Piles

Large Diameter Cast Cast-In-In


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Situ Piles (Bored Piles)

Size : 450mm to 2m (Up to 3.0m for specialcase)

Lengths : Varies

Structural Capacity : 80Ton to 2,300Tons

Concrete Grade : 20MPa to 35MPa (Tremie)

Joints : None

Installation Method : Drill then Cast-In-Situ

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Mani MohanDrillingLower Reinforcement


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Place Tremie Concrete

TYPES OF PILE SHOES


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TYPES OF PILE SHOES

Flat Ended Shoe

Oslo Point

Cast-Iron Pointed Tip

Cross Fin Shoe

H-Section

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Mani MohanCross Fin Shoe


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Use Oslo Point Shoe to Minimize

Damage


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Damage

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Design and Construction Issues #2


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g

Issue #2

Presence of Cavity

Solution #2

Detect Cavities through Cavity Probing then

perform Compaction Grouting

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Design and Construction Issues #3


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Issue #3Differential Settlement

Solution #3

Carry out analyses to check the settlement

compatibility if different piling system is

adopted

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Typical Design and Construction Issues

#4


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#4

Issue #4Costly conventional piling design piled to set

to deep layer in soft ground.

Solution #4

Strip footings / Raft

Floating Piles

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Mani MohanConventional Foundation for Low Rise Buildings


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Foundation for Low Rise Buildings (Soil Settlement)


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Conceptual Design of FOUNDATION

SYSTEM


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SYSTEM

Low Rise Buildings (Double-Storey Houses):

= Strip Footings or Raft or Combination.

Medium Rise Buildings :-

= Floating Piles System.

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Advantages of Floating Piles System


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Cost Effective. No Down drag problems on the Piles.

Insignificant Differential Settlement betweenBuildings and Platform.

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Pier Foundation Its a foundation for carrying a heavy structural load which is


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It s a foundation for carrying a heavy structural load which isconstructed in site in a deep excavation.

It consists of a cylindrical column of large

diameter to support and transfer large


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diameter to support and transfer large

superimposed loads to the firm strata below. The difference between pile foundation and

pier foundation is that, the pile foundation

transfer the load through friction and/orbearing. But pier foundation transfer the load

only through bearing. Moreover pier

foundation is shallower in depth than the pile

foundation.

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Caisson Foundation


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A caisson is basically a structure with a hollow portion,which may be circular or box shaped in plan, having one or

more chambers/cells.

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It is a permanent

substructure that, whilebeing sunk into position,

permits excavation to

proceed inside and also

provides protection for the

workers against water

pressure and collapse of soil.

Types of Caisson Foundations According to material: concrete, RC, steel, masonry, timber


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According to cross-sectional shape: circular (having no

directional property), rectangular, round-ended.

According to the number of cells/chambers: single-celled,double-celled, and multiple-celled

According to installation:* Open caisson (well foundation): a caisson that has no top or

bottom cover during its sinking.

* Closed caisson (pneumatic caisson): A pneumatic caisson is like abox or cylinder in shape; but the top is closed and thus compressed aircan be forced inside to avoid water and soil from entering the bottomof the shaft.

* A floating or box caisson consists of an open box with sides andclosed bottom, but no top. It is usually built on shore and floated tothe site where it is weighted and lowered

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Open Caisson ( well Foundation)


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It is a box type ofstructure which is open atthe top and at thebottom.

Open Caisson arenormally used on sandysoils or soft bearingstratum and where no

firm bed is available at ahigher depth.

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Component of well foundation


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Stening

Stening is constructed in concrete or masonry

work.

Use ofstening is to provide dead load during

sinking operation.

Well Cap

R.C.C Slab covering provided over the top

of well is termed as well cap.

Top plug

Covering provided over the sand filling.Sand is filled in between top plug and

bottom plug. Top plug also acts as a part

of shuttering for laying the well cap.

Bottom Plug:

The lower portion of well is sealed by the concrete is called

b l


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as bottom plug.

Sand filling

The portion between top and bottom plug is filled with sand

so as to increase the self weight of the well and makes safe

during earthquake.

Well Curbs

It is made of concrete or brick. Cutting edge of well or

caisson is attached to well curb. It is shaped in the form of

wedge shape to facilitate easy sinking.

Advantages and Disadvantages of

Open Caissons


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p

Advantages: This type of Caisson can be extended up to large

depths.

Cost of Construction is relatively less on bed level or

lower side.Disadvantages:

Ifany obstruction of boulders or logs areencountered, then progress of work becomes slow.

The help of divers may be required for excavationnear haunches at the cutting edges.

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Closed Caisson (Pneumatic Caisson)


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This type of caisson is open atthe bottom and close at thetop. Pneumatic caisson isspecially used at the placewhere it is not possible toconstruct the well.

It is suitable for the depth of water more than 12 m.

In the construction of Pneumatic Caisson, thecompressed air is used toremove water from theworking chamber and thefoundation work is carried outin dry condition

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Assignment???
http://en.wikipedia.org/wiki/File:Caisson_Schematic.svg


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Components of Pneumatic Caisson

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Advantages of

Pneumatic Caissons


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Quality control is good because work is done indry conditions.

In situ soil tests are possible to determine thebearing capacity.

There is direct and easy passage to reach thebottom of caisson, hence any obstruction caneasily be removed.

Concrete gain more strength due to dryconditions.

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Disadvantages of

Pneumatic Caissons


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Construction of pneumatic caissons is muchexpensive than open caissons.

During working the various constructional activities,a proper care has to be taken, otherwise it may lead

to fatal accidents. Maximum depth below water table is limited to 30m to 40 m. Beyond 40 m depth, construction is notpossible.

There is more chances of caisson diseases toworkmen working under high pressure.

Labor cost is high.

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Assignment???


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What is Caisson disease.

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Floating Caisson


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After Casting the box Caisson, it is taken to the site byfloating in water, then it is called as 'floating' Caisson.

Sinking Process can be made-faster by increasing theself weight of caisson, self weight is increased by

adding sand or gravel inside the caisson. Floating caisson are not provided by cutting edge ascompared to the other caisson. This type of caisson issimple rest on a hard or level strata. In this way, loadcarrying capacity is a function ofthe resistance at thebase since there is no skin friction on sides.

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Advantages and Disadvantages of

floating Caissons


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Advantage:

Since concrete are pre-casted, good quality control is possible

Its installation is quick and more convenient

These types of caissons are less expensive as compared to othertypes of caissons.

Disdvantage:

The foundation bed require leveled surface before installation of afloating caisson

Rip-rap should be provided to protect the caisson from scouring

Floating caisson or box caisson is only advantageous when hardstrata is available.

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The excavated

dirt was being

piled up on the


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An underground

garage was being

dug to a depth

of 4.6 meters

p p

to a height of

10 meters


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unit i - foundation

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