unit i - foundation
TRANSCRIPT
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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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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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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,
by preparing distance-time graphs and using analytical methods. Mani Mohan
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Using Seismic Refraction toMap the Subsurface
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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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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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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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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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