Bearing Capacity of Soil and Settlement Criterion

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FOUNDATION SYSTEM


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Bearing Capacity of Shallow

foundation

A foundation is required for distributing the loads of the

superstructure on a large area.

The foundation should be designed such that

The soil below does not fail in shear &

Settlement is within the safe limits.

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Basic Definitions

1. Ultimate Bearing Capacity (qu) : The ultimate bearing capacity isthe gross pressure at the base of the foundation at which soil fails

in shear.

2. Net ultimate Bearing Capacity (qnu) :It is the net increase inpressure at the base of foundation that cause shear failure of the

soil.

qnu = quDf(overburden pressure)

3. Net Safe Bearing Capacity (qns) : It is the net soil pressure which

can be safely applied to the soil considering only shear failure.

=

FOS usually taken as 2.00 -3.00

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Basic Definitions

4. Gross Safe Bearing Capacity (qs) :It is the maximum pressurewhich the soil can carry safely without shear failure.

=

+ Df

5. Net Safe Settlement Pressure (qnp) :It is the net pressure which

the soil can carry without exceeding allowable settlement.

6. Net Allowable Bearing Pressure (qna): It is the net bearing

pressure which can be used for design of foundation.

Thus,qna = qns ; if qnp > qns

qna = qnp ; if qns > qnp

It is also known as Allowable Soil Pressure (ASP).

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Modes of Shear Failure

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GENERAL SHEAR FAILURELOCAL SHEAR FAILURE

PUNCHING SHEAR FAILURELOCAL SHEAR FAILURE


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General Shear Failure

The load - Settlement curve in case of footingresting on surface of dense sand or stiff

clays shows pronounced peak & failure occurs

at very small stain.

A loaded base on such soils sinks or tilts

suddenly in to the ground showing a surface

heave of adjoining soil

The shearing strength is fully mobilized all

along the slip surface & hence failure planes

are well defined.

The failure occurs at very small vertical strains

accompanied by large lateral strains.

ID > 65 ,N>35, > 360, e < 0.55

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GENERAL SHEAR FAILURE


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Local Shear Failure

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When load is equal to a certain value qu(1),

The foundation movement is accompanied by

sudden jerks.

The failure surface gradually extend out wards

from the foundation.

The failure starts at localized spot beneath the

foundation & migrates outward part by part

gradually leading to ultimate failure.

The shear strength of soil is not fully mobilized

along planes & hence failure planes are notdefined clearly.

The failure occurs at large vertical strain & very

small lateral strains.

ID = 15 to 65 , N=10 to 30 , 0.75

LOCAL SHEAR FAILURE


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Punching Shear Failure

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The loaded base sinks into soil like a punch. The failure surface do not extend up to the

ground surface.

No heave is observed.

Large vertical strains are involved withpractically no lateral deformation.

Failure planes are difficult to locate

PUNCHING SHEAR FAILURE


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Definition of Foundation - Foundations are the

building component which transfers building

loads to the soil. The lowest part of a structure

is generally referred to as foundation.

Function of foundation

To transfer load of the superstructure to the soil

on which it is resting.

Anchor the building's superstructure to prevent

uplifting due to wind and earthquake forces.

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REQUIREMENTS of a Foundatiom

A properly designed foundation is one that transfers the structuralload throughout the soil without overstressing of soil which canresult in either excessive settlement or shear failure, both of whichcan damage the structure.

The foundation should be designed such that The soil below does not fail in shear: The foundation, including the

underlying soil and rock, must be safe against a structural failure thatcould result in collapse.

Settlement is within the safe limits: During the life of the building, thefoundation must not settle in such way as to damage the structure or

impair its function. The foundation must be feasible both technically and economically

and practical to build without adverse effect to surrounding property.

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Types of Foundation

There are two basic types of foundations:

SHALLOW - Shallow foundations transfer the load

to soil at the base of the substructure.

DEEP - Deep foundations transfer loads far below

the substructure. These foundations penetrate

incompetent soil until a satisfactory bearingstratum is reached.

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SHALLOW FOUNDATION

Shallow foundations transfer the load to soil at thebase of the substructure.

Characteristics of shallow foundations are they are:

cheap and

they require suitable soil: mostly for firm soils or lightloads

Advantages

Cost (affordable)

Construction Procedure (simple)

Material (mostly concrete)

Labour (doesnt need expertise)

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SHALLOW FOUNDATION

Spread or Isolated footing Also known as a footer or footing

Its an enlargement at the bottom of a column/ bearing wall

that spreads the applied structural loads over a sufficiently

large soil area. Each column & each bearing wall has its own spread footing, so

each structure may include dozens of individual footings.

PLAN

ELEVATION

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SHALLOW FOUNDATION

Strip footing It is a continuous spread footing

Used to support bearing walls

Wall Footing

ELEVATION

ISOMETRIC VIEW

PLAN

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SHALLOW FOUNDATION

Combined footing: support more than one column useful when columns are located too close

together for each to have its own footing

ELEVATION

PLAN

Combined Rectangular Footing

Combined Trapezoidal Footing

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SHALLOW FOUNDATION

Strap or cantilever footing The strap simply act as a connecting beam and does not take

any soil reaction

Economical than combined footing when the allowable soilpressure is relatively high and the distance between the column

Is large

PLANELEVATION

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SHALLOW FOUNDATION

Raft footing A foundation system in which essentially the entire building is placed

on a large continuous footing.

It is a flat concrete slab, heavily reinforced with steel, which carriesthe downward loads of the individual columns or walls.

Raft foundations are used to spread the load from a structure over alarge area, normally the entire area of the structure.

It is normally consists of a concrete slab which extends over the entireloaded area.

It may be stiffened by ribs or beams incorporated into thefoundation.

Raft foundations have the advantage of reducing differentialsettlements as the concrete slab resists differential movementsbetween loading positions.

They are often needed on soft or loose soils with low bearing capacityas they can spread the loads over a larger area.

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SHALLOW FOUNDATION

Mat or raft footing

Raft Foundation

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SHALLOW FOUNDATION

Mat footing The structural loads are so high or the soil condition so poor

that spread footings would be exceptionally large. As a general

rule of thumb, if spread footings would cover more than 50% of

the building footprint area, a mat or some type of deep

foundation will usually be more economical.

The soil is very erratic & prone to excessive differential

settlements. The structure continuity and flexural strength of a

mat will bridge over these irregularities. The same is true of

mats on highly expansive soils prone to differential heaves The structural loads are erratic and thus increase the likelihood

of excessive differential settlements. Again, the structural

continuity and flexural strength of the mat will absorb these

irregularities.

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SHALLOW FOUNDATION

Mat footing The lateral loads are not uniformly distributed through the

structure and thus may cause differential horizontal movements

in spread footings and pile caps. The continuity of a mat will

resist such movement.

The uplift loads are larger than spread footings can

accommodate. The greater weight and continuity of a mat may

provide sufficient resistance.

The bottom of the structure is located below the groundwater

table, so waterproofing is an important concern. Because matsare monolithic, they are much easier to waterproof. The weight

of the mat also helps resist hydrostatic uplift forces from the

groundwater.

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SHALLOW FOUNDATION

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SHALLOW FOUNDATION

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DEEP FOUNDATION

Extend several dozen feet below the building

a. Piles

b. Piers

c. Caissons

d. Compensated Foundation

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Factors that affect the choice of a

foundation type

Primary factors

Subsurface soil and groundwater conditions;

Structural requirements: including foundation loads,building configurations, and depth.

Secondary factors Construction methods, including access and working space;

Environmental factors, including noise, traffic, and disposalof earth and water

Building codes and regulations

Impact on adjacent property

Time available for construction

Construction risks.

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Foundation loads

Dead Load : Refers to the overall weight of the structure. Includesweight of the materials permanently attached to the structure(such as flooring) and fixed service equipment (such as airconditioning)

Live load: Refers to the weight of the applied bodied that are notpermanent parts of the structure. Applied to the structure duringpart of its useful life (e.g. people, warehouse goods). Specified bycode.

Wind loads : Acts on all exposed parts of the structure. Calculatedusing building codes.

Earthquake Forces : Building code is consulted.

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Depth and location of foundation

Depth and location of foundation depends on

1. Zone of significant volume changes in soil.

2. Adjacent structures and property lines.

3. Ground water

4. Underground defects

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Zone of significant volume changes in soil :Clays having high plasticity shrink and swell considerably up on drying

and wetting respectively. Volume change is greatest near ground.

Decreases with increasing depth. Volume changes usually insignificant

below a depth from 1.5-3.0 m and does not occur below volume

changes.

Ground water

Presence of water reduces soil bearing capacity, larger footing size

more cost. During construction pumping is necessary adds to the

cost of construction.Underground defects

Footing location affected by underground defects. Faults, caves, mines,

sewer lines , underground cables and utilities.

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Adjacent structures and property lines.Structures may be damaged by the construction of

new foundations, as a result of vibrations,

undermining by excavation or lowering of the

water table. After new foundations have been

constructed, the (new) loads they place on the soil

may cause settlement of previously existing

structures as a result of new stress pattern in the

surrounding soil.

In general, deeper the foundations and closer tothe old structure, greater will be the potential for

damage to old structures.

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Part extending

property line

Property line

450 Existing

Footing

New Footing

Limit for bottom of

deeper Footing

Documents

Bearing Capacity of Soil and Settlement Criterion