189184148 Bearing Capacity

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    BEARING CAPACITY OF SOIL

    Dr. S. K. Prasad

    Professor of Civil Engineering

    S. J. College of Engineering, Mysore

    7.0 Syllabus

    1. Definition of ultimate, net and safe bearing capacities, Allowable bearing pressure

    2. Terzaghis and Brinch Hansens bearing capacit e!uations " Assumptions and

    #imitations

    $. Bearing capacit of footings sub%ected to eccentric loading

    &. 'ffect of ground water table on bearing capacit

    (. )late load test, *tandard )enetration Test, +one )enetration Test- Hours

    7.1 Definii!ns

    Bearing capacit is the power of foundation soil to hold the forces from the

    superstructure without undergoing shear failure or e/cessi0e settlement.

    oundation soil is that portion of ground which is sub%ected to additional

    stresses when foundation and superstructure are constructed on the ground. The

    following are a few important terminologies related to bearing capacit of soil.

    Super Structure

    Foundation

    Foundation Soil

    Ground Level

    ig. .1 3 4ain components of a structure including soil

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    7.1.1 "li#ae Bea$in% Ca&a'iy ()f* 3 5t is the ma/imum pressure that a

    foundation soil can withstand without undergoing shear failure.

    7.1.+ Ne uli#ae Bea$in% Ca&a'iy ()n* ,5t is the ma/imum e/tra pressure

    in addition to initial o0erburden pressure that a foundation soil can withstand

    without undergoing shear failure.

    !n6 !f7 !o

    Here, !orepresents the o0erburden pressure at foundation le0el and is e!ual to

    8D for le0el ground without surcharge where 8 is the unit weight of soil and D

    is the depth to foundation bottom from 9round #e0el.

    7.1.- Safe Bea$in% Ca&a'iy ()s* ,5t is the safe e/tra load the foundation soil is

    sub%ected to in addition to initial o0erburden pressure.

    on

    s qFqq +=

    Here. represents the factor of safet.

    7.1. All!/able Bea$in% P$essu$e ()a* , 5t is the ma/imum pressure the

    foundation soil is sub%ected to considering both shear failure and settlement.

    7.1. F!unai!nis that part of the structure which is in direct contact with

    soil. oundation transfers the forces and moments from the super structure to

    the soil below such that the stresses in soil are within permissible limits and it

    pro0ides stabilit against sliding and o0erturning to the super structure. 5t is a

    transition between the super structure and foundation soil. The %ob of a

    geotechnical engineer is to ensure that both foundation and soil below are safe

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    against failure and do not e/perience e/cessi0e settlement. ooting and

    foundation are snonmous.

    7.+ 2!es !f s3ea$ failu$e

    Depending on the stiffness of foundation soil and depth of foundation, the

    following are the modes of shear failure e/perienced b the foundation soil.

    1. 9eneral shear failure :ef ig. .1a

    2. #ocal shear failure :ef ig. .1b

    $. )unching shear failure :ef ig. .1c

    *hear failure in foundation soil ) " ; cur0e in different foundation soils

    ig. . 1 3 ooting on ground that e/periences a 9eneral shear failure, b #ocal shear

    failure and c )unching shear failure

    .2.1 9eneral *hear ailureThis tpe of failure is seen in dense and stiff soil. The following are some

    characteristics of general shear failure.

    1. +ontinuous, well defined and distinct failure surface de0elops between the

    edge of footing and ground surface.

    2. Dense or stiff soil that undergoes low compressibilit e/periences this

    failure.$. +ontinuous bulging of shear mass ad%acent to footing is 0isible.

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    &. ailure is accompanied b tilting of footing.

    (. ailure is sudden and catastrophic with pronounced pea< in ) " ; cur0e.

    =. The length of disturbance beond the edge of footing is large.

    . *tate of plastic e!uilibrium is reached initiall at the footing edge and

    spreads graduall downwards and outwards.

    -. 9eneral shear failure is accompanied b low strain >(? in a soil with

    considerable @ @$=o and large $C ha0ing high relati0e densit

    5D C?.

    .2.2 #ocal *hear ailure

    This tpe of failure is seen in relati0el loose and soft soil. The following are

    some characteristics of general shear failure.

    1. A significant compression of soil below the footing and partial de0elopment

    of plastic e!uilibrium is obser0ed.

    2. ailure is not sudden and there is no tilting of footing.

    $. ailure surface does not reach the ground surface and slight bulging of soil

    around the footing is obser0ed.

    &. ailure surface is not well defined.

    (. ailure is characterized b considerable settlement.

    =. ell defined pea< is absent in ) " ; cur0e.

    . #ocal shear failure is accompanied b large strain 1C to 2C? in a soil

    with considerabl low @ @>2-

    o

    and low > ( ha0ing low relati0edensit 5D 2C?.

    .2.$ )unching *hear ailure

    This tpe of failure is seen in loose and soft soil and at deeper ele0ations. The

    following are some characteristics of general shear failure.

    1. This tpe of failure occurs in a soil of 0er high compressibilit.

    2. ailure pattern is not obser0ed.

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    $. Bulging of soil around the footing is absent.

    &. ailure is characterized b 0er large settlement.

    (. +ontinuous settlement with no increase in ) is obser0ed in ) " ; cur0e.

    ig. .2 presents the conditions for different failure modes in sand soil carring

    circular footing based on the contributions from Eesic 1F=$ G 1F$

    ig. .2 3 4odes of failure at different :elati0e densities G depths of

    foundations

    7.+. Disin'i!n be/een Gene$al S3ea$ 4 L!'al !$ Pun'3in% S3ea$

    Failu$es

    The basic distinctions between general shear failure and punching shear failure

    are presented in Table .1.

    Table .1 3 Distinction between 9eneral *hear G #ocal *hear ailures

    Gene$al S3ea$ Failu$e L!'al5Pun'3in% S3ea$ Failu$e

    ccurs in denseIstiff soil

    @$=o, $C, 5DC?, +u1CC 2-o, >(, 5D>2C?, +u>(C (? :esults in large strain 2C?

    ailure pattern well defined G clear ailure pattern not well definedell defined pea< in )7; cur0e o pea< in )7; cur0e

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    Bulging formed in the neighbourhood of

    footing at the surface

    o Bulging obser0ed in the

    neighbourhood of footing

    '/tent of horizontal spread of

    disturbance at the surface large

    '/tent of horizontal spread of

    disturbance at the surface 0er smallbser0ed in shallow foundations bser0ed in deep foundations

    ailure is sudden G catastrophic ailure is gradual

    #ess settlement, but tilting failure

    obser0ed

    +onsiderable settlement of footing

    obser0ed

    7.- Te$6a%3is bea$in% Ca&a'iy T3e!$y

    Terzaghi 1F&$ was the first to propose a comprehensi0e theor for e0aluating

    the safe bearing capacit of shallow foundation with rough base.

    .$.1 Assumptions

    1. *oil is homogeneous and 5sotropic.

    2. The shear strength of soil is represented b 4ohr +oulombs +riteria.

    $. The footing is of strip footing tpe with rough base. 5t is essentiall a two

    dimensional plane strain problem.

    &. 'lastic zone has straight boundaries inclined at an angle e!ual to @ to the

    horizontal.

    (. ailure zone is not e/tended abo0e, beond the base of the footing. *hear

    resistance of soil abo0e the base of footing is neglected.

    =. 4ethod of superposition is 0alid.

    . )assi0e pressure force has three components ))+produced b cohesion, ))!

    produced b surcharge and ))Jproduced b weight of shear zone.

    -. 'ffect of water table is neglected.

    F. ooting carries concentric and 0ertical loads.

    1C.ooting and ground are horizontal.

    11.#imit e!uilibrium is reached simultaneousl at all points. +omplete shear

    failure is mobilized at all points at the same time.

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    12.The properties of foundation soil do not change during the shear failure

    .$.2 #imitations

    1. The theor is applicable to shallow foundations

    2. As the soil compresses, @ increases which is not considered. Hence full

    plastic zone ma not de0elop at the assumed @.

    $. All points need not e/perience limit e!uilibrium condition at different loads.

    &. 4ethod of superstition is not acceptable in plastic conditions as the ground is

    near failure zone.

    ig. .$ 3 Terzaghis concept of ooting with fi0e distinct failure zones infoundation soil

    .$.$ +oncept

    A strip footing of width B graduall compresses the foundation soil underneath

    due to the 0ertical load from superstructure. #et !fbe the final load at which the

    foundation soil e/periences failure due to the mobilization of plastic

    e!uilibrium. The foundation soil fails along the composite failure surface and

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    the region is di0ided in to fi0e zones, Kone 1 which is elastic, two numbers of

    Kone 2 which are the zones of radial shear and two zones of Kone $ which are

    the zones of linear shear. +onsidering horizontal force e!uilibrium and

    incorporating empirical relation, the e!uation for ultimate bearing capacit is

    obtained as follows.

    Lltimate bearing capacit, BNDNcNq qcf (.C++=

    5f the

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