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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
