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B.tech Admission in India By: admission.edhole.com
Bearing CapacityBearing Capacity
foundations are designed to foundations are designed to transmit load from the structure they transmit load from the structure they
support to the soilsupport to the soil foundations are generally grouped foundations are generally grouped
into two categories:into two categories:
A. Shallow FoundationsA. Shallow Foundations
B. Deep FoundationsB. Deep Foundationsadmission.edhole.com
Shallow FoundationsShallow Foundations the most common (and cheapest) the most common (and cheapest)
type of shallow foundations aretype of shallow foundations are SPREAD FOOTINGSSPREAD FOOTINGS
square spread square spread footings to footings to support individual support individual columns (also columns (also circular)circular) McCarthy, 6McCarthy, 6thth Ed. Ed.
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Strip Footings to support wall loadsStrip Footings to support wall loads
Rectangular and Trapezoidal Footings for Rectangular and Trapezoidal Footings for two columns (combined footing) or two columns (combined footing) or machine basemachine base
McCarthy, 6McCarthy, 6thth Ed. Ed.
McCarthy, 6McCarthy, 6thth Ed. Ed.
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RAFT or MAT FoundationsRAFT or MAT Foundations
To lower the bearing pressure and reduce To lower the bearing pressure and reduce differential settlement on soils with low differential settlement on soils with low bearing capacity or erratic or variable bearing capacity or erratic or variable conditionsconditions
McCarthy, 6McCarthy, 6thth Ed. Ed.
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FLOATING FoundationsFLOATING Foundations
where deep deposits of compressible, where deep deposits of compressible, cohesive soil are present and piles are cohesive soil are present and piles are impracticalimpractical
building’s substructure is a combination building’s substructure is a combination mat and caisson to create a rigid boxmat and caisson to create a rigid box
weight of earth displaced by foundation is weight of earth displaced by foundation is equal to total weight of structure, thereby equal to total weight of structure, thereby minimizing settlement from consolidationminimizing settlement from consolidation
McCarthy, 6McCarthy, 6thth Ed. Ed.
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Deep FoundationsDeep Foundations used when soil near surface has used when soil near surface has
poor load-bearing capacitypoor load-bearing capacity
they transmit load through weak they transmit load through weak soil strata (overburden) to stronger, soil strata (overburden) to stronger, load-bearing stratum (eg., bedrock, load-bearing stratum (eg., bedrock, dense sand and gravel, etc.)dense sand and gravel, etc.)
loose soil
bedrock
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Types of Deep Types of Deep FoundationsFoundations
where load-bearing stratum where load-bearing stratum no more than 5 m deepno more than 5 m deep
not used much any morenot used much any more
PIERSPIERS
McCarthy, 6McCarthy, 6thth Ed. Ed.
CAISSONCAISSONSS
McCarthy, 6McCarthy, 6thth Ed. Ed.
where over-where over-burden no burden no more than 8 - more than 8 - 9 m thick9 m thick
replacing replacing pierspiersadmission.edhole.com
PILESPILES
deep over-deep over-burden more burden more than 8 - 9 m than 8 - 9 m thickthick
Various types Various types and placement and placement methodsmethods
Craig, 6Craig, 6thth Ed. Ed.
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Structural RequirementsStructural Requirements
1.1. Factor of Safety against General Shear Factor of Safety against General Shear Failure of supporting soil is normally Failure of supporting soil is normally required to be in the range 2.5 – 3.0required to be in the range 2.5 – 3.0
2.2. Tolerable amount of settlement; in Tolerable amount of settlement; in particular, differential settlement should particular, differential settlement should not cause significant damage to not cause significant damage to structure nor interfere with functionstructure nor interfere with function
3.3. Secondary to these, during construction, Secondary to these, during construction, there should be no adverse affect on there should be no adverse affect on adjacent structures or servicesadjacent structures or servicesadmission.edhole.com
Ultimate Bearing Capacity,Ultimate Bearing Capacity, q qff
The least pressure that would cause shear The least pressure that would cause shear failure of supporting soil immediately failure of supporting soil immediately below and adjacent to a foundationbelow and adjacent to a foundation
Craig, 6Craig, 6thth Ed.Ed.
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modes of modes of failure:failure:
on low compressibility (dense or stiff) soilson low compressibility (dense or stiff) soils plastic equilibrium throughout support and plastic equilibrium throughout support and
adjacent soil massesadjacent soil masses heaving on both sides of foundationheaving on both sides of foundation final slip (movement of soil) on one side final slip (movement of soil) on one side
only causing structure to tiltonly causing structure to tilt
General Shear General Shear FailureFailure
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on highly compressible soilson highly compressible soils only partial development of plastic only partial development of plastic
equilibriumequilibrium only slight heaving on sidesonly slight heaving on sides significant compression of soil under significant compression of soil under
footing but no tiltingfooting but no tilting
Local Shear Local Shear FailureFailure
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on loose, uncompacted soilson loose, uncompacted soils vertical shearing around edges of footingvertical shearing around edges of footing high compression of soil under footing, high compression of soil under footing,
hence large settlementshence large settlements no heaving, no tiltingno heaving, no tilting
Punching Shear Punching Shear FailureFailure
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Terzaghi’s TheoryTerzaghi’s Theory
strip footing of infinite length and width Bstrip footing of infinite length and width B uniform surcharge, quniform surcharge, q00 on surface of on surface of
isotropic, homogeneous soilisotropic, homogeneous soil Rankine active wedge, ABC: forces Rankine active wedge, ABC: forces Passive zones, ADE (Passive zones, ADE () & BGF () & BGF ())
Craig, 6Craig, 6thth Ed.Ed.
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transition betweentransition between ::ACD & BCG ACD & BCG (zones or radial shear or slip fans)(zones or radial shear or slip fans)
above EDCGF: plastic equilibriumabove EDCGF: plastic equilibrium below EDCGF: elastic equilibriumbelow EDCGF: elastic equilibrium
Craig, 6Craig, 6thth Ed.Ed.
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Neglecting the shear strength of the soil Neglecting the shear strength of the soil above depth D implies that this soil is a above depth D implies that this soil is a surcharge: qsurcharge: q00 = = DD
Terzaghi’s general equation:Terzaghi’s general equation:
the more general case is a footing at the more general case is a footing at depth Ddepth D
qqff = = 0.50.5BNBN + + cNcNcc + + DNDNqqContribution Contribution
of:of:Soil Self Soil Self
WeightWeightShear Shear StrengtStrengt
hh
SurchargSurchargee
Craig, 6Craig, 6thth Ed.Ed.
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Bearing Capacity FactorsBearing Capacity Factors NN, N, Ncc and N and Nqq are bearing capacity factors are bearing capacity factors
and are derived from various sourcesand are derived from various sources
Craig, 6Craig, 6thth Ed.Ed.
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General Shear Failure of Footings (Ultimate General Shear Failure of Footings (Ultimate Bearing Capacity)Bearing Capacity)
qccf DNSNcSNγBq )()(5.0
)45(tan 22)tan( eN q
)cot()1( qc NN
)4.1tan()1( qNN
FOOTING TYPE
Sγ Sc
Strip 1.0 1.0
Square 0.8 1.2
Circular 1.6 1.2
Rectangular )(2.01L
B )(2.01
L
B
theory was theory was developed for strip developed for strip footingsfootings
to adapt to square, to adapt to square, circular and circular and rectangular rectangular shapes, Terzaghi & shapes, Terzaghi & Peck developed Peck developed shape factors here shape factors here which are still which are still widely used today:widely used today:admission.edhole.com
Allowable Bearing Allowable Bearing CapacityCapacity
the allowable bearing capacity, qthe allowable bearing capacity, qaa is the is the value used in the design of footing sizevalue used in the design of footing size
in North America, a factor of safety in North America, a factor of safety against general shear failure, F is applied against general shear failure, F is applied to the ultimate bearing capacity, qto the ultimate bearing capacity, qff::
F
qq fa
qcc
a DNF
NScNSBq
)()(5.0
in Britain, F is not applied to the in Britain, F is not applied to the surcharge:surcharge:admission.edhole.com
Skempton’s NSkempton’s Ncc Values Values if undrained shear if undrained shear
strength strength parameters are parameters are used for the design used for the design then a special case then a special case arises:arises:
since since uu = 0, N = 0, Nqq = 1 = 1 and:and: DNcq cuf
values of Nvalues of Ncc are are acquired from acquired from Skempton’s Skempton’s ChartChart Craig, 6Craig, 6thth
Ed.Ed.
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