CE 632 Shallow Foundations Part-1 Handout
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Transcript of CE 632 Shallow Foundations Part-1 Handout
8/8/2019 CE 632 Shallow Foundations Part-1 Handout
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CE-632Foundation Analysis and
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Shallow FoundationsShallow Foundations
Foundation Analysis and Design: Dr. Amit Prashant
SUMMARY of TerminologySUMMARY of Terminology
Net Loading IntensityPressure at the level of foundation causing actual
settlement due to stress increase. This includes
Gross Loading IntensityTotal pressure at the level of foundation
including the weight of superstructure,
foundation, and the soil above foundation.
superstructure Foundati on soil
Foundation
g
Q Q Qq
A
+ +=
nq q Dγ = −
2
the weight of superstructure and foundation only.
Ultimate Bearing capacity:
Maximum gross intensity of loading that thesoil can support against shear failure is
called ultimate bearing capacity.
Net Ultimate Bearing Capacity:
Maximum net intensity of loading that the
soil can support at the level of foundation.
from
Bearing capacity calculation
uq
nu u f q q Dγ = −
Foundation Analysis and Design: Dr. Amit Prashant
SUMMARY of TerminologySUMMARY of Terminology
Gross Safe Bearing capacity:
Maximum gross intensity of loading that the soil
can safely support without the risk of shear failure.
Net Safe Bearing capacity:
Maximum net intensity of loading that the soil can
safely support without the risk of shear failure.nu
ns
FOS=
gs n s f q q Dγ = +
3
Safe Bearing Pressure:
Maximum net intensity of loading that can be
allowed on the soil without settlement
exceeding the permissible limit.
Allowable Bearing Pressure:
Maximum net intensity of loading that can
be allowed on the soil with no possibility of
shear failure or settlement exceeding the
permissible limit.
from settlement analysissq ρ
Minimum of
bearing capacity and
settlement analysis
a net q −
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Foundation Analysis and Design: Dr. Amit Prashant
Common Types of FootingCommon Types of Footing
Strip footing
Spread Footing
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Foundation Analysis and Design: Dr. Amit Prashant
Common Types of FootingCommon Types of Footing
Combined Footing
Raft or Mat footing
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Foundation Analysis and Design: Dr. Amit Prashant
General Requirements of FoundationGeneral Requirements of Foundation
Location and Depth of Foundation
Bearing Capacity of Foundation DONEONE
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Settlement of Foundation DONEONE
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Foundation Analysis and Design: Dr. Amit Prashant
Location and depth of FoundationLocation and depth of Foundation
The following considerations are necessary for deciding thelocation and depth of foundation As per IS:1904-1986, minimum depth of foundation shall be 0.50 m.
Foundation shall be placed below the zone of The frost heave
Excessive volume change due to moisture variation (usually existswithin 1.5 to 3.5 m depth of soil from the top surface)
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Topsoil or organic material
Peat and Muck
Unconsolidated material such as waste dump
Foundations adjacent to flowing water (flood water, rivers, etc.) shall beprotected against scouring. The following steps to be taken for designin such conditions
Determine foundation type
Estimate probable depth of scour, effects, etc.
Estimate cost of foundation for normal and various scour conditions
Determine the scour versus risk, and revise the design accordingly
Foundation Analysis and Design: Dr. Amit Prashant
Location and depth of FoundationLocation and depth of Foundation
IS:1904-1986 recommendationsfor foundations adjacent toslopes and existing structures When the ground surface slopes
downward adjacent to footing, thesloping surface should not cut the 1V
2H
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ne o s r u on o e oa(2H:1V).
In granular soils, the line joining the
lower adjacent edges of upper andlower footings shall not have aslope steeper than 2H:1V.
In clayey soil, the line joining thelower adjacent edge of the upper footing and the upper adjacentedge of the lower footing should notbe steeper than 2H:1V.
1V
2H
Foundation Analysis and Design: Dr. Amit Prashant
Location and depth of FoundationLocation and depth of Foundation
Other recommendations for footing adjacent to existing
structures
Minimum horizontal distance between the foundations shall not
be less than the width of larger footing to avoid damage to
existin structure
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If the distance is limited, the principal of 2H:1V distribution
should be used so as to minimize the influence to old structure
Proper care is needed during excavation phase of foundation
construction beyond merely depending on the 2H:1V criteria for
old foundations. Excavation may cause settlement to oldfoundation due to lateral bulging in the excavation and/or shear
failure due to reduction in overburden stress in the surrounding
of old foundation
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Foundation Analysis and Design: Dr. Amit Prashant
Location and depth of FoundationLocation and depth of Foundation
Footings on surface rock or sloping rock faces For the locations with shallow rock beds, the foundation can be
laid on the rock surface after chipping the top surface. If the rock bed has some slope, it may be advisable to provide
dowel bars of minimum 16 mm diameter and 225 mm embedment
into the rock at 1 m spacing.
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A raised water table may cause damage to the foundation
by Floating the structure
Reducing the effective stress beneath the foundation
Water logging around the building may also cause wet basements. In
such cases, proper drainage system around the foundation may be
required so that water does not accumulate.
Foundation Analysis and Design: Dr. Amit Prashant
Loads on FoundationLoads on Foundation
Permanent Load: This is actual service load/sustained loads of astructure which give rise stresses and deformations in the soil belowthe foundation causing its settlement.
Transient Load: This momentary or sudden load imparted to astructure due to wind or seismic vibrations. Due to its transitorynature, the stresses in the soil below the foundation carried by suchloads are allowed certain ercenta e increase over the allowable
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safe values.
Dead Load: It includes the weight of the column/wall, footings,foundations, the overlaying fill but excludes the weight of thedisplaced soil
Live Load: This is taken as per the specifications of IS:875 (pt-2) –1987.
Foundation Analysis and Design: Dr. Amit Prashant
Loads for Proportioning and Design of FoundationLoads for Proportioning and Design of FoundationIS:1904IS:1904 -- 19861986
Following combinations shall be used
Dead load + Live load
Dead Load + Live load + Wind/Seismic load
For cohesive soils only 50% of actual live load is consideredfor design (Due to settlement being time dependent)
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For wind/seismic load < 25% of Dead + Live load
Wind/seismic load is neglected and first combination is used tocompare with safe bearing load to satisfy allowable bearing pressure
For wind/seismic load ≥ 25% of Dead + Live load
It becomes necessary to ensure that pressure due to secondcombination of load does not exceed the safe bearing capacity bymore than 25%. When seismic forces are considered, the safebearing capacity shall be increased as specified in IS: 1893 (Part-1)-2002 (see next slide). In non-cohesive soils, analysis for liquefaction
and settlement under earthquake shall also be made.
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Foundation Analysis and Design: Dr. Amit Prashant
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Foundation Analysis and Design: Dr. Amit Prashant
Safe Bearing Capacity: National Building Code ofSafe Bearing Capacity: National Building Code ofIndia (1983)India (1983)
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Foundation Analysis and Design: Dr. Amit Prashant
Safe BearingSafe BearingCapacity: NationalCapacity: NationalBuilding Code ofBuilding Code ofIndia (1983)India (1983)
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Foundation Analysis and Design: Dr. Amit Prashant
Other considerations for Shallow Foundation DesignOther considerations for Shallow Foundation Design
For economical design, it is preferred to have square footing for verticalloads and rectangular footing for the columns carrying moment
Allowable bearing pressure should not be very high in comparison to thenet loading intensity leading to an uneconomical design.
It is preferred to use SPT or Plate load test for cohesion less soils andundrained shear strength test for cohesive soils.
In case of lateral loads or moments, the foundation should also be
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.less than 1.75 against sliding and 2.0 against overturning. Whenwind/seismic loads are considered the FOS is taken as 1.5 for both thecases.
Wall foundation width shall not be less than [wall thickness + 30 cm].
Unreinforced foundation should have angular spread of load from thesupported column with the following criteria
2V:1H for masonry foundation
3V:2H for lime concrete
1V:1H for cement concrete foundation
The bottom most layer should have a thickness of atleast 150 mm.
Foundation Analysis and Design: Dr. Amit Prashant
Combined FootingsCombined Footings
Combined footing is preferred when
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The columns are spaced too closely that if isolated footing isprovided the soil beneath may have a part of common influencezone.
The bearing capacity of soil is such that isolated footing designwill require extent of the column foundation to go beyond theproperty line.
Types of combined footings Rectangular combined footing
Trapezoidal combined footing
Strap beam combined footing
Foundation Analysis and Design: Dr. Amit Prashant
Rectangular Combined FootingRectangular Combined Footing
If two or more columns are carryingalmost equal loads, rectangular combined footing is provided
Proportioning of foundation willinvolve the following steps
Area of foundation 1 2Q Q A
+=
Q1 Q2
Q1+Q2
x
L1 S L2
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Location of the resultant force
For uniform distribution of pressure under the foundation, theresultant load should pass through the center of foundation base.
Length of foundation,
Offset on the other side,
The width of foundation,
a net q −
2
1 2
Q S x
Q Q=
+
( )12 L L S= +
2 1 0 L L S L= − − >
B A L=
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Foundation Analysis and Design: Dr. Amit Prashant
Trapezoidal Combined FootingTrapezoidal Combined Footing
If one of the columns is carrying muchlarger load than the other one,trapezoidal combined footing is provided
Proportioning of foundation will involvethe following steps if L, and L1 are known
Area of foundation1 2
Q Q A
+=
Q1 Q2
Q1+Q2
x
L1 S L2
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Location of the resultant force
For uniform distribution of pressure under the foundation, theresultant load should pass through the center of foundation base.This gives the relationship,
Area of the footing,
a net −
2
1 2
Q S x
Q Q=
+
1 21
1 2
2
3
B B L x L
B B
⎛ ⎞++ = ⎜ ⎟
+⎝ ⎠1 2
2
B B L A
+=
Solution of these
two equations
gives B1 and B2
B1 B2
Foundation Analysis and Design: Dr. Amit Prashant
Strap Combined FootingStrap Combined Footing
Strap footing is used toconnect an eccentrically loadedcolumn footing to an interior column so that the moment canbe transferred through thebeam and have uniform stressdistribution beneath both thefoundations.
Q1 Q2
M2
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This type of footing is preferredover the rectangular or trapezoidal footing if distancebetween the columns isrelatively large.
Some design considerations:
Strap must be rigid: Istrap/Ifooting > 2.
Footings should be proportioned to have approximately equal soil
pressure in order to avoid differential settlement
Strap beam should not have contact with soil to avoid soil reaction to it.
Foundation Analysis and Design: Dr. Amit Prashant
Example: Strap FootingExample: Strap Footing
Q1 Q2
S
21
R1 R2B1
B2
L
dc1
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Foundation Analysis and Design: Dr. Amit Prashant
Raft or Mat FoundationsRaft or Mat Foundations
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Structures like chimneys, silos, cooling towers, buildings
with basements where continuous water proofing isneeded
For foundations where differential settlement can be amajor concern
For soft soils strata or site with pockets of weak soil
In situations where individual footings may touch or overlap each other.
Foundation Analysis and Design: Dr. Amit Prashant
Types of Raft FoundationTypes of Raft Foundation
Plane Slab Rafts: For fairly small and uniform spacing of columns
and when the supporting soil is not too compressible.
Beam and Slab: For large column spacing and unequal column
loads.
Slab with Column Pedestals: For columns with heav loads which
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may require large shear strength or flexural strength of slab.
Cellular Rafts: For compensated foundations to avoid differential
settlements in weak soils.
Piled Rafts: For heavy structures on soft soils in order to share theloads with piles.
Strip Rafts or Grid Rafts: For economical design where a completeslab may be avoided.
Foundation Analysis and Design: Dr. Amit Prashant
General Considerations for Raft FoundationGeneral Considerations for Raft Foundation
The depth of foundation shall not be less than 1.0 m.
Punching shear failure for raft foundation on cohesionlesssoils is not an option so it shall not be considered for analysis. The design is mostly governed by settlementcriteria.
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,deep seated failure shall be analyzed. The effect of longterm settlement due to consolidation shall also beconsidered.
The uplift due to sub-soil water shall be considered indesign. The construction below water table shall bechecked for floatation
Foundations subjected to heavy vibratory loading should
preferably be isolated
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Foundation Analysis and Design: Dr. Amit Prashant
Rigidity of SoilRigidity of Soil--Structure SystemStructure System
Performance of raft depends on the relative rigidity of itsthree components Super structure
Raft
Soil
Distribution of contact pressures depends on the relative
25
r g y o e oun a on w respec o so
It is important that the rigidity of superstructure also matcheswith the rigidity of foundation Rigid Superstructure with Rigid Foundation: Does not allow
differential settlement so it is good
Rigid Superstructure with Flexible Foundation: Large deformations inthe foundation which is not suitable for superstructure
Flexible Superstructure with Rigid Foundation: It may acceptable butnot necessary
Flexible Superstructure with Flexible Foundation: This is also good
Foundation Analysis and Design: Dr. Amit Prashant
Flexural Rigidity of Structure,Flexural Rigidity of Structure, EI EI
( )
( )
22
22 2
' '1
2 ' ' '
u ll lb
b u f
I I b E I b EI E I
H I I I l
⎡ ⎤+⎢ ⎥= + +
+ +⎢ ⎥⎣ ⎦∑
Terms on next slideTerms on next slide
26
The summation is to bedone over all the floors,including foundationbeam of raft.
For top layer Iu'becomes zero.
For foundation beamsIf ' replaces Ib‘ and Il
becomes zero
Foundation Analysis and Design: Dr. Amit Prashant
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Foundation Analysis and Design: Dr. Amit Prashant
Relative Stiffness of Structure and Foundation SoilRelative Stiffness of Structure and Foundation Soil
Relative Stiffness Factor:
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For K > 0.5, the foundation may be considered as rigid with the ratio of
differential to total settlement (δ) being equal to zero.
For K = 0, ratio δ may be taken as 0.1, and for K < 0.5, it can be taken as
0.35 for square and 0.5 for long mat foundations.
Foundation Analysis and Design: Dr. Amit Prashant
Characteristic Length and Critical Column SpacingCharacteristic Length and Critical Column Spacing
The characteristic coefficient λ, as used in classical solution of beams
on elastic foundation, can be obtained as
1Characteristic Length Parameter,
e L
λ =
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Foundation Analysis and Design: Dr. Amit Prashant
Characteristic Length and Critical Column SpacingCharacteristic Length and Critical Column Spacing
( )
1.75
3 2
e
e
L L
L Lπ
< × ⇒
> × ⇒
Foundations may be treated as short beam, hence rigid
Foundations may be treated as long beam, hence flexible
Rigidity of foundation can be defined by the spacing
between columns, L
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( )1.75 3 2e e L L Lπ × < < × ⇒ Foundations may be treated as finite beam
with intermediate rigidity
0.8
3
0.8 3
e
e
e e
L L
L L
L L L
< × ⇒
> × ⇒
× < > × ⇒
Hetenyi’s (1946) recommendations
Rigid Foundation
Flexible Foundation
Intermediate Flexibility of Foundation
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Foundation Analysis and Design: Dr. Amit Prashant
Determination of Modulus of Elasticity from PlateDetermination of Modulus of Elasticity from PlateLoad TestLoad Test
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s f E qB I
s
ν −=
intensity of contact pressureq = Posson's ratioν =
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ate w t
settlements
=
=
2
2
f f p
sf sP
P f
B B B E E
B B
⎛ ⎞+= ⎜ ⎟⎜ ⎟
⎝ ⎠
Elastic Modulus of granular soil belowfoundation considering scale effects:
n uence actor f =
Foundation Analysis and Design: Dr. Amit Prashant
Modulus ofModulus of SubgradeSubgrade Reaction from Plate Load TestReaction from Plate Load Test
C o h e s i o n l e s s
S o i l
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C o h e s i v e
S o i l
Foundation Analysis and Design: Dr. Amit Prashant
Raft Foundation: Rigid Beam AnalysisRaft Foundation: Rigid Beam Analysis Assumptions
Foundation is rigid relative to soil and compressible layer is relativelyshallow.
The contact pressure is planar such that centroid of the contactpressure coincides with the line of action of resultant force.
The above conditions are satisfied if
Relative stiffness factor > 0.5
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Spacing between the columns isless than 1.75xLe
Types of Analysis
Flat Slab Analysis: Flat plate with regular layout of vertical loads can be analyzedassuming beam with the width of distance between mid-span width
when ground settlement due tostructural loads are not large. Strip
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Foundation Analysis and Design: Dr. Amit Prashant
Types of Analysis (Continued…)
Equivalent Frame Analysis: If adjacent loads or column
spacing exceed 20% of their higher value, the perpendicular beams as defined above may betreated as part of a frame structure
Raft Foundation: Rigid Beam AnalysisRaft Foundation: Rigid Beam Analysis
34
and the analysis is performed for this equivalent frame.
Beam and Slab Analysis: For the raft with addedperpendicular beams to increaserigidity of foundation, an elementalanalysis may be performed bycutting it into pieces as beams andslabs. Slab is designed as two wayslab and beam as T-beam.
Foundation Analysis and Design: Dr. Amit Prashant
Types of Analysis (Continued…)
Deep Cellular Raft: This involves raft slab with beams andstructural walls. Cross walls act as beam between the columnswith net loading from soil pressure minus the self weight of walland slab section beneath. A simple elemental analysis may beperformed as in the previous case. An arbitrary value of ±wL2/10
Raft Foundation: Rigid Beam AnalysisRaft Foundation: Rigid Beam Analysis
35
for positive and negative moment can give reasonable results.
Foundation Analysis and Design: Dr. Amit Prashant
Pressure distribution:
Raft Foundation: Rigid Beam AnalysisRaft Foundation: Rigid Beam Analysis
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Foundation Analysis and Design: Dr. Amit Prashant
Modification Factor for ColumnLoads and Soil Reaction :
The total soil reaction =
Q1 Q2 Q3 Q4
ey
ex
Q
1. .
avq B L
Total load from columns =
Raft Foundation:Raft Foundation:RigidRigid Beam AnalysisBeam Analysis
37
Q1 Q2 Q3 Q4
avq
1 B
L
1 2 3 4Q Q Q Q+ + +
Total load from columns is slightly
different than the total soil reactiondue to the fact that no consideration
has been given to the shear
between adjacent strips. Thereforecolumn loads and soil reaction need
to adjusted.
Foundation Analysis and Design: Dr. Amit Prashant
Modification Factor for Column Loads and Soil Reaction :
Average Load =( )1 1 2 3 4. .
2
avq B L Q Q Q Q+ + + +
Average Load⎛ ⎞
Raft Foundation: Rigid Beam AnalysisRaft Foundation: Rigid Beam Analysis
38
modified
1. .av av
avq B L⎝ ⎠
Column load modification factor
1 2 3 4
Average LoadF
Q Q Q Q=
+ + +
Modified Column Loads: ( )i imQ FQ=
Foundation Analysis and Design: Dr. Amit Prashant
Raft Foundation: Rigid BeamRaft Foundation: Rigid Beam AnalysisAnalysis -- ExampleExample
1 200Q kN =
5 370Q kN =
4 230Q kN =
8 420Q kN =
2 370Q kN = 3 420Q kN =
6 740Q kN = 7 900Q kN =
1.0m
6.0m
xe
39
13 200Q kN = 16 200Q kN =
9 370Q kN = 12 370Q kN =
14 370Q kN = 15 370Q kN =
10 740Q kN = 11 740Q kN =
6.0m
6.0m
1.0m
1.0m 1.0m6.0m6.0m6.0m
y