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ENCE4610

Foundation

Analysis

and

DesignShallow FoundationsTotal and Differential Settlement

Schmertmanns Method

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Strength

Requirements Geotechnical Strength

Requirements Design to prevent failure bysoil shear failure

Geotechnical strength forshear failure is referred to asthe bearing capacity of the

soil Analysis usually performed by

ASD analysis; LRFD becomingmore common

Structural StrengthRequirements

Design to avoid structuralfailure of foundationcomponents

Similar to other structuralanalyses

Most common strengthrequirement: avoid bearingcapacity failure


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Serviceability

Considerations Most common issue

in serviceability:settlement



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TypesofSettlement Definitions of

Settlemento Absolute settlement,

usually associatedwith uniform/totalsettlement

o Angular distortionsettlement, usuallyassociated withdifferential settlement(ratio of settlement todistance betweenfoundations andstructures)

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FactorstoDetermine

Acceptable

Settlement Connections with existing

structures Utility Lines

Total settlement ofpermanent facilities canharm or sever connectionsto outside utilities such as

water, natural gas, andsewer lines. Water and sewer lines may

leak contributing tolocalised wetting of the soilprofile and aggravating

differential displacement. Leaking gas from breaks

caused by settlement canlead to explosions.

Surface Drainage

Access

Aesthetics Material of structure (steel,

concrete)

Usage Requirements

Settlement ofbridges/overpasses vs.settlement of embankments,the bump in the bridge

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Typical

Values

of

Acceptable

SettlementThisimagecannotcurrently bedisplayed.



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Example

of

Settlement

Calculations Given Steel framed office building, 20' column spacing Supported on spread footings founded on clayey

soil

Find Allowable total settlement

Allowable differential settlement

Solution Typical total settlement specification = 4 (Frames

structure)

Use = 1/500 (Steel and concrete frame); du

=(1/500)(20') = 0.04' = 0.5

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Schmertmanns

Method:

Procedure

and

Example Given

o 6 x 24 footing, shown below

o 2 ksf applied bearing pressureo Soil Profile and foundation

depth as shown below

Note that N160 arecorrected for both

overburden and hammerefficiency

Findo Settlement in inches at the

end of constructiono Settlement in inches one (1)

year after the end ofconstruction

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Schmertmanns MethodStep1:DrawtheStrainInfluenceDiagram,

Compute

IzbatSurface Strain influence

diagrams for squareand continuousfoundations areshown at the right

Compute Lf/Bf(Equivalent Footing)o Uniform loading, so Lf/Bf

= L/B = 24/6 = 4

o For L/B = 1, Iz|z = 0 = 0.1

o For L/B = 10, Iz|z = 0 = 0.2

o By linear interpolation, forL/B = 4, Iz|z = 0 = 0.133

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Schmertmanns MethodStep

2:

Draw

the

Strain

Influence

Diagram,

Compute

Maximum

Depth

of

Influence

Compute DI

o Uniform loading, so Lf/Bf =

L/B = 24/6 = 4

o For L/B = 1, DI = 2Bfo For L/B = 10, DI = 4Bfo By linear interpolation, for

L/B = 4, DI = 8Bf/3

o For B = 6, DI = (8)(6)/(3) =16

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3:

Draw

the

Strain

Influence

Diagram,

Determine

Depth

of

Peak

Strain

Influence

Factor

Compute DIP

o Uniform loading, so Lf/Bf =

L/B = 24/6 = 4

o For L/B = 1, DIP = Bf/2

o For L/B = 10, DIP = Bfo By linear interpolation, for

L/B = 4, DIP = 2Bf/3

o For B = 6, DIP = (2)(6)/(3) =4

o Alternate: DIP = DI/4

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4:

Draw

the

Strain

Influence

Diagram,

Determine

Peak

Strain

Influence

Factor

Compute IZPo DIP = (2)(6)/(3) = 4o This is 4 below the foundation;

since the foundation is 3below the surface, the depthof the peak strain influence

factor is 3 + 4 = 7 below thesoil surface (important foreffective stress computations)

o IZP=0.5 + 0.1(p/pop)0.5

o Increase in stress at depth of

footing

p = 2 ksf (3)(0.115kcf) = 1.655 ksfo pop = (3)(0.115) + (3)(0.125) +

(1)(120) = 0.840 ksfo IZP = 0.5 + 0.1(1.665/0.840)

0.5 =0.64

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SchmertmansMethod

Step

5:

Draw

the

Strain

Influence

Diagram

Helpful Guidelines:o The depth of the peak value

of the strain influence isfixed. To aid in thecomputation, develop thelayering such that one ofthe layer boundaries occursat this depth even though itrequires that an actual soil

layer be sub-divided.o Limit the top layer as well as

the layer immediately belowthe peak value of influencefactor, Izp, to 2/3Bf or less toadequately represent thevariation of the influencefactor within DIP.

o Limit maximum layerthickness to 10 ft (3 m) orless.

o Match the layer boundarywith the subsurface profile

layering.

Layer Boundaries are SOLID

Layer Mid-Points are DASHED

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6:

Determine

the

Values

of

Elastic

Modulus

Estimate from SPT Value

o Layer 1: Sandy Silt, Es =4(N160) = (4)(25) = 100 tsf =200 ksf

o Layer 2: Coarse Sand, Es =10(N160) = (10)(30) = 300 tsf =

600 ksfo Layer 3: Coarse Sand, Es =

10(N160) = (10)(30) = 300 tsf =600 ksf

o Layer 4: Sandy Gravel, Es =12(N1

60) = (12)(68) = 816 tsf =

1632 ksf

Values computed in thisfashion must be correctedby a factor X

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6:

Determine

the

Values

of

Elastic

Modulus

Modulus of Elasticity

Correction Factor Xo X = 1.25 for Lf/Bf = 1

o X = 1.75 for Lf/Bf > 10

o

By linear interpolation, forLf/Bf = 4, X = 1.42

Corrected Values of Eso 1: (100)(1.42) = 142 tsf

o 2: (300)(1.42) = 426 tsfo 3: (300)(1.42) = 426 tsf

o 4: (816)(1.42) = 1159 tsf

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Schmertmanns MethodStep7:ComputeBasicTotalSettlement

Basic Formula for

Schmertmanns Methodo We first concentrate on

computing thesummation, which willrepresent the settlement

divided by the appliedbearing pressure

=

= =

s

zci

n

iii

XE

IHH

HpCCS1

21

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Schmertmanns MethodStep8:DetermineEmbedmentandCreepFactors

896.01655

115'35.01

5.01

1

1

=

=

=

pcf

pcfC

p

pC o

Embedment Factor Creep Factor

2.11.0

1log2.01

year,oneofendAt

1on,constructiofendFor

1.0log2.01

102

2

102

=+=

=

+=

C

C

tC

years

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Schmertmanns Method

Step

9:

Determine

Settlement

at

End

of

ConstructionStep10:DetermineSettlementatEndofOneYear

End of Construction

End of One Year

"130.0

)ksf2tsf1)(in/tsf1760.0)(ksf1.655)(1)(896.0(

1

21

=

=

= =

i

i

n

i

ii

S

/S

HpCCS

"156.0

)ksf2tsf1)(in/tsf1760.0)(ksf1.655)(2.1)(896.0(

1

21

=

=

= =

i

i

n

i

ii

S

/S

HpCCS

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Chartfor

Interpolated

Values

S ttl t B i

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Settlement

vs.

Bearing

Capacity

(Shear

Failure)

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Bearing

Capacity

ChartsExample

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C t B i

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Comments

on

Bearing

Capacity

Chart

Example

Li h l L d d F i d

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LightlyLoadedFootingsand

Presumptive

Bearing

Pressures Lightly loaded footings are

those which meet thefollowing criteria:o Square, circular, or rectangular

footings subjected to vertical loadsless than 200 kN (45 kips)

o Continuous footings subjected tovertical loads less than 60 kN/m (4kips/ft)

Include typical one andtwo-story wood framebuildings and other similarstructures

A conservative approach;

normally easier in thesecases to design aconservative structure thanto perform the analysis

The use of presumptive bearingcapacities for shallow foundationsbearing in soils is not recommended forfinal design of shallow foundations for

transportation structures, especiallybridges. Guesses about the geologyand nature of a site and theapplication of a presumptive valuefrom generalizations in codes or in thetechnical literature are not a substitutefor an adequate site-specific

subsurface investigation andlaboratory testing program. As anexception, presumptive bearing valuesare sometimes used for the preliminaryevaluation of shallow foundationfeasibility and estimation of footingdimensions for preliminary

constructability or cost evaluations.

Presumpti e Bearing Pressures

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PresumptiveBearingPressures

SandsAllowable Bearing Pressure

Tons Per sq ft

Type of Bearing Material Consistency In Place Range RecommendedValue for Use

Well graded mixture of fine and coarse-grained soil: glacial till, hardpan, boulder clay(GW-GC, GC, SC)

Very compact 8 to 12 10.0

Gravel, gravel-sand mixtures, boulder gravelmixtures (SW, SP, SW, SP)


Medium to compact 4 to 7 5.0

Loose 2 to 6 3.0

Coarse to medium sand, sand with littlegravel (SW, SP)



Loose 1 to 3 1.5

Fine to medium sand, silty or clayey mediumto coarse sand (SW, SM, SC)



Loose 1 to 2 1.5

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Clays

and

Silts

Allowable Bearing Pressure TonsPer sq ft

Type of Bearing Material Consistency InPlace

Range RecommendedValue for Use

Homogeneous inorganic clay,

sandy or silty clay (CL, CH)

Very stiff to hard 3 to 6 4.0

Medium to stiff 1 to 3 2.0

Soft .5 to 1 0.5

Inorganic silt, sandy or clayey silt,varved silt-clay-fine Sand

Very stiff to hard 2 to 4 3.0

Medium to stiff 1 to 3 1.5

Soft .5 to 1 0.5

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Noteso Compacted fill, placed with control of moisture, density, and lift

thickness, has allowable bearing pressure of equivalent natural soil.

o Allowable bearing pressure on compressible fine grained soils isgenerally limited by considerations of overall settlement of structure.

o Allowable bearing pressure on organic soils or uncompacted fills isdetermined by investigation of individual case.

o If tabulated recommended value for rock exceeds unconfinedcompressive strength of intact specimen, allowable pressures equals

unconfined compressive strength.

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