Lesson 08-Chapter 8 Shallow Foundations.pdf

8/9/2019 Lesson 08-Chapter 8 Shallow Foundations.pdf

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SOILS AND FOUNDATIONS SOILS AND FOUNDATIONS

Testing

Experience

Theory

Lesson 08 Lesson 08 Chapter 8Chapter 8 – – Shallow FoundationsShallow Foundations


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TopicsTopics

gg Topic 1 (Section 8.0, 8.1, 8.2, 8.3, 8.4)Topic 1 (Section 8.0, 8.1, 8.2, 8.3, 8.4)

-- General and Bearing Capacity General and Bearing Capacity gg Topic 2 (Section 8.5, 8.6, 8.7, 8.8, 8.9)Topic 2 (Section 8.5, 8.6, 8.7, 8.8, 8.9)

-- Settlement Settlement -- Spread footings on embankments,Spread footings on embankments, IGMsIGMs , rocks, rocks-- Effect of deformations on bridge structuresEffect of deformations on bridge structures

gg Topic 3 (Section 8.10)Topic 3 (Section 8.10)-- ConstructionConstruction


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Learning OutcomesLearning Outcomes

gg At the end of this session, the participant will At the end of this session, the participant will

be able to:be able to:-- Identify different types of shallow foundationsIdentify different types of shallow foundations-- Recall foundation design procedureRecall foundation design procedure-- Contrast factors that influence bearing capacityContrast factors that influence bearing capacity

in sand and clay in sand and clay -- Compute bearing capacity in sand and clay Compute bearing capacity in sand and clay -- Describe allowable bearing pressure for rockDescribe allowable bearing pressure for rock

foundationsfoundations


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Stresses Imposed by StructuresStresses Imposed by Structures

gg

Abutment and piers may have shallow or deep Abutment and piers may have shallow or deepfoundationsfoundations


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General Approach to FoundationGeneral Approach to Foundation

DesignDesigngg Duty of Foundation Designer Duty of Foundation Designer

-- Establish the most economical design that safelyEstablish the most economical design that safelyconforms to prescribed structural criteria andconforms to prescribed structural criteria and properly accounts for the intended function of properly accounts for the intended function of

the structurethe structuregg Rational method of designRational method of design

-- Evaluate various foundation typesEvaluate various foundation types


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Recommended Foundation DesignRecommended Foundation Design

Approach Approachgg Step 1:Step 1:

Determine:Determine:-- Direction, type and magnitude of foundationDirection, type and magnitude of foundationloadsloads

-- Tolerable deformationsTolerable deformations-- Special constraintsSpecial constraints•• UnderclearanceUnderclearance requirementsrequirements

••

Structure type, span lengthsStructure type, span lengths

•• Time constraints on constructionTime constraints on construction•• Extreme event loading Extreme event loading •• Construction load requirementsConstruction load requirements


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Recommended Foundation DesignRecommended Foundation Design

Approach Approachgg Step 3:Step 3:

Consider alternate foundation typesConsider alternate foundation types


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Foundation AlternativesFoundation Alternatives

gg Shallow FoundationsShallow Foundationsgg Deep FoundationsDeep Foundations

-- Piles, shaftsPiles, shafts


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Foundation Cost Foundation Cost

gg Express foundation capacity in terms of $ Express foundation capacity in terms of $

gg TOTAL cost of foundation system divided by theTOTAL cost of foundation system divided by theload supported by the foundation in tonsload supported by the foundation in tons

gg TOTAL cost of a foundation must include ALLTOTAL cost of a foundation must include ALL

costs associated with the foundationscosts associated with the foundations-- Need for excavation support system, pile caps, etc.Need for excavation support system, pile caps, etc.-- Environmental restrictionsEnvironmental restrictions

-- All other factors as applicable All other factors as applicable


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Foundation Cost Foundation Cost

gg If estimated costs of alternative foundationIf estimated costs of alternative foundation

systems during design are within 15%, thesystems during design are within 15%, thealternate foundation designs should bealternate foundation designs should beconsidered for inclusion in contractconsidered for inclusion in contractdocumentsdocuments


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Types of Shallow FoundationsTypes of Shallow Foundations

gg Isolated Spread FootingsIsolated Spread Footings

-- Length (L) to width (B) ratio, L/B < 10 Length (L) to width (B) ratio, L/B < 10


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Types of Shallow FoundationsTypes of Shallow Foundations

gg Combined Strip Spread FootingsCombined Strip Spread Footings

-- Length (L) to width (B) ratio, L/BLength (L) to width (B) ratio, L/B ≥ ≥ 10 10


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Shallow Foundations for BridgeShallow Foundations for Bridge

Abutments Abutments


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Shallow Foundations for RetainingShallow Foundations for Retaining

WallsWalls


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Mat FoundationsMat Foundations

REINFORCED CONCRETE MAT


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Spread Footing Design ProcedureSpread Footing Design Procedure

gg Geotechnical design of spread footing is aGeotechnical design of spread footing is a

two part processtwo part process

gg First Part:First Part:-- Establish an allowable stress to prevent shearEstablish an allowable stress to prevent shearfailure in soil failure in soil

gg Second Part:Second Part:

-- Estimate the settlement under the applied stressEstimate the settlement under the applied stress


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Allowable Bearing Capacity Allowable Bearing Capacity

gg Allowable bearing capacity is lesser of: Allowable bearing capacity is lesser of:

Applied stress that will result in shear failure Applied stress that will result in shear failuredivided by FS divided by FS

-- Ultimate limit criterionUltimate limit criterion

OR OR

Applied stress that results in a specified amount of Applied stress that results in a specified amount ofsettlement of the structuresettlement of the structure-- Serviceability criterionServiceability criterion


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Bearing Capacity Chart Bearing Capacity Chart

Effective Footing Width, ft (m)

A l l o w a b

l e B e a r i n g

C a p a c

i t y , k

s f ( k P a )

Ultimate Bearing Capacity, q ult

Contours of AllowableBearing Capacity for agiven settlement

S1S2S3

Allowable Bearing Capacity,

FS

qq ultall =


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Design Process Flow Chart Design Process Flow Chart

gg Figure 8 Figure 8 - - 10 10


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Bearing Capacity Bearing Capacity

gg Bearing capacity failure occurs when theBearing capacity failure occurs when the

shear strength of foundation soil is exceeded shear strength of foundation soil is exceeded gg Similar to slope stability failureSimilar to slope stability failure

II

I III

DC

A EB

Q

L = ∞ q


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BearingBearing

CapacityCapacityFailureFailure

MechanismsMechanismsgg General shear General shear gg Local shear Local shear gg Punching shear Punching shear

(a) GENERAL SHEAR

(b) LOCAL SHEAR

(c) PUNCHING SHEAR

LOAD

S E T T L E M E N T

LOAD

S E T T L E M E N T

LOAD

S E T T L E M E N T

SURFACE TEST

TEST GREADEPTH


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Footing Dimension Terminology Footing Dimension Terminology

gg BB f f = Width of footing = Width of footing

-- Least lateral dimensionLeast lateral dimension

gg

LLf f = Length of footing = Length of footing

gg

DDf f = Depth of= Depth ofembedment of footing embedment of footing L

f

D

Bf


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Basic Bearing Capacity EquationBasic Bearing Capacity Equation

gg Equation 8 Equation 8 - - 8 8

cc = cohesion= cohesion

qq = surcharge at footing base= surcharge at footing baseNNcc ,, NNqq , N, N γ = Bearing capacity factors= Bearing capacity factors

γ = unit weight of foundation soil = unit weight of foundation soil

))(Nf )(B(0.5)q(Nq)c(Ncultq γγ++=


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Assumptions of Basic Bearing Assumptions of Basic Bearing

Capacity Equation (Section 8.4.3)Capacity Equation (Section 8.4.3)gg Strip (continuous) footing Strip (continuous) footing gg Rigid footing Rigid footing gg General shear General shear gg Concentric loading (i.e., loading through theConcentric loading (i.e., loading through the

centroid centroid of the footing)of the footing)gg

Footing bearing on level surface ofFooting bearing on level surface ofhomogeneous soil homogeneous soil gg No impact of groundwater No impact of groundwater


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Bearing Capacity FactorsBearing Capacity Factors

1

10

100

1000

0 5 10 15 20 25 30 35 40 45Friction Angle, degrees

B e a r i n g

C a p a c i

t y F a c t o r s

Nq

N c

Nγ

Figure 8-15Table 8-1


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Example 8 Example 8 - - 11

γsub = 63 pcf

d = D = 5 ′ γT = 125 pcf

B = 6 ′

= 20 ° c = 500 psf


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Example 8 Example 8 - - 11

gg SolutionSolution


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Effect of Variation of Soil PropertiesEffect of Variation of Soil Properties

and Footing Dimensions (Table 8 and Footing Dimensions (Table 8 - - 2)2)CohesiveSoil

CohesionlessSoil

φ= 0c = 1000 psf

qult (psf)

φ= 30 oc = 0

qult (psf)

A. Initial situation : γ = 120 pcf, D f = 0',Bf = 5', deep water table

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B. Effect of embedment : D f = 5', γ=120 pcf, B f = 5', deep water table

C. Effect of width : B f = 10' γ = 120 pcf,D f = 0', deep water table

D. Effect of water table at surface :γ = 57.6 pcf, D f = 0', B f = 5'

Properties and Dimensionsγ = γa = effective unit weight

γ b = submerged unit weightD f = embedment depthBf = footing width (assume strip footing)


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Effect of Variation of Soil PropertiesEffect of Variation of Soil Properties

and Footing Dimensions (Table 8 and Footing Dimensions (Table 8 - - 2)2)CohesiveSoil

CohesionlessSoil

φ= 0c = 1000 psf

qult (psf)

φ= 30 oc = 0

qult (psf)

A. Initial situation : γ = 120 pcf, D f = 0',Bf = 5', deep water table

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B. Effect of embedment : D f = 5', γ=120 pcf, B f = 5', deep water table

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C. Effect of width : B f = 10' γ = 120 pcf,D f = 0', deep water table

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D. Effect of water table at surface :γ = 57.6 pcf, D f = 0', B f = 5'

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Properties and Dimensionsγ = γa = effective unit weight

γ b = submerged unit weightD f = embedment depthBf = footing width (assume strip footing)


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Student Exercise 5 Student Exercise 5

gg Find the allowable bearing capacity assumingFind the allowable bearing capacity assuming

a FS=3 for the condition shown below for aa FS=3 for the condition shown below for a10’x50’ footing with rough base10’x50’ footing with rough base

30 ′

4 ′

10 ′

Final Grade

Sandγ = 115 pcf φ = 35 °C = 0


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Bearing Capacity Correction FactorsBearing Capacity Correction Factors

gg Footing shapeFooting shape

-- Adjusted for eccentricity Adjusted for eccentricity gg Depth of water tableDepth of water tablegg

Embedment depthEmbedment depthgg Sloping ground surfaceSloping ground surfacegg

Inclined baseInclined basegg Inclined loading Inclined loading


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gg SolutionSolution


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Estimation ofEstimation of for Bearing Capacityfor Bearing Capacity

Factors (Table 8 Factors (Table 8 - - 3)3)Description

Very

LooseLoose Medium Dense

Very

DenseCorrected N-value

N160

0 4 10 30 50

Friction angleφ Degrees

25 – 30

27 – 32

30 – 3535 –

4038 –

43

Moist unit weight

(γ) pcf

70 –

100

90 –

115

110 –

130

120 –

140

130 –

150


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Shape Correction FactorsShape Correction Factors

gg Basic equation assumes strip footing whichBasic equation assumes strip footing which

meansmeans LL f f /B /B f f ≥≥ 1010

gg

For footings withFor footings with LL f f /B /B f f


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Effective Footing DimensionsEffective Footing Dimensions

B ′ f = B f – 2e B ; L′

f = L f – 2e L ; A′ = B ′ f L

′

f


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Pressure DistributionsPressure Distributions

Structural designStructural design Sizing the footing Sizing the footing


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Shape Correction FactorsShape Correction Factors

FactorFriction

AngleCohesionTerm (s c)

UnitWeight

Term (s γ)

SurchargeTerm (s q)

φ= 0 1.0 1.0

φ> 0

ShapeFactors,sc, sγ, sq

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ φ+ tanL

B1

f

f

⎟⎟

⎠

⎞⎜⎜

⎝

⎛ +f

f

L5

B1

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +c

q

f

f

N

N

L

B1

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ −f

f

L

B4.01

gg In routine foundation design, use of effectiveIn routine foundation design, use of effective

dimensions in shape factors is not practical dimensions in shape factors is not practical


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Embedment DepthEmbedment Depthgg To account for theTo account for the

shearing resistance inshearing resistance in

the soil above thethe soil above thefooting basefooting base

FrictionAngle,

(degrees) D f /B f d q

32

12

48

1.201.30

1.351.40

37

1

248

1.20

1.251.301.35

421248

1.151.201.251.30

See Note

Note: The depth correctionfactor should be used onlywhen the soils above thefooting bearing elevation areas competent as the soils

beneath the footing level;

otherwise, the depth correctionfactor should be taken as 1.0.


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Sloping Ground SurfaceSloping Ground Surface

gg Modify the bearing capacity equation asModify the bearing capacity equation as

follows:follows:

gg

Useful in designing footings constructedUseful in designing footings constructedwithin bridge approach fillswithin bridge approach fills

))(N)(B(0.5)(Ncqqf cqult

γγ+=


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Footing in SlopeFooting in Slope


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Footing Near SlopeFooting Near Slope


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Inclined BaseInclined Basegg Footings with inclined base should beFootings with inclined base should be

avoided oravoided or limted limted to angles less than 8 to angles less than 8 - - 10 10 º º

gg Sliding may be an issue for inclined basesSliding may be an issue for inclined bases⎟ ⎠ ⎞⎜⎝ ⎛ α− 3.1471 ⎟⎟ ⎠ ⎞⎜⎜⎝ ⎛ φ−− tan N b1

bc

qq

CohesionTerm (c)

Unit WeightTerm ( γ )

SurchargeTerm (q)

bc bγ bqφ= 0 1.0 1.0

φ> 0 (1-0.017 α tanφ)2 (1-0.017 α tanφ)2

φ= friction angle, degrees;α = footing inclination from horizontal, upward +, degrees

Base

InclinationFactors, bc, b γ, bq

Factor FrictionAngle


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Inclined Loading Inclined Loading

gg If shear (horizontal) component is checkedIf shear (horizontal) component is checked

for sliding resistance, the inclinationfor sliding resistance, the inclinationcorrection factor is omitted correction factor is omitted gg

Use effective footing dimensions inUse effective footing dimensions inevaluation of the vertical component of theevaluation of the vertical component of theload load


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Comments on Use of BearingComments on Use of Bearing

Capacity Correction FactorsCapacity Correction Factorsgg For settlement For settlement - - controlled allowable bearingcontrolled allowable bearing

capacity, the effect application of correctioncapacity, the effect application of correctionfactors may be negligiblefactors may be negligible

gg Application of correction factors is Application of correction factors issecondary to the adequate assessment ofsecondary to the adequate assessment ofthe shear strength characteristics of thethe shear strength characteristics of thefoundation soil through correctly performedfoundation soil through correctly performed

subsurface explorationsubsurface exploration


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Local or Punching Shear Local or Punching Shear

c* = 0.67cc* = 0.67c

*=tan*=tan --11(0.67tan(0.67tan ))

gg Loose sandsLoose sandsgg Sensitive claysSensitive clays

gg CollapsibleCollapsiblesoilssoils

gg

Brittle claysBrittle clays


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Bearing Capacity Factors of Safety Bearing Capacity Factors of Safety

gg q q all all = allowable bearing capacity = allowable bearing capacity gg

q q ult ult = ultimate bearing capacity = ultimate bearing capacity gg Typical FS = 2.5 to 3.5 Typical FS = 2.5 to 3.5 gg FS is a function of FS is a function of

-- Confidence in shear strength parameter, c andConfidence in shear strength parameter, c and φ-- Importance of structureImportance of structure-- Consequences of failureConsequences of failure

FS

qq ultall =


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Overstress AllowancesOverstress Allowances

gg For short For short - - duration infrequentlyduration infrequently occuring occuring

loads, an overstress of 25 to 50 % may beloads, an overstress of 25 to 50 % may beallowed for allowable bearing capacity allowed for allowable bearing capacity


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Practical Aspects of BearingPractical Aspects of Bearing

Capacity Capacity


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be able to:be able to:-- Identify different types of shallow foundationsIdentify different types of shallow foundations-- Recall foundation design procedureRecall foundation design procedure-- Contrast factors that influence bearing capacityContrast factors that influence bearing capacity

in sand and clay in sand and clay -- Compute bearing capacity in sand and clay Compute bearing capacity in sand and clay -- Describe allowable bearing pressure for rockDescribe allowable bearing pressure for rock

foundationsfoundations


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Any Questions? Any Questions?

T H E R OA D T O

U N D E R S T A N D IN G

S O IL S

A N D

F O U N D A T IO N S


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Shallow FoundationsShallow Foundations

Lesson 08Lesson 08 - - Topic 2 Topic 2 Settlement, footings on embankments,Settlement, footings on embankments, IGMsIGMs ,,

rocks, effect of deformations on bridge structuresrocks, effect of deformations on bridge structuresSection 8.5 to 8.9Section 8.5 to 8.9

L i O


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be able to:be able to:-- Calculate immediate settlements in granularCalculate immediate settlements in granularsoilssoils

-- Calculate consolidation settlements in saturatedCalculate consolidation settlements in saturatedfinefine - - grained soilsgrained soils

-- Describe tolerances and consequences ofDescribe tolerances and consequences ofdeformations on bridge structuresdeformations on bridge structures

l f dS l f S d F i


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Settlement of Spread FootingsSettlement of Spread Footings

gg Immediate (short Immediate (short - - term)term)gg Consolidation (long Consolidation (long - - term)term)

d lI di S l


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Immediate Settlement Immediate Settlement

gg Hough’s method Hough’s method

-- Conservative by a factor of 2 (FHWA, 1987)Conservative by a factor of 2 (FHWA, 1987)gg Schmertmann’sSchmertmann’s method method -- More rational More rational -- Based on nonlinear theory of elasticity andBased on nonlinear theory of elasticity and

measurementsmeasurements

ChCh


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ChartsCharts

Figure 2 Figure 2 - - 1111gg DDss = 4B to 6B= 4B to 6B

for continuousfor continuousfootings wherefootings whereLLf f /B /B f f ≥≥ 1010

gg DDss = 1.5B to 2B= 1.5B to 2Bfor squarefor squarefootings wherefootings where

LLf f /B /B f f == 11


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S hS h M h dM h d


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SchmertmannSchmertmann Method Method

gg I I z z Strain Influence Factor Strain Influence Factor gg

E E Elastic Modulus, Table 5 Elastic Modulus, Table 5 - - 20 20 gg X X Modification factor for E Modification factor for E gg C C 11 Correction factor for strain relief Correction factor for strain relief gg

C C 2 2 Correction factor for creep deformationCorrection factor for creep deformation

∑=

ΔΔ=n

1i

i21i H pCCS ⎟ ⎠ ⎞

⎜⎝ ⎛ =Δ

XEI

HH zci

5.0

p

p5.01C o1 ≥⎟⎟

⎠

⎞⎜⎜

⎝

⎛

Δ−= ( )

⎠

⎞⎜

⎝

⎛ +=1.0

yearstlog2.01C 102


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5.0

op p

p1.05.0zpI

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛ Δ+=

below) b(see

Plane Strain L f /B f ≥ 10

AxisymmetricLf /B f =1

Lf = Length of footing

Bf = least width of footing

o p

op pBf /2 (for axisymmetric case)Bf (for plane strain case)

Bf

p p −=Δ

Depth to Peak StrainInfluence Factor, I z

E l 8E l 8 22


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Example 8 Example 8 - - 2 2 gg Given: 6’x24’ footing on soil profile shownGiven: 6’x24’ footing on soil profile shown

below. Determine settlement at end ofbelow. Determine settlement at end of

construction and 10 years after constructionconstruction and 10 years after construction

Clayey Silt

Sandy Silt

Coarse Sand

Sandy Gravel

γt= 115 pcf; N1

60= 8

γt = 125 pcf; N1 60 = 25

γt = 120 pcf; N1 60 = 30

γt = 128 pcf; N1 60 = 68

3 ft3 ft

5 ft

25 ft

Bf = 6 ft

Ground Surface

D S i I fl DiD St i I fl Di


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Draw Strain Influence DiagramDraw Strain Influence Diagram

gg

Calculate peakCalculate peak

Iz Iz

= 0.64= 0.64Plane Strain L f /B f ≥ 10

AxisymmetricLf /B f =1

0

4

8

12

16

20

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Influence Factor (Iz)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Influence Factor (Iz)

D e p

t h b e l o w

f o o

t i n g B

2B

3B

Strain Influence DiagramStrain Influence Diagram


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Strain Influence DiagramStrain Influence Diagram

Divide into layersDivide into layers0

4

8

12

16

20

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Influence Factor (Iz)

0

4

8

12

16

200.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Influence Factor (Iz)

D e p

t h b e

l o w

f o o

t i n g

( f t )

Layer 1

Layer 2

Layer 3

Layer 4

D t i El ti M d l ED t i El ti M d l E


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Determine Elastic Modulus, E Determine Elastic Modulus, E ss

gg Use Table 5 Use Table 5 - - 20, Page 5 20, Page 5 - - 90 90

gg Calculate X Calculate X - - factor, X = 1.42 factor, X = 1.42

Layer 1: Sandy Silt: E = 4N1 60 tsfLayer 2: Coarse Sand: E = 10N1 60 tsfLayer 3: Coarse Sand: E = 10N1 60 tsfLayer 4: Sandy Gravel: E = 12N1 60 tsf

S t T bl f S ttl tSetup Table for Settlement


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Setup Table for SettlementSetup Table for Settlement

ComputationComputationLayer H c N1 60 E XE Z 1 IZ at Z i

(inches) (tsf) (tsf) (ft) (in/tsf)

1 36 25 100 142 1.5 0.31 0.07592 12 30 300 426 3.5 0.56 0.0152

3 48 30 300 426 6 0.55 0.0599

4 96 68 816 1,159 12 0.22 0.0176

Σ H i= 0.1686

Zi H

XE

IH =

Comp te Correction Factors CCompute Correction Factors C CC


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Compute Correction Factors C Compute Correction Factors C 11 , C , C 2 2

gg At end of construction, t=0.1 year At end of construction, t=0.1 year

gg At t=10 years At t=10 years

0.896 psf 1655

pcf 115ft30.51Δ p p0.51C o1 =⎟⎟ ⎠

⎞⎜⎜

⎝ ⎛ ×−=

⎟⎟

⎠ ⎞

⎜⎜

⎝ ⎛ −=

( )⎟ ⎠ ⎞⎜

⎝ ⎛ +=

1.0yearstlog2.01C 102

0.11.01.0

log2.01C 102 =⎟ ⎠

⎞⎜

⎝

⎛ +=

4.11.010

log2.01C 102 =⎟ ⎠ ⎞⎜⎝ ⎛ +=

D t i I di t S ttl tDetermine Immediate Settlement


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Determine Immediate Settlement Determine Immediate Settlement

gg At end of construction, t = 0.1 year At end of construction, t = 0.1 year

gg At t = 10 years At t = 10 years

( )( )

inches125.0S

tsf in

1686.0

tsf psf

2000

psf 16550.1896.0S

H pCCS

i

i

i21i

=

⎟

⎠

⎞⎜

⎝

⎛

⎟

⎟⎟

⎠

⎞

⎜

⎜⎜

⎝

⎛ =

Δ= ∑

inches175.00.14.1

inches125.0S i =⎟ ⎠ ⎞

⎜⎝ ⎛ =

C lid ti S ttl tConsolidation Settlement


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Consolidation Settlement Consolidation Settlement

gg Same procedures as in Chapter 7 (ApproachSame procedures as in Chapter 7 (Approach

Roadway Deformations)Roadway Deformations)


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Example 8Example 8 33


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Example 8 Example 8 - - 33 p0 = (14 ′ × 130 pcf) + (5 ′ × 65 pcf) = 2,145 psf

psf 208ksf 0.208ft625130kips

ft)15ft(10kips130

Δ p 2 ===+=

⎟⎟ ⎠ ⎞⎜⎜

⎝ ⎛ ++= 0

010

0

c

pΔ p plog

e1CHΔH

⎟⎟ ⎠ ⎞⎜⎜⎝ ⎛ +⎟ ⎠ ⎞⎜⎝ ⎛ +=Δ psf 2145 psf 208 psf 2145log

0.7510.410ftH 10

″=′=Δ 1.109.0H

Student Exercise 6Student Exercise 6


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gg Find footing settlement (immediate +Find footing settlement (immediate +

consolidation) for the following caseconsolidation) for the following case

Sand and Gravel Avg. N ′ = 40

5 ′

25 ′

45 ′Clayey Silt

C C = 0.25 e 0 = 0.90

(Normally Consolidated)

Student Exercise 6Student Exercise 6


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Student Exercise 6 Student Exercise 6 Pressure - psf

–

.

Spread Footings on EmbankmentsSpread Footings on Embankments


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Spread Footings on EmbankmentsSpread Footings on Embankments

gg Section 8.6 Section 8.6

gg If spread footings are placed onIf spread footings are placed onembankments, structural fills that includeembankments, structural fills that includesand and gravel sized particles should besand and gravel sized particles should beused that are compacted properly (minimumused that are compacted properly (minimum95% of standard Proctor energy)95% of standard Proctor energy)

Settlement of Footings on StructuralSettlement of Footings on Structural


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Settlement of Footings on StructuralSettlement of Footings on Structural

FillsFillsgg In absence of other data, use N1In absence of other data, use N1 60 60 = 32 for= 32 for

the structural to estimate settlement ofthe structural to estimate settlement offootings on compacted structural fill footings on compacted structural fill


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Vertical Stress DistributionVertical Stress Distribution


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Vertical Stress DistributionVertical Stress Distribution

Vertical StressVertical StressVertical Stress

0 1 2 3 4 50 1 2 3 40 1 2 3 4 55

D e p

t h

D e p

t h

Bridge Pier Bridge Pier Bridge Pier

EarthEmbankment

EarthEarthEmbankmentEmbankment

h=20’h=20’h=20’ h=40’h=40’h=40’

0

20

40

60

80

100


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Footings onFootings on IGMsIGMs and Rocksand Rocks


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Footings onFootings on IGMsIGMs and Rocksand Rocks

gg Use theory of elasticity Use theory of elasticity

m

2f dv E

)1(B pC ν−Δ=δ

where: δv = vertical settlement at surface

Cd = shape and rigidity factors (Table 8-12)

Δ p = change in stress at top of rock surface due to applied footing loadBf = footing width or diameter ν = Poisson’s ratio (refer to Table 5-23 in Chapter 5)

Em = Young’s modulus of rock mass (see Section 5.12.3 in Chapter 5)

Effect ofEffect of


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Effect ofEffect of

Deformations onDeformations onBridgeBridge

StructuresStructuresgg Section 8.9Section 8.9

Tilt (Rotation)

DifferentialSettlement

DifferentialSettlement

Tolerable Movements for BridgesTolerable Movements for Bridges


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Tolerable Movements for BridgesTolerable Movements for Bridges

(Table 8 (Table 8 - - 13)13)Limiting Angular

Distortion, /S

Type of Bridge

0.004 Multiple-span (continuous

span) bridges0.005 Single-span bridges

Note:δ is differential settlement, S is the span length. The quantity, δ/S , isdimensionless and is applicable when the same units are used for δ

and S, i.e., if δ is expressed in inches then S should also be expressedin inches.


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Any Questions?Any Questions?


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T H E R O A D T O


S O IL S

A N D

F OU N D A T IO N S


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Shallow FoundationsShallow Foundations

Lesson 08Lesson 08 - - Topic 3Topic 3ConstructionConstruction

Section 8.10 Section 8.10


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Key Elements of Shallow FoundationKey Elements of Shallow Foundation


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Key Elements of Shallow FoundationKey Elements of Shallow Foundation

ConstructionConstructiongg Table 8 Table 8 - - 15 15

gg Contractor set Contractor set - - upup

gg ExcavationExcavationgg Shallow foundationShallow foundationgg

Post installationPost installation-- Monitoring Monitoring

Structural FillStructural Fill


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Structural Fill Structural Fill

gg Tests for gradation and durability of fill atTests for gradation and durability of fill at

sufficient frequency to ensure that thesufficient frequency to ensure that thematerial meets the specificationmaterial meets the specificationgg Compaction testsCompaction testsgg If surcharge fill is used for preIf surcharge fill is used for pre - - loading verifyloading verify

the unit weight of surchargethe unit weight of surcharge

MonitoringMonitoring


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Monitoring Monitoring

gg Check elevations of footing, particularlyCheck elevations of footing, particularly

when footings are on embankment fillswhen footings are on embankment fillsgg Periodic surveying during the service life ofPeriodic surveying during the service life of

the footing, particularly if the subsurface hasthe footing, particularly if the subsurface hassoft soils within the depth of influencesoft soils within the depth of influence

gg Impacts on neighboring facilitiesImpacts on neighboring facilitiesgg Use instrumentation as necessary Use instrumentation as necessary



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be able to:be able to:-- Discuss elements of shallow foundationDiscuss elements of shallow foundationconstruction/inspectionconstruction/inspection

Any Questions?Any Questions?


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T H E R O A D T O


S O IL S

A N D

FO U N D A T IO N S

Interstate 0Interstate 0 – – Apple Freeway Apple Freeway


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Interstate 0 Apple Freeway pp y

Note: Scale shown in Station FormNote: Scale shown in Station Form

BaselineStationing

BaselineStationing

S.B.AppleFrwy

N.B.AppleFrwy

Proposed Toeof Slope

Proposed Toe

of Slope

ExistingGround SurfaceExistingGround Surface

12

Proposed Final GradeProposed Final GradeProposedAbutmentProposedAbutment

Interstate 0Interstate 0

9090 9191 9292 9393

Apple FreewayApple FreewaySubsurfaceInvestigations

Terrain reconnaissanceSite inspectionSubsurface borings

Basic Soil Properties Visual description


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Apple Freeway Apple Freeway

ExerciseExercisegg Appendix A Appendix A

-- Section A.7 Section A.7

Basic Soil Properties Visual descriptionClassification testsSoil profile

Laboratory Testing P o diagramTest requestConsolidation resultsStrength results

SlopeStability

Design soil profileCircular arc analysisSliding block analysisLateral squeeze analysis

Approach RoadwaySettlement

Design soil profileMagnitude and rate ofsettlementSurchargeVertical drains

Spread FootingDesign

Design soil profile

Pier bearing capacityPier settlementAbutment settlementSurchargeVertical drains

Driven Pile Design Design soil profileStatic analysis – pier

Pipe pileH – pile

Static analysis – abutmentPipe pileH – pile

Driving resistanceLateral movement - abutment

ConstructionMonitoring

Wave equationHammer approvalEmbankment instrumentation

APPLE FREEWAY APPLE FREEWAY



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″ N″

7′

4611

2122403733

BAF -2

Clay

15′10′

4′

Sand

Assumptions:

• Footing embeded 4 ′ below ground• Footing width = 1/3 pier height = 7 ′ • Footing length = 100 ′

L/W = 100/7 > 9 Continuous

PIER BEARING CAPACITY PIER BEARING CAPACITY

″ N″

7′

4611

2122403733

BAF -2

Clay

15′10′

4′

Sand

″ N″

7′

4611

2122403733

BAF -2

Clay

15′10′

4′

Sand

7′

4611

2122403733

BAF -2

Clay

15′10′

4′

Sand

7′

4611

2122403733

BAF -2

Clay

15′10′

4′

Sand

7′

4611

2122403733

BAF -2

Clay

15′10′

4′

Sand

4611

2122403733

BAF -2

Clay

15′10′

4′

Sand

Assumptions:

• Footing embeded 4 ′ below ground• Footing width = 1/3 pier height = 7 ′ • Footing length = 100 ′

L/W = 100/7 > 9 Continuous

PIER BEARING CAPACITY PIER BEARING CAPACITY

10


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EAST ABUTMENT SETTLEMENT EAST ABUTMENT SETTLEMENT


EAST ABUTMENT SETTLEMENT EAST ABUTMENT SETTLEMENT


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P f

P o

P abut

P c

10 ′

20 ′

30 ′

40 ′

1000 2000 3000 4000 5000 6000

Pressure (psf)

0

4920 5850

6200 5650

5550 4470

Gravel Layer

Clay

Sand

50 ′

Time (days)

0

H 2 ″

1″

500 400 300 200 100

H = 2.59 ″

D e p

t h ( f t )

D e p

t h ( f t )

P f

P o

P abut

P c

10 ′

20 ′

30 ′

40 ′

1000 2000 3000 4000 5000 6000

Pressure (psf)

0

4920 5850

6200 5650

5550 4470

Gravel Layer

Clay

Sand

50 ′

Time (days)

0

H 2 ″

1″

500 400 300 200 100

H = 2.59 ″

D e p

t h ( f t )

D e p

t h ( f t )

P f

P o

P abut

P c

10 ′

20 ′

30 ′

40 ′

1000 2000 3000 4000 5000 6000

Pressure (psf)

0

4920 5850

6200 5650

5550 4470

Gravel Layer

Clay

Sand

50 ′

Time (days)

0

H 2 ″

1″

500 400 300 200 100

H = 2.59 ″

D e p

t h ( f t )

D e p

t h ( f t )


EAST ABUTMENT SETTLEMENT TREATMENT EAST ABUTMENT SETTLEMENT TREATMENT


EAST ABUTMENT SETTLEMENT TREATMENT EAST ABUTMENT SETTLEMENT TREATMENT


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H ABUT

12.66 ″ emb.

0

H

15 ″

10 ″

5 ″

Time – days

400 300 200 100

15.25 ″ Emb. + Abut

Assume Wick Drains Installed

*0.25 ″ Remaining 30 days after abutment loaded

Begin Abutment Footing Construction

15.25 ″Total H

30 ′ Fill to10 ′ Surcharge

0.83 ″

13.7 ″ t90

Time – Days

H –Total

*Assume 10 ′Surcharge Used

240 days 400 days

15 ″

10 ″

5 ″

0 100 200 300 500 400

H ABUT

12.66 ″ emb.

0

H

15 ″

10 ″

5 ″

Time – days

400 300 200 100

15.25 ″ Emb. + Abut




H ABUT

12.66 ″ emb.

0

H

15 ″

10 ″

5 ″

Time – days

400 300 200 100

15.25 ″ Emb. + Abut




H ABUT

12.66 ″ emb.

0

H

15 ″

10 ″

5 ″

Time – days

400 300 200 100

15.25 ″ Emb. + Abut




15.25 ″Total H


0.83 ″

13.7 ″ t90

Time – Days

H –Total


240 days 400 days

15 ″

10 ″

5 ″

0 100 200 300 500 400

15.25 ″Total H


0.83 ″

13.7 ″ t90

Time – Days

H –Total


240 days 400 days

15 ″

10 ″

5 ″

0 100 200 300 500 400


0.83 ″

13.7 ″ t90

Time – Days

H –Total


240 days 400 days

15 ″

10 ″

5 ″

0 100 200 300 500 400

SPREAD FOOTING DESIGN SPREAD FOOTING DESIGN

SPREAD FOOTING DESIGN SPREAD FOOTING DESIGN


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Design Soil Profile

Strength and consolidation values selected for all soil layers.Footing elevation and width chosen.

Pier Bearing Capacity

Q allowable = 3 tons/sq.ft.

Pier Settlement

Settlement = 2.8", t 90 = 220 days.

Abutment Settlement

Settlement - 2.6", t 90 = 433 days.

Vertical Drains

t 90 = 60 days - could reduce settlement to 0.25" after abutmenconstructed and loaded.

Surcharge

10' surcharge: t 90 = 240 daysbefore abutment constructed.

De sign Soil Profile

Strength and consolidation values selected for all soil layers.Footing elevation and width chosen.

Pier Bearing Capacity

Q allowable = 3 tons/sq.ft.

Pier Settlement

Settlement = 2.8", t 90 = 220 days.

Abutment Settlement

Settlement - 2.6", t 90 = 433 days.

Vertical Drains

t 90 = 60 days - could reduce settlement to 0.25" after abutmenconstructed and loaded.

Surcharge

10' surcharge: t 90 = 240 daysbefore abutment constructed.


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Documents

Lesson 08-Chapter 8 Shallow Foundations.pdf