lesson 08-chapter 8 shallow foundations
TRANSCRIPT
SOILS AND FOUNDATIONSSOILS AND FOUNDATIONS
Testing
Experience
Theory
Lesson 08Lesson 08Chapter 8 Chapter 8 –– Shallow FoundationsShallow Foundations
TopicsTopics
ggTopic 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 CapacityGeneral and Bearing Capacity
ggTopic 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)-- SettlementSettlement-- Spread footings on embankments, Spread footings on embankments, IGMsIGMs, rocks, rocks-- Effect of deformations on bridge structuresEffect of deformations on bridge structures
ggTopic 3 (Section 8.10)Topic 3 (Section 8.10)-- Construction Construction
Shallow FoundationsShallow Foundations
Lesson 08 Lesson 08 -- Topic 1Topic 1General and Bearing CapacityGeneral and Bearing Capacity
Section 8.0 to 8.4Section 8.0 to 8.4
Learning OutcomesLearning Outcomes
ggAt 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 capacity Contrast factors that influence bearing capacity
in sand and clayin sand and clay-- Compute bearing capacity in sand and clayCompute bearing capacity in sand and clay-- Describe allowable bearing pressure for rock Describe allowable bearing pressure for rock
foundationsfoundations
Stresses Imposed by StructuresStresses Imposed by Structures
ggAbutment and piers may have shallow or deep Abutment and piers may have shallow or deep foundationsfoundations
General Approach to Foundation General Approach to Foundation DesignDesignggDuty of Foundation DesignerDuty of Foundation Designer
-- Establish the most economical design that safely Establish the most economical design that safely conforms to prescribed structural criteria and conforms to prescribed structural criteria and properly accounts for the intended function of properly accounts for the intended function of the structurethe structure
ggRational method of designRational method of design-- Evaluate various foundation typesEvaluate various foundation types
Recommended Foundation Design Recommended Foundation Design ApproachApproachggStep 1:Step 1:
Determine:Determine:-- Direction, type and magnitude of foundation Direction, type and magnitude of foundation
loadsloads-- Tolerable deformationsTolerable deformations-- Special constraintsSpecial constraints
•• UnderclearanceUnderclearance requirementsrequirements•• Structure type, span lengthsStructure type, span lengths•• Time constraints on constructionTime constraints on construction•• Extreme event loadingExtreme event loading•• Construction load requirementsConstruction load requirements
Recommended Foundation Design Recommended Foundation Design ApproachApproachggStep 2:Step 2:
Evaluate subsurface investigation and Evaluate subsurface investigation and laboratory testing data for reliability and laboratory testing data for reliability and completenesscompleteness
Choose design method consistent with Choose design method consistent with quality and quantity of subsurface dataquality and quantity of subsurface data
Recommended Foundation Design Recommended Foundation Design ApproachApproachggStep 3:Step 3:
Consider alternate foundation typesConsider alternate foundation types
Foundation AlternativesFoundation Alternatives
ggShallow FoundationsShallow FoundationsggDeep FoundationsDeep Foundations
-- Piles, shaftsPiles, shafts
Foundation CostFoundation Cost
gg Express foundation capacity in terms of $Express foundation capacity in terms of $
gg TOTAL cost of foundation system divided by the TOTAL cost of foundation system divided by the load supported by the foundation in tonsload supported by the foundation in tons
gg TOTAL cost of a foundation must include ALL TOTAL 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 applicableAll other factors as applicable
Foundation CostFoundation Cost
gg If estimated costs of alternative foundation If estimated costs of alternative foundation systems during design are within 15%, the systems during design are within 15%, the alternate foundation designs should be alternate foundation designs should be considered for inclusion in contract considered for inclusion in contract documentsdocuments
Loads and Limit StatesLoads and Limit States
ggLoadsLoads-- Permanent and TransientPermanent and Transient-- Codes specify load combinationsCodes specify load combinations
ggFoundation limit statesFoundation limit states-- Ultimate Ultimate
•• Bearing capacity, eccentricity, sliding, global stability, Bearing capacity, eccentricity, sliding, global stability, structural capacitystructural capacity
-- ServiceabilityServiceability•• Excessive settlement, excessive lateral displacement, Excessive settlement, excessive lateral displacement,
structural deterioration of foundationstructural deterioration of foundation
Types of Shallow FoundationsTypes of Shallow Foundations
gg Isolated Spread FootingsIsolated Spread Footings-- Length (L) to width (B) ratio, L/B < 10Length (L) to width (B) ratio, L/B < 10
Types of Shallow FoundationsTypes of Shallow Foundations
ggCombined Strip Spread FootingsCombined Strip Spread Footings-- Length (L) to width (B) ratio, L/B Length (L) to width (B) ratio, L/B ≥≥ 1010
Shallow Foundations for Bridge Shallow Foundations for Bridge AbutmentsAbutments
Shallow Foundations for Retaining Shallow Foundations for Retaining WallsWalls
Combined FootingsCombined Footings
21
Original Ground
Abutment Fill
Toe of Side Slope
Toe of End Slope
Mat FoundationsMat Foundations
REINFORCED CONCRETE MAT
Spread Footing Design ProcedureSpread Footing Design Procedure
ggGeotechnical design of spread footing is a Geotechnical design of spread footing is a two part processtwo part process
ggFirst Part:First Part:-- Establish an allowable stress to prevent shear Establish an allowable stress to prevent shear
failure in soilfailure in soil
ggSecond Part:Second Part:-- Estimate the settlement under the applied stressEstimate the settlement under the applied stress
Allowable Bearing CapacityAllowable 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 failure divided by FSdivided by FS-- Ultimate limit criterionUltimate limit criterion
OROR
Applied stress that results in a specified amount of Applied stress that results in a specified amount of settlement of the structuresettlement of the structure-- Serviceability criterionServiceability criterion
Bearing Capacity ChartBearing Capacity Chart
Effective Footing Width, ft (m)
Allo
wab
le B
earin
g C
apac
ity, k
sf (k
Pa)
Ultimate Bearing Capacity, qult
Contours of Allowable Bearing Capacity for a given settlement
S1 S2 S3
Allowable Bearing Capacity,
FSqq ult
all =
Design Process Flow ChartDesign Process Flow Chart
ggFigure 8Figure 8--1010
Bearing CapacityBearing Capacity
ggBearing capacity failure occurs when the Bearing capacity failure occurs when the shear strength of foundation soil is exceededshear strength of foundation soil is exceeded
ggSimilar to slope stability failureSimilar to slope stability failure
II
I III
DC
A EB
Q
L = ∞ q
ψ
Bearing Bearing Capacity Capacity Failure Failure MechanismsMechanismsggGeneral shearGeneral shearggLocal shearLocal shearggPunching shearPunching shear
(a) GENERAL SHEAR
(b) LOCAL SHEAR
(c) PUNCHING SHEAR
LOAD
SETT
LEM
ENT
LOAD
SETT
LEM
ENT
LOAD
SETT
LEM
ENT
SURFACE TEST
TEST ATGREATERDEPTH
Footing Dimension TerminologyFooting Dimension Terminology
ggBBff = Width of footing= Width of footing-- Least lateral dimensionLeast lateral dimension
ggLLff = Length of footing= Length of footing
ggDDff = Depth of = Depth of embedment of footingembedment of footing
Lf
Df
Bf
Basic Bearing Capacity EquationBasic Bearing Capacity Equation
ggEquation 8Equation 8--88
cc = cohesion= cohesionqq = surcharge at footing base= surcharge at footing baseNNcc, , NNqq, N, Nγγ = Bearing capacity factors= Bearing capacity factorsγγ = unit weight of foundation soil= unit weight of foundation soil
))(Nf)(B( 0.5 )q(N q )c(N c ultq γγ++=
Assumptions of Basic Bearing Assumptions of Basic Bearing Capacity Equation (Section 8.4.3)Capacity Equation (Section 8.4.3)ggStrip (continuous) footingStrip (continuous) footingggRigid footingRigid footingggGeneral shearGeneral shearggConcentric loading (i.e., loading through the Concentric loading (i.e., loading through the
centroidcentroid of the footing)of the footing)ggFooting bearing on level surface of Footing bearing on level surface of
homogeneous soilhomogeneous soilggNo impact of groundwaterNo impact of groundwater
Bearing Capacity FactorsBearing Capacity Factors
1
10
100
1000
0 5 10 15 20 25 30 35 40 45
Friction Angle, degrees
Bea
ring
Cap
acity
Fac
tors
Nq
Nc
Nγ
Figure 8-15Table 8-1
Example 8Example 8--11
γsub = 63 pcf
d = D = 5′ γT = 125 pcf
B = 6′
φ = 20° c = 500 psf
Example 8Example 8--11
ggSolutionSolution
Effect of Variation of Soil Properties Effect of Variation of Soil Properties and Footing Dimensions (Table 8and Footing Dimensions (Table 8--2)2)
Cohesive Soil
CohesionlessSoil
φ = 0c = 1000 psf
qult (psf)
φ = 30o
c = 0qult (psf)
A. Initial situation: γ = 120 pcf, Df = 0', Bf = 5', deep water table
5140 6720
B. Effect of embedment: Df = 5', γ=120 pcf, Bf = 5', deep water table
C. Effect of width: Bf = 10' γ = 120 pcf, Df = 0', deep water table
D. Effect of water table at surface: γ = 57.6 pcf, Df = 0', Bf = 5'
Properties and Dimensionsγ = γa = effective unit weightγb = submerged unit weightDf = embedment depthBf = footing width (assume strip footing)
Effect of Variation of Soil Properties Effect of Variation of Soil Properties and Footing Dimensions (Table 8and Footing Dimensions (Table 8--2)2)
Cohesive Soil
CohesionlessSoil
φ = 0c = 1000 psf
qult (psf)
φ = 30o
c = 0qult (psf)
A. Initial situation: γ = 120 pcf, Df = 0', Bf = 5', deep water table
5140 6720
B. Effect of embedment: Df = 5', γ=120 pcf, Bf = 5', deep water table
5740 17760
C. Effect of width: Bf = 10' γ = 120 pcf, Df = 0', deep water table
5140 13440
D. Effect of water table at surface: γ = 57.6 pcf, Df = 0', Bf = 5'
5140 3226
Properties and Dimensionsγ = γa = effective unit weightγb = submerged unit weightDf = embedment depthBf = footing width (assume strip footing)
Student Exercise 5Student Exercise 5
ggFind the allowable bearing capacity assuming Find the allowable bearing capacity assuming a FS=3 for the condition shown below for a a FS=3 for the condition shown below for a 10’x50’ footing with rough base10’x50’ footing with rough base
30′
4′
10′
Final Grade
Sandγ = 115 pcfφ = 35°C = 0
Bearing Capacity Correction FactorsBearing Capacity Correction Factors
ggFooting shapeFooting shape-- Adjusted for eccentricityAdjusted for eccentricity
ggDepth of water tableDepth of water tableggEmbedment depthEmbedment depthggSloping ground surfaceSloping ground surfacegg Inclined baseInclined basegg Inclined loadingInclined loading
Student Exercise 5Student Exercise 5
ggSolutionSolution
Modified Bearing Capacity EquationModified Bearing Capacity EquationEquationEquation 88--1111
gg sscc, s, sγγ, s, sqq shape correction factorsshape correction factors
gg bbcc, b, bγγ, , bbqq base inclination correction factorsbase inclination correction factors
gg CCwqwq, , CCwwγγ groundwater correction factorsgroundwater correction factors
gg ddqq embedment correction factorembedment correction factor
gg NNcc, N, Nγγ, , NNqq bearing capacity factors as function of bearing capacity factors as function of φφ
γγγγγ++= bsCNB5.0dbsCqN bscNq WfqqqqWqcccult
Estimation of Estimation of φφ for Bearing Capacity for Bearing Capacity Factors (Table 8Factors (Table 8--3)3)
Description VeryLoose Loose Medium Dense Very
Dense
Corrected N-value N160
0 4 10 30 50
Friction angleφ Degrees
25 –30
27 –32 30 – 35 35 –
4038 –
43
Moist unit weight (γ) pcf
70 –100
90 –115
110 –130
120 –140
130 –150
Shape Correction FactorsShape Correction Factors
ggBasic equation assumes strip footing which Basic equation assumes strip footing which means means LLff/B/Bff ≥≥ 1010
ggFor footings with For footings with LLff/B/Bff << 1010 apply shape apply shape correction factorscorrection factors
ggCompute the effective shape of the footing Compute the effective shape of the footing based on eccentricitybased on eccentricity
Effective Footing DimensionsEffective Footing Dimensions
B′f = Bf – 2eB ; L′f = Lf – 2eL ; A′= B′f L′f
Pressure DistributionsPressure Distributions
Structural designStructural design Sizing the footingSizing the footing
Shape Correction FactorsShape Correction Factors
FactorFriction Angle
Cohesion Term (sc)
Unit Weight
Term (sγ)
Surcharge Term (sq)
φ = 0 1.0 1.0
φ > 0
Shape Factors,sc, sγ, sq ⎟⎟
⎠
⎞⎜⎜⎝
⎛φ+ tan
LB1
f
f
⎟⎟⎠
⎞⎜⎜⎝
⎛+
f
fL5B1
⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛+
c
q
f
fNN
LB1 ⎟⎟
⎠
⎞⎜⎜⎝
⎛−
f
fLB4.01
gg In routine foundation design, use of effective In routine foundation design, use of effective dimensions in shape factors is not practicaldimensions in shape factors is not practical
Location of Groundwater tableLocation of Groundwater table
ggTo correct the unit weightTo correct the unit weight
DW CWγ CWq0 0.5 0.5Df 0.5 1.0
> 1.5Bf + Df 1.0 1.0Note: For intermediate positions of the groundwater table, interpolate between the values shown above.
Embedment DepthEmbedment DepthggTo account for the To account for the
shearing resistance in shearing resistance in the soil above the the soil above the footing basefooting base
Friction Angle, φ(degrees) Df/Bf dq
32
1248
1.201.301.351.40
37
1248
1.201.251.301.35
42
1248
1.151.201.251.30
See Note
Note: The depth correction factor should be used only when the soils above the footing bearing elevation are as competent as the soils beneath the footing level; otherwise, the depth correction factor should be taken as 1.0.
Sloping Ground SurfaceSloping Ground Surface
ggModify the bearing capacity equation as Modify the bearing capacity equation as follows:follows:
ggUseful in designing footings constructed Useful in designing footings constructed within bridge approach fillswithin bridge approach fills
))(N)(B( 0.5 )(N c q qfcqult γγ+=
Footing in SlopeFooting in Slope
Footing Near SlopeFooting Near Slope
Inclined BaseInclined BaseggFootings with inclined base should be Footings with inclined base should be
avoided or avoided or limtedlimted to angles less than 8to angles less than 8--1010ººggSliding may be an issue for inclined basesSliding may be an issue for inclined bases
⎟⎠⎞
⎜⎝⎛ α
−3.147
1⎟⎟⎠
⎞⎜⎜⎝
⎛
φ
−−
tanNb1
bc
Cohesion Term (c)
Unit Weight Term (γ)
Surcharge Term (q)
bc bγ bq
φ = 0 1.0 1.0
φ > 0 (1-0.017α tanφ)2 (1-0.017α tanφ)2
φ= friction angle, degrees; α = footing inclination from horizontal, upward +, degrees
Base Inclination Factors,bc, bγ, bq
FactorFriction Angle
Inclined LoadingInclined Loading
gg If shear (horizontal) component is checked If shear (horizontal) component is checked for sliding resistance, the inclination for sliding resistance, the inclination correction factor is omittedcorrection factor is omitted
ggUse effective footing dimensions in Use effective footing dimensions in evaluation of the vertical component of the evaluation of the vertical component of the loadload
Comments on Use of Bearing Comments on Use of Bearing Capacity Correction FactorsCapacity Correction FactorsggFor settlementFor settlement--controlled allowable bearing controlled allowable bearing
capacity, the effect application of correction capacity, the effect application of correction factors may be negligiblefactors may be negligible
ggApplication of correction factors is Application of correction factors is secondary to the adequate assessment of secondary to the adequate assessment of the shear strength characteristics of the the shear strength characteristics of the foundation soil through correctly performed foundation soil through correctly performed subsurface explorationsubsurface exploration
Local or Punching ShearLocal or Punching Shear
c* = 0.67cc* = 0.67cφφ*=tan*=tan--11(0.67tan(0.67tanφφ))
ggLoose sandsLoose sandsggSensitive claysSensitive claysggCollapsible Collapsible
soilssoilsggBrittle claysBrittle clays
Bearing Capacity Factors of SafetyBearing Capacity Factors of Safety
ggqqallall = allowable bearing capacity= allowable bearing capacityggqqultult = ultimate bearing capacity= ultimate bearing capacityggTypical FS = 2.5 to 3.5Typical FS = 2.5 to 3.5ggFS is a function ofFS is a function of
-- Confidence in shear strength parameter, c and Confidence in shear strength parameter, c and φφ-- Importance of structureImportance of structure-- Consequences of failureConsequences of failure
FSq
q ultall =
Overstress AllowancesOverstress Allowances
ggFor shortFor short--duration infrequently duration infrequently occuringoccuringloads, an overstress of 25 to 50 % may be loads, an overstress of 25 to 50 % may be allowed for allowable bearing capacityallowed for allowable bearing capacity
Practical Aspects of Bearing Practical Aspects of Bearing CapacityCapacity
Presumptive Allowable Bearing Presumptive Allowable Bearing CapacityCapacityggNOT recommended for soilsNOT recommended for soilsggSee Tables 8See Tables 8--8, 88, 8--9 and 89 and 8--10 for rocks10 for rocks
Learning OutcomesLearning Outcomes
ggAt 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 capacity Contrast factors that influence bearing capacity
in sand and clayin sand and clay-- Compute bearing capacity in sand and clayCompute bearing capacity in sand and clay-- Describe allowable bearing pressure for rock Describe allowable bearing pressure for rock
foundationsfoundations
Any Questions?Any Questions?
THE ROAD TOUNDERSTANDING
SOILSAND
FOUNDATIONS
Shallow FoundationsShallow Foundations
Lesson 08 Lesson 08 -- Topic 2Topic 2Settlement, 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
Learning OutcomesLearning Outcomes
ggAt the end of this session, the participant will At the end of this session, the participant will be able to:be able to:-- Calculate immediate settlements in granular Calculate immediate settlements in granular
soilssoils-- Calculate consolidation settlements in saturated Calculate consolidation settlements in saturated
finefine--grained soilsgrained soils-- Describe tolerances and consequences of Describe tolerances and consequences of
deformations on bridge structuresdeformations on bridge structures
Settlement of Spread FootingsSettlement of Spread Footings
gg Immediate (shortImmediate (short--term)term)ggConsolidation (longConsolidation (long--term)term)
Immediate SettlementImmediate Settlement
ggHough’s methodHough’s method-- Conservative by a factor of 2 (FHWA, 1987)Conservative by a factor of 2 (FHWA, 1987)
ggSchmertmann’sSchmertmann’s methodmethod-- More rationalMore rational-- Based on nonlinear theory of elasticity and Based on nonlinear theory of elasticity and
measurementsmeasurements
ChartsChartsFigure 2Figure 2--1111ggDDss = 4B to 6B = 4B to 6B
for continuous for continuous footings where footings where LLff/B/Bff ≥≥ 1010
ggDDss = 1.5B to 2B = 1.5B to 2B for square for square footings where footings where LLff/B/Bff == 11
Trend of Analytical Trend of Analytical Results and Results and MeasurementsMeasurements
Legend:Legend:
Square footings Square footings where where LLff/B/Bff =1=1
Continuous footings Continuous footings where where LLff/B/Bff ≥≥ 1010
Vertical Strain, %
Dep
th b
elow
Foo
ting
2B
4B
SchmertmannSchmertmann MethodMethod
gg IIzz Strain Influence FactorStrain Influence Factorgg EE Elastic Modulus, Table 5Elastic Modulus, Table 5--2020gg XX Modification factor for EModification factor for Egg CC11 Correction factor for strain reliefCorrection factor for strain reliefgg CC22 Correction factor for creep deformationCorrection factor for creep deformation
∑=
ΔΔ=n
1ii21i HpCCS ⎟
⎠⎞
⎜⎝⎛=Δ
XEI
HH zci
5.0p
p5.01C o
1 ≥⎟⎟⎠
⎞⎜⎜⎝
⎛Δ
−=( )
⎟⎠⎞
⎜⎝⎛+=
1.0yearstlog2.01C 102
5.0
opp
p1.05.0zpI⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛Δ
+=
below)b(see
Plane Strain Lf/Bf ≥ 10
AxisymmetricLf/Bf =1
Lf = Length of footingBf = least width of footing
op
oppBf /2 (for axisymmetric case)Bf (for plane strain case)
Bf
oppp −=Δ
Depth to Peak Strain Influence Factor, Izp
Example 8Example 8--22ggGiven: 6’x24’ footing on soil profile shown Given: 6’x24’ footing on soil profile shown
below. Determine settlement at end of below. 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; N160 = 8
γt = 125 pcf; N160 = 25
γt = 120 pcf; N160 = 30
γt = 128 pcf; N160 = 68
3 ft
3 ft
5 ft
25 ft
Bf = 6 ft
Ground Surface
Draw Strain Influence DiagramDraw Strain Influence Diagram
ggCalculate peak Calculate peak IzIz = 0.64= 0.64Plane Strain Lf/Bf ≥ 10
AxisymmetricLf/Bf =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
4
8
12
16
200.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Influence Factor (Iz)
Dep
th b
elow
foot
ing B
2B
3B
Strain Influence DiagramStrain Influence DiagramDivide into layersDivide into layers
0
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)
Dep
th b
elow
foot
ing
(ft)
Layer 1
Layer 2
Layer 3
Layer 4
Determine Elastic Modulus, EDetermine Elastic Modulus, Ess
ggUse Table 5Use Table 5--20, Page 520, Page 5--9090
ggCalculate XCalculate X--factor, X = 1.42factor, X = 1.42
Layer 1: Sandy Silt: E = 4N160 tsf Layer 2: Coarse Sand: E = 10N160 tsf Layer 3: Coarse Sand: E = 10N160 tsf Layer 4: Sandy Gravel: E = 12N160 tsf
Setup Table for Settlement Setup Table for Settlement ComputationComputation
Layer Hc N160 E XE Z1 IZ at Zi
(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 0.01523 48 30 300 426 6 0.55 0.05994 96 68 816 1,159 12 0.22 0.0176
Σ Hi= 0.1686
cZ
i HXEI
H =
Compute Correction Factors CCompute Correction Factors C11 , C, C22
ggAt end of construction, t=0.1 yearAt end of construction, t=0.1 year
ggAt t=10 yearsAt t=10 years
0.896psf1655
pcf115ft30.51Δpp
0.51C o1 =⎟⎟
⎠
⎞⎜⎜⎝
⎛ ×−=⎟⎟
⎠
⎞⎜⎜⎝
⎛−=
( )⎟⎠⎞
⎜⎝⎛+=
1.0yearstlog2.01C 102
0.11.01.0log2.01C 102 =⎟
⎠⎞
⎜⎝⎛+=
4.11.0
10log2.01C 102 =⎟⎠⎞
⎜⎝⎛+=
Determine Immediate SettlementDetermine Immediate Settlement
ggAt end of construction, t = 0.1 yearAt end of construction, t = 0.1 year
ggAt t = 10 yearsAt t = 10 years
( )( )
inches125.0S
tsfin1686.0
tsfpsf2000
psf16550.1896.0S
HpCCS
i
i
i21i
=
⎟⎠⎞
⎜⎝⎛
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛=
Δ= ∑
inches175.00.14.1inches125.0Si =⎟
⎠⎞
⎜⎝⎛=
Consolidation SettlementConsolidation Settlement
ggSame procedures as in Chapter 7 (Approach Same procedures as in Chapter 7 (Approach Roadway Deformations)Roadway Deformations)
Example 8Example 8--33
gg Calculate consolidation settlement for following case:Calculate consolidation settlement for following case:130 kips
Rock
Normally consolidated clay γsub = 65 pcf, e0 = 0.75, Cc = 0.410′
4′
10′
GravelγT = 130 pcf
Example 8Example 8--33p0 = (14′ × 130 pcf) + (5′ × 65 pcf) = 2,145 psf
psf208ksf0.208ft625
130kipsft)15ft(10
kips130Δp 2 ===+
=
⎟⎟⎠
⎞⎜⎜⎝
⎛ ++
=0
010
0
c
pΔpp
loge1
CHΔH
⎟⎟⎠
⎞⎜⎜⎝
⎛ +⎟⎠⎞
⎜⎝⎛
+=Δ
psf2145psf208psf2145log
0.7510.410ftH 10
″=′=Δ 1.109.0H
Student Exercise 6Student Exercise 6
ggFind 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 CC = 0.25e0 = 0.90
(Normally Consolidated)
Student Exercise 6Student Exercise 6Pressure - psf
Dep
th –
ft.
Spread Footings on EmbankmentsSpread Footings on Embankments
ggSection 8.6Section 8.6gg If spread footings are placed on If spread footings are placed on
embankments, structural fills that include embankments, structural fills that include sand and gravel sized particles should be sand and gravel sized particles should be used that are compacted properly (minimum used that are compacted properly (minimum 95% of standard Proctor energy)95% of standard Proctor energy)
Settlement of Footings on Structural Settlement of Footings on Structural FillsFillsgg In absence of other data, use N1In absence of other data, use N16060 = 32 for = 32 for
the structural to estimate settlement of the structural to estimate settlement of footings on compacted structural fillfootings on compacted structural fill
Vertical Stress DistributionVertical Stress Distribution
Vertical StressVertical StressVertical Stress0 1 2 3 4 50 1 2 3 4 0 1 2 3 4 55
Dep
thD
epth
Bridge PierBridge PierBridge Pier
Earth Embankment
Earth Earth EmbankmentEmbankment
h=20’h=20’h=20’ h=40’h=40’h=40’
0
20
40
60
80
100
Footings on Footings on IGMsIGMs and Rocksand Rocks
ggUse theory of elasticityUse theory of elasticity
m
2fd
v E)1(BpC ν−Δ
=δ
where: δv = vertical settlement at surface Cd = shape and rigidity factors (Table 8-12) Δp = change in stress at top of rock surface due to applied footing load Bf = 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 of Effect of Deformations on Deformations on Bridge Bridge StructuresStructuresggSection 8.9Section 8.9
Tilt (Rotation)
Differential Settlement
Differential Settlement
Tolerable Movements for BridgesTolerable Movements for Bridges(Table 8(Table 8--13)13)
Limiting Angular Distortion, δ/S
Type of Bridge
0.004 Multiple-span (continuous span) bridges
0.005 Single-span bridges
Note: δ is differential settlement, S is the span length. The quantity, δ/S, is dimensionless and is applicable when the same units are used for δand S, i.e., if δ is expressed in inches then S should also be expressed in inches.
Construction Point Concept for Construction Point Concept for Evaluation of SettlementsEvaluation of SettlementsggDivide the loadings based on sequence of Divide the loadings based on sequence of
constructionconstructionggKey construction point is when the final load Key construction point is when the final load
bearing member is constructed, e.g., when a bearing member is constructed, e.g., when a bridge deck is constructedbridge deck is constructed
ggTable 8Table 8--1414-- Put in a slidePut in a slide
Learning OutcomesLearning Outcomes
ggAt the end of this session, the participant will At the end of this session, the participant will be able to:be able to:-- Calculate immediate settlements in granular Calculate immediate settlements in granular
soilssoils-- Calculate consolidation settlements in saturated Calculate consolidation settlements in saturated
finefine--grained soilsgrained soils-- Describe tolerances and consequences of Describe tolerances and consequences of
deformations on bridge structuresdeformations on bridge structures
Any Questions?Any Questions?
THE ROAD TOUNDERSTANDING
SOILSAND
FOUNDATIONS
Shallow FoundationsShallow Foundations
Lesson 08 Lesson 08 -- Topic 3Topic 3ConstructionConstruction
Section 8.10Section 8.10
Learning OutcomesLearning Outcomes
ggAt the end of this session, the participant will At the end of this session, the participant will be able to:be able to:-- Discuss elements of shallow foundation Discuss elements of shallow foundation
construction/inspectionconstruction/inspection
Key Elements of Shallow Foundation Key Elements of Shallow Foundation ConstructionConstructionggTable 8Table 8--1515
ggContractor setContractor set--upupggExcavationExcavationggShallow foundationShallow foundationggPost installationPost installation
-- MonitoringMonitoring
Structural FillStructural Fill
ggTests for gradation and durability of fill at Tests for gradation and durability of fill at sufficient frequency to ensure that the sufficient frequency to ensure that the material meets the specificationmaterial meets the specification
ggCompaction testsCompaction testsgg If surcharge fill is used for preIf surcharge fill is used for pre--loading verify loading verify
the unit weight of surchargethe unit weight of surcharge
MonitoringMonitoring
ggCheck elevations of footing, particularly Check elevations of footing, particularly when footings are on embankment fillswhen footings are on embankment fills
ggPeriodic surveying during the service life of Periodic surveying during the service life of the footing, particularly if the subsurface has the footing, particularly if the subsurface has soft soils within the depth of influencesoft soils within the depth of influence
gg Impacts on neighboring facilitiesImpacts on neighboring facilitiesggUse instrumentation as necessaryUse instrumentation as necessary
Learning OutcomesLearning Outcomes
ggAt the end of this session, the participant will At the end of this session, the participant will be able to:be able to:-- Discuss elements of shallow foundation Discuss elements of shallow foundation
construction/inspectionconstruction/inspection
Any Questions?Any Questions?
THE ROAD TOUNDERSTANDING
SOILSAND
FOUNDATIONS
Interstate 0 Interstate 0 –– Apple FreewayApple FreewayNote: Scale shown in Station FormNote: Scale shown in Station Form
Baseline Stationing
Baseline Stationing
S.B. Apple Frwy
N.B. Apple Frwy
Proposed Toe of SlopeProposed Toe of Slope
Existing Ground SurfaceExisting Ground Surface
12
Proposed Final GradeProposed Final GradeProposed AbutmentProposed Abutment
Interstate 0Interstate 0
9090 9191 9292 9393
Apple Freeway Apple Freeway ExerciseExerciseggAppendix AAppendix A
-- Section A.7Section A.7
Subsurface Investigations
Terrain reconnaissance Site inspection Subsurface borings
Basic Soil Properties Visual description
Classification tests Soil profile
Laboratory Testing Po diagram
Test request Consolidation results Strength results
Slope Stability
Design soil profile Circular arc analysis Sliding block analysis Lateral squeeze analysis
Approach Roadway Settlement
Design soil profile Magnitude and rate of settlement Surcharge Vertical drains
Spread Footing Design
Design soil profile Pier bearing capacity Pier settlement Abutment settlement Surcharge Vertical drains
Driven Pile Design Design soil profile
Static analysis – pier Pipe pile H – pile Static analysis – abutment Pipe pile H – pile Driving resistance Lateral movement - abutment
Construction Monitoring
Wave equation Hammer approval Embankment instrumentation
″N″
7′
46112122403733
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
APPLE FREEWAYAPPLE FREEWAY
PIER BEARING CAPACITYPIER BEARING CAPACITY
″N″
7′
46112122403733
BAF -2
Clay
15′10′
4′
Sand
″N″
7′
46112122403733
BAF -2
Clay
15′10′
4′
Sand
7′
46112122403733
BAF -2
Clay
15′10′
4′
Sand
7′
46112122403733
BAF -2
Clay
15′10′
4′
Sand
7′
46112122403733
BAF -2
Clay
15′10′
4′
Sand
46112122403733
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
APPLE FREEWAYAPPLE FREEWAY
PIER BEARING CAPACITYPIER BEARING CAPACITY
10
Compute N1Compute N16060 valuesvalues
Depth (ft.)
p0
(psf) p0
(tsf) N
(bpf) Hammer
Efficiency (Ef) Ef / 60
N60
(bpf) Cn
N160 (bpf)
5 550 0.275 11 65 1.083 12 1.43 17 7 770 0.385 21 65 1.083 23 1.32 30 8 880 0.440 22 65 1.083 24 1.28 30 10 1100 0.550 40 65 1.083 43 1.20 52 12 1195 0.598 37 65 1.083 40 1.17 47 14 1290 0.645 33 65 1.083 36 1.15 41
Average corrected blow count = 36
APPLE FREEWAY PIER SETTLEMENTAPPLE FREEWAY PIER SETTLEMENT
Time (days)
Δ H
3″
2″
1″
25020015010050
Δ H = 2.85″
SANDSAND
CLAY CLAY --11
CLAYCLAY--22
APPLE FREEWAY PIER SETTLEMENTAPPLE FREEWAY PIER SETTLEMENT
Time (days)
Δ H
3″
2″
1″
25020015010050
Δ H = 2.85″
Time (days)
Δ H
3″
2″
1″
25020015010050
Δ H = 2.85″
SANDSAND
CLAY CLAY --11
CLAYCLAY--22
SANDSAND
CLAY CLAY --11
CLAYCLAY--22
APPLE FREEWAY APPLE FREEWAY
EAST ABUTMENT SETTLEMENTEAST ABUTMENT SETTLEMENT
Pf
Po
Pabut
Pc
10′
20′
30′
40′
1000 2000 3000 4000 5000 6000
Pressure (psf)
0
4920 5850
62005650
55504470
Gravel Layer
Clay
Sand
50′
Time (days)
0
Δ H2″
1″
500400300200100
Δ H = 2.59″
Dep
th (f
t)D
epth
(ft)
APPLE FREEWAY APPLE FREEWAY
EAST ABUTMENT SETTLEMENTEAST ABUTMENT SETTLEMENT
Pf
Po
Pabut
Pc
10′
20′
30′
40′
1000 2000 3000 4000 5000 6000
Pressure (psf)
0
4920 5850
62005650
55504470
Gravel Layer
Clay
Sand
50′
Time (days)
0
Δ H2″
1″
500400300200100
Δ H = 2.59″
Dep
th (f
t)D
epth
(ft)
Pf
Po
Pabut
Pc
10′
20′
30′
40′
1000 2000 3000 4000 5000 6000
Pressure (psf)
0
4920 5850
62005650
55504470
Gravel Layer
Clay
Sand
50′
Time (days)
0
Δ H2″
1″
500400300200100
Δ H = 2.59″
Dep
th (f
t)D
epth
(ft)
ΔHABUT
12.66″ emb. Δ
0
Δ H
15″
10″
5″
Time – days
400300200100
15.25″ Emb. + Abut
Assume Wick Drains Installed
*0.25″ Δ Remaining 30 days after abutment loaded
Begin Abutment Footing Construction
APPLE FREEWAYAPPLE FREEWAY
EAST ABUTMENT SETTLEMENT TREATMENTEAST ABUTMENT SETTLEMENT TREATMENT
15.25″Total ΔH
30′ Fill to 10′ Surcharge
0.83″
13.7″ t90
Time – Days
ΔH –Total
*Assume 10′Surcharge Used
240 days 400 days
15″
10″
5″
0100 200 300 500400
ΔHABUT
12.66″ emb. Δ
0
Δ H
15″
10″
5″
Time – days
400300200100
15.25″ Emb. + Abut
Assume Wick Drains Installed
*0.25″ Δ Remaining 30 days after abutment loaded
Begin Abutment Footing Construction
ΔHABUT
12.66″ emb. Δ
0
Δ H
15″
10″
5″
Time – days
400300200100
15.25″ Emb. + Abut
Assume Wick Drains Installed
*0.25″ Δ Remaining 30 days after abutment loaded
Begin Abutment Footing Construction
ΔHABUT
12.66″ emb. Δ
0
Δ H
15″
10″
5″
Time – days
400300200100
15.25″ Emb. + Abut
Assume Wick Drains Installed
*0.25″ Δ Remaining 30 days after abutment loaded
Begin Abutment Footing Construction
APPLE FREEWAYAPPLE FREEWAY
EAST ABUTMENT SETTLEMENT TREATMENTEAST ABUTMENT SETTLEMENT TREATMENT
15.25″Total ΔH
30′ Fill to 10′ Surcharge
0.83″
13.7″ t90
Time – Days
ΔH –Total
*Assume 10′Surcharge Used
240 days 400 days
15″
10″
5″
0100 200 300 500400
15.25″Total ΔH
30′ Fill to 10′ Surcharge
0.83″
13.7″ t90
Time – Days
ΔH –Total
*Assume 10′Surcharge Used
240 days 400 days
15″
10″
5″
0100 200 300 500400
30′ Fill to 10′ Surcharge
0.83″
13.7″ t90
Time – Days
ΔH –Total
*Assume 10′Surcharge Used
240 days 400 days
15″
10″
5″
0100 200 300 500400
Design Soil Profile
Strength and consolidation values selected for all soil layers. Footing elevation and width chosen.
Pier Bearing Capacity
Qallowable = 3 tons/sq.ft.
Pier Settlement
Settlement = 2.8", t90 = 220 days.
Abutment Settlement
Settlement - 2.6", t90 = 433 days.
Vertical Drains
t90 = 60 days - could reduce settlement to 0.25" after abutmentconstructed and loaded.
Surcharge
10' surcharge: t90 = 240 daysbefore abutment constructed.
SPREAD FOOTING DESIGNSPREAD FOOTING DESIGN
Design Soil Profile
Strength and consolidation values selected for all soil layers. Footing elevation and width chosen.
Pier Bearing Capacity
Qallowable = 3 tons/sq.ft.
Pier Settlement
Settlement = 2.8", t90 = 220 days.
Abutment Settlement
Settlement - 2.6", t90 = 433 days.
Vertical Drains
t90 = 60 days - could reduce settlement to 0.25" after abutmentconstructed and loaded.
Surcharge
10' surcharge: t90 = 240 daysbefore abutment constructed.
SPREAD FOOTING DESIGNSPREAD FOOTING DESIGN
Any Questions?Any Questions?
THE ROAD TOUNDERSTANDING
SOILSAND
FOUNDATIONS