deep foundations

MnDOTMnDOT Deep Foundation Deep Foundation Design Using LRFD Design Using LRFD

MethodologyMethodology

LRFD Bridge Design WorkshopLRFD Bridge Design WorkshopJune 12, 2007June 12, 2007

David Dahlberg, P.E.David Dahlberg, P.E.LRFD EngineerLRFD Engineer

Presentation OverviewPresentation Overview

Previous Pile Design MethodPrevious Pile Design MethodAASHTO LRFD Pile Design AASHTO LRFD Pile Design MethodMethodNew New MnDOTMnDOT LRFD MethodLRFD MethodPile Pile DowndragDowndragPile Lateral Load CapacityPile Lateral Load CapacityDrilled Shaft DesignDrilled Shaft Design

Previous Pile Design MethodPrevious Pile Design Method

Based on Allowable Stress Design (ASD)Based on Allowable Stress Design (ASD)∑ ∑ QQii ≤ ≤ QQultult / FS/ FS

wherewhere

Q = service loadQ = service loadQQultult = ultimate capacity= ultimate capacityFS = factor of safetyFS = factor of safety


Need to consider four things:Need to consider four things:Capacity of soilCapacity of soilStructural capacity of pileStructural capacity of pileDriveabilityDriveability of pile (max driving stresses)of pile (max driving stresses)Field verification during driving operation to Field verification during driving operation to ensure required resistance is obtainedensure required resistance is obtained


Design soil allowable capacity determination Design soil allowable capacity determination based on combination of:based on combination of:

Static analysis w/ F.S (done by Static analysis w/ F.S (done by geotechsgeotechs))Correlation of borings with field verification Correlation of borings with field verification method (done by Regional Construction method (done by Regional Construction Engineer)Engineer)


Typical pile was 12” Typical pile was 12” diadia. CIP w/0.25” wall. CIP w/0.25” wall60 to 75 ton allowable maximum load60 to 75 ton allowable maximum load(based on considering past practice,(based on considering past practice,AASHTO, experience, and AASHTO, experience, and driveabilitydriveabilityof the pile)of the pile)


Majority of pile capacities based on field Majority of pile capacities based on field measured initial drive capacitymeasured initial drive capacitySoil/pile setup used when warranted by Soil/pile setup used when warranted by soil profilesoil profile

Only in low initial capacity situationsOnly in low initial capacity situations


Field verification during driving:Field verification during driving:MnDOTMnDOT Modified ENR FormulaModified ENR Formula

CIP pilesCIP piles

H H –– pilespiles

PDA sometimes usedPDA sometimes used

MWM1.0W

2.0SE5.3P

++

⋅+

=

MWM2.0W

2.0SE5.3P

++

⋅+

=

AASHTO LRFD Design MethodAASHTO LRFD Design Method

Requires use of factored loads & nominal Requires use of factored loads & nominal resistanceresistance

∑ ∑ ηηi i ⋅⋅ γγi i ⋅⋅QQii ≤ ≤ φφ⋅⋅RRnn

wherewhere

ηη = load modifier= load modifierγγ = load factor= load factorQ = service loadQ = service loadφφ = resistance factor= resistance factorRRnn = nominal (ultimate) resistance= nominal (ultimate) resistance


Need to consider four things:Need to consider four things:Capacity of soilCapacity of soilStructural capacity of pileStructural capacity of pileDriveabilityDriveability of pile (max driving stresses)of pile (max driving stresses)Field verification during driving operation to Field verification during driving operation to ensure required resistance is obtainedensure required resistance is obtained


Capacity of soil:Capacity of soil:Estimated by geotechnical engineer using static Estimated by geotechnical engineer using static pile analysispile analysisResistance factors Resistance factors φφstatstat from LRFDfrom LRFDTable 10.5.5.2.3Table 10.5.5.2.3--1 1


LRFD Resistance Factors for PilesLRFD Resistance Factors for PilesLRFD Table 10.5.5.2.3LRFD Table 10.5.5.2.3--11


Structural capacity of Structural capacity of pile:pile:

CIP piles per LRFD CIP piles per LRFD 6.9.5.16.9.5.1φφcc ·(A·(Asfsfffyy+0.85f’+0.85f’cc·A·Acc))

H piles per LRFD 6.9.4.1H piles per LRFD 6.9.4.1φφcc ··AAssffyy

Resistance factors for Resistance factors for axial resistance per LRFD axial resistance per LRFD 6.15.2 and 6.5.4.26.15.2 and 6.5.4.2


LRFD Resistance Factors for Steel PilesLRFD Resistance Factors for Steel Pilesfound in LRFD 6.5.4.2found in LRFD 6.5.4.2


DriveabilityDriveability (max driving resistance):(max driving resistance):Per LRFD 10.7.8: Per LRFD 10.7.8:

0.9· 0.9· φφdada·f·fyy

Resistance factor per LRFDResistance factor per LRFDTable 10.5.5.2.3Table 10.5.5.2.3--1 and LRFD 6.5.4.21 and LRFD 6.5.4.2


LRFD Resistance Factor for LRFD Resistance Factor for DriveabilityDriveabilityLRFD Table 10.5.5.2.3LRFD Table 10.5.5.2.3--11

LRFD 6.5.4.2LRFD 6.5.4.2


Field verification during driving Field verification during driving operation to ensure required resistance operation to ensure required resistance is obtained:is obtained:

Verification by static load test, dynamic Verification by static load test, dynamic testing (PDA), wave equation, or dynamic testing (PDA), wave equation, or dynamic formulaformulaUses resistance factor Uses resistance factor φφdyndyn fromfromLRFD Table 10.5.5.2.3LRFD Table 10.5.5.2.3--11

LRFD LRFD Resistance Resistance Factors for Factors for PilesPilesLRFD Table LRFD Table 10.5.5.2.310.5.5.2.3--11


New New MnDOTMnDOT LRFD MethodLRFD Method

Capacity of soil:Capacity of soil:Look in the Foundation ReportLook in the Foundation ReportTypical Foundation Report should include:Typical Foundation Report should include:

Project descriptionProject descriptionField investigation and foundation conditionsField investigation and foundation conditionsFoundation analysisFoundation analysisRecommendationsRecommendationsAdditional sections as neededAdditional sections as needed


Foundation analysis should include:Foundation analysis should include:Nominal Resistance (ultimate capacity) Nominal Resistance (ultimate capacity) estimates provided by Foundations Unitestimates provided by Foundations UnitInitial drive and setInitial drive and set--up graph which shows up graph which shows resistance as a function of depthresistance as a function of depth


Pile Resistance Pile Resistance φφRRnn for designfor designDetermined considering LRFD structural Determined considering LRFD structural capacity of pile, maximum LRFD driving capacity of pile, maximum LRFD driving resistance, and past experienceresistance, and past experience

Pile Capacity TablePile Capacity Table


Field verification during drivingField verification during drivingTypically will use Typically will use MnDOTMnDOT dynamic formula dynamic formula modified to provide nominal resistance as modified to provide nominal resistance as the outputthe output

Will use PDA on larger projects by running Will use PDA on larger projects by running a PDA on the test piles to calibrate the a PDA on the test piles to calibrate the MnDOTMnDOT dynamic formula for other pilesdynamic formula for other piles


Field Verification during driving:Field Verification during driving:MnDOTMnDOT Nominal Resistance Pile Driving Nominal Resistance Pile Driving Formula (forFormula (for both CIP & Hboth CIP & H--piles)piles)

Incorporated by special provisionIncorporated by special provisionSB2005SB2005--2452.22452.2

MWM1.0W

2.0SE5.10Rn +

+⋅

+=


LRFD LRFD Resistance Resistance Factors for Factors for PilesPilesLRFD Table LRFD Table 10.5.5.2.310.5.5.2.3--11


Resistance factors:Resistance factors:Compare LRFD to ASDCompare LRFD to ASDLRFD: ∑ LRFD: ∑ γγQ ≤ Q ≤ φφRRnnASD: ∑ Q ≤ ASD: ∑ Q ≤ RRnn /F.S. Then F.S.= /F.S. Then F.S.= γγ / / φφ

Average Average γγ ≈≈ 1.41.4

For For MnDOTMnDOT formula, formula, φφdyndyn = 1.4/3.0 ≈ 0.45= 1.4/3.0 ≈ 0.45

For PDA, For PDA, φφdyndyn = 1.4/2.25 ≈ 0.60= 1.4/2.25 ≈ 0.60


Comparisons made with Comparisons made with MnDOTMnDOT Formula, Formula, WEAP, Gates Formula, and PDA data WEAP, Gates Formula, and PDA data


Field verificationField verificationPDAPDA

φφdyndyn = 0.65= 0.65

MnDOTMnDOT Nominal Nominal Resistance PileResistance PileDriving FormulaDriving Formula

φφdyndyn = 0.40= 0.40


Monitoring method determines required Monitoring method determines required driving resistance for the Contractor driving resistance for the Contractor For example, assume a factored design For example, assume a factored design load of 100 tons/pile:load of 100 tons/pile:

PDA verificationPDA verificationRRnn = = QQuu/ / φφdyndyn = 100/0.65 = 154 tons= 100/0.65 = 154 tons

MnDOTMnDOT Ultimate formulaUltimate formulaRRnn = = QQuu/ / φφdyndyn = 100/0.40 = 250 tons= 100/0.40 = 250 tons


ExampleExample


Pile Capacity TablePile Capacity Table

Bridge Plan Bridge Plan Load Tables Load Tables


Implementation for T.H.Implementation for T.H.

MnDOTMnDOT Foundation Unit (Maplewood Lab)Foundation Unit (Maplewood Lab)Providing ultimate capacity estimates Providing ultimate capacity estimates

Regional Bridge Construction EngineersRegional Bridge Construction EngineersProvide pile type with maximum resistanceProvide pile type with maximum resistanceIdentify verification Identify verification method(smethod(s) to use) to use

DesignersDesignersDesign with LRFD methods and loadsDesign with LRFD methods and loadsFactored loads presented on plansFactored loads presented on plansCompare with past ASD designsCompare with past ASD designs

Implementation for State AidImplementation for State Aid

Geotechnical EngineerGeotechnical EngineerProviding ultimate capacity estimates Providing ultimate capacity estimates

DesignerDesignerProvide pile type with maximum resistanceProvide pile type with maximum resistanceIdentify verification Identify verification method(smethod(s) to use) to useDesign with LRFD methods and loadsDesign with LRFD methods and loadsFactored loads presented on plansFactored loads presented on plansCompare with past ASD designsCompare with past ASD designs

ResearchResearch

Two projects rolled into one:Two projects rolled into one:Development of Resistance Factor for Development of Resistance Factor for MnDOTMnDOT Pile Driving FormulaPile Driving Formula

Study of Pile Setup Evaluation MethodsStudy of Pile Setup Evaluation Methods

Research begins this year Research begins this year

DowndragDowndrag

DowndragDowndrag is the is the downward load induced downward load induced in the pile by the settling in the pile by the settling soil as it grips the pile soil as it grips the pile due to negative side due to negative side frictionfrictionCovered in LRFD 3.11.8, Covered in LRFD 3.11.8, 10.7.1.6.2, 10.7.2.5, and 10.7.1.6.2, 10.7.2.5, and 10.7.3.710.7.3.7

DowndragDowndrag

Estimated Estimated downdragdowndrag load will be given in load will be given in the Foundation Report the Foundation Report For piles driven to rock or a dense layer For piles driven to rock or a dense layer (end bearing piles), nominal pile (end bearing piles), nominal pile resistance should be based on pile resistance should be based on pile structural capacitystructural capacity

DowndragDowndrag

For piles controlled by side friction, For piles controlled by side friction, downdragdowndrag may cause pile settlement, may cause pile settlement, which will result in reduction of the which will result in reduction of the downdragdowndrag loadloadAmount of pile settlement difficult to Amount of pile settlement difficult to calculate, so calculate, so downdragdowndrag on friction piles to on friction piles to be considered on a case by case basis be considered on a case by case basis

DowndragDowndrag

Transient loads reduce Transient loads reduce downdragdowndrag, so do not , so do not combine live load (or other transient loads) with combine live load (or other transient loads) with downdragdowndragConsider a load combination with DC + LL and Consider a load combination with DC + LL and also a load combination that includes DC + DD, also a load combination that includes DC + DD, but do not consider LL and DD within the same but do not consider LL and DD within the same load combinationload combinationDiscuss with Regional Construction Engineer Discuss with Regional Construction Engineer before using battered piles before using battered piles

Pile Lateral Load CapacityPile Lateral Load Capacity

Past Practice Using ASDPast Practice Using ASDService loads resisted by:Service loads resisted by:

battered pile componentbattered pile component++

12 kips/pile resistance12 kips/pile resistance

Current Practice Using LRFDCurrent Practice Using LRFDFactored loads resisted by:Factored loads resisted by:

battered pile componentbattered pile component++

18 kips/pile resistance18 kips/pile resistance


Parametric study conducted:Parametric study conducted:12” & 16” diameter CIP piles12” & 16” diameter CIP pilesHP10x42, HP12x53 and HP14x73HP10x42, HP12x53 and HP14x73Single layer of Single layer of noncohesivenoncohesive soil with soil with varied friction angles of 30˚, 32˚, 34˚, varied friction angles of 30˚, 32˚, 34˚, 36˚, and 38˚36˚, and 38˚ENSOFT program LENSOFT program L--Pile 5.0.30 used for Pile 5.0.30 used for this study this study


Piles under combined axial compressive Piles under combined axial compressive load and moment due to axial and lateral load and moment due to axial and lateral loads at the top of pilesloads at the top of piles

LRFD 6.9.2.2 interaction equation:LRFD 6.9.2.2 interaction equation:

0.1M

M98

PP

nf

u

nc

u ≤⎟⎟⎠

⎞⎜⎜⎝

⎛+

φφ


Inserting known values for Inserting known values for PPuu, , φφccPPnn, , φφffMMnn, , interaction equation solved for interaction equation solved for MMuu

Lateral load applied at top of pile and Lateral load applied at top of pile and increased until the calculated maximum increased until the calculated maximum MMuu was reached in the pilewas reached in the pile


Results:Fy Wall t φRnh

(ksi) (in.) (kips)12" CIP 45 all 2416" CIP 45 1/4 2816" CIP 45 5/16 4016" CIP 45 3/8 4016" CIP 45 1/2 40

HP 10x42 50 NA 24HP 12x53 47.8 NA 32HP 14x73 43.9 NA 40

Pile Type


Results:Max deflection due to factored loadswas approximately 0.5”

Serviceability does not govern

Drilled Shaft DesignDrilled Shaft Design

Design process is interactiveDesigner, Regional Construction Engineer, and geotechnical engineer need to discuss:

Proposed construction methodPermanent vs. temporary casingShaft diameterVertical & horizontal loads for multiple row shaft foundationLoads & moment for single shaftsRock sockets


Resistance factors vary:

Tip/side resistanceLoad testsBase grouting


Existing foundation load tables given in MnDOT Bridge Design Manual Appendix 2-H do not include drilled shaftsSpread footing load tables were used in the pastNew load tables to be created for drilled shafts

QuestionsQuestions

deep foundations

Documents

regional construction

ensure required

max driving

mndot lrfd

soil structural

past asd designs

pile capacity

mndot formula