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LRFD and Geotechnical Design andConstruction

September 6, 2007

Jeff Horsfall

Wisconsin Department of Transportation


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Discussion Topics

•WisDOT Geotechnical guidelinesand variances from AASHTO code

•Process Description•Plan Documents

•New Pile Driving Specification


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Session Goals:

How the WisDOT Policies vary from

the AASHTO Code and Guidelines

Process for WisDOT to evaluatebridge foundations using LRFD

What to put on the Bridge PlansWhat is the new pile specification


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“ The Code is more what you call guidelinesthan an actual rule.”

Captain Barbosa to Miss Swan

aboard the Black Pearl


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WisDOT Geotechnical

Guidelines, variances from AASHTO

code and the old Bridge Manual

Old Bridge Manual•Chapter 10 Substructures - General

•Chapter 11 Piling

Revised Bridge Manual•Chapter 10 Geotechnical Investigation

•Chapter 11 Foundation Support


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Revised Bridge Manual

Chapter 10 Geotechnical Investigation•Geotechnical Bulletin #1

• AASHTO Table 10.4.2-1


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WisDOT Policy and

exceptions to the AASHTO

•Shallow Foundations


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WisDOT exception to AASHTO:

It is WisDOT’s current policy to designshallow foundations using the Allowable

Stress Design (ASD) method.

Wi DOT li i


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WisDOT policy item:

It is WisDOT’s policy to place the bottom ofshallow foundations at embedment depths of

4 feet unless founded on competent bedrock.

Wi DOT li it


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WisDOT policy item:

It is WisDOT’s policy to permit a maximum of1 inch of horizontal movement at top of

substructure unit and 1.5 inches of totalestimated settlement.

Wi DOT li it


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WisDOT policy item:

The normal ground water level is used todetermine the allowable bearing capacity of

footings.

At stream crossings, the mean annual high

water level is used to compute a factor ofsafety for bearing capacity. This factor of

safety must be reasonable, but can be lessthan that specified for design (FS=3).


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WisDOT Policy and

exceptions to the AASHTO

•Deep Foundations

Wi DOT li it


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WisDOT policy item:

WisDOT’s minimum pile spacing is 2’-6” or 2.5pile diameters, whichever is greater.

WisDOT’s maximum pile spacing is 8’-0” ,

based on abutment or pier structural designs.

WisDOT policy item:


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WisDOT policy item:

It is WisDOT’s policy to use a minimum 6-inchpile embedment in the footings, but there are

conditions where more embedment is

required.

WisDOT policy item:


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WisDOT policy item:

It is WisDOT’s policy to place the bottom ofpile-supported footings below the final

ground surface at a minimum depth of 2.5 feetfor sill abutments and at a minimum depth of

4 feet for piers and other types of abutments.

WisDOT policy item:


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WisDOT policy item:

It is WisDOT policy to limit the yield strengthto 36 ksi for steel piles.

WisDOT policy item:


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WisDOT policy item:

It is WisDOT’s policy to limit the horizontalmovement at top of substructure unit to 0.5

inch or less at the service limit state.

WisDOT policy item:


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WisDOT policy item:

WisDOT requires at least four (4) piles pergroup to support each substructure unit,

including each column for multi-columnbents.

WisDOT does not reduce geotechnicalresistance factors to satisfy redundancy

requirements to determine axial pileresistance. Hence, redundancy of resistance

factors in LRFD [10.5.5.2.3] is not applicable

to WisDOT structures.



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Recommended values of nominal lateralresistance for battered or vertical piles used

by the Structures Design Office, are asfollows:

• A detailed analysis

• A minimum value based on pile type• A table of typical values


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WisDOT policy item:


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WisDOT policy item:

It is WisDOT policy to initially compute adesign resistance capacity for piling and then

to drive the piles such that the drivenresistance capacity meets or exceeds the

design capacity.

Hence for piles prone to scour, the driven pile

capacity will exceed the design capacity tocompensate for extreme events when a

portion of the embedded pile length is

exposed.

WisDOT policy item:


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WisDOT policy item:

WisDOT policy requires a multi-column bentto be designed as a redundant rigid frame.

Hence when a bent contains at least 4

columns then the resistance factors should

be increased by 20 percent.


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Process Description

Process


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Process


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Soil Boring Example


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Soil Boring Example

SPT Hammer Efficiency Correction, AASHTO Code


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SPT Hammer

EfficiencyCorrection

FHWA Driven Software


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FHWA Driven Software


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Site Investigation Report Table


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Site Investigation Report Table

Drivability Evaluation - GRLWEAP Software


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Drivability Evaluation GRLWEAP Software


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10 ¾” Diameter CIP Pile using a D-16-32


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Plan Documents

Current Language

Abutments to be supported on HP 10 x 42


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steel piling driven to a minimum bearing valueof 55 tons per pile.

Estimated Length 70 feet.

New Language


steel piling driven to a Required DrivingResistance of 275 tons per pile as determined

by the modified Gates dynamic formula.

Estimated Length 70 feet.

New Language for General Plan Notes

The factored axial resistance of piles in


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The factored axial resistance of piles in

compression used for design is the requireddriving resistance multiplied by a resistance

factor of 0.4 using the modified Gates

dynamic formula to determine driven pile

capacity.

New Pile Driving Specification


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Purpose: To revise the currentspecification, which has three sections

•508 Timber Piling•510 Cast in Place Concrete Piling

•511 Steel Piling

to one comprehensive specification

•508 Driven Piles (modified Gates or

WEAP)

Why make the Change ?


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1. The old specification is written forwood piles and then steel piles.

2. The old specification uses the

modified ENR formula to determine an

allowable bearing, FS could ranged

from 0.6 to 20.3. The old specification does little to

address the pile driving system.4. The old specification do not discuss

pre-cast concrete piles.

5. Pile specification are over threesections


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Pile Hammer

System

WisDOT Contacts ?


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Scott Becker, Structure [email protected]

608-266-5161

Bob Arndorfer, Geotechnical Unit

[email protected]

Jeff Horsfall, Geotechnical Unit

[email protected]

608-243-5993


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Questions and Comments