Increase Use of Spread Footings on Soils to Support Highway Bridges - - PowerPoint PPT Presentation

2019 Midwest Geotechnical Conference Columbus, OH September 17-19

Increase Use of Spread Footings on Soils to Support Highway Bridges

By: Dr. Naser Abu-Hejleh, P.E. FHWA Resource Center, Geo/Hydro Team Geotechnical Engineering Specialist

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Spread Footing

Soil

Spread Footings on Soils to Support Bridges

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Advantages of Spread Footings

Cost/time savings in foundation design, construction and maintenance:

• Simpler and more flexible design and construction
• Use common materials, equipment, and labour
• Construction: safer and fewer problems/claims
• Maintenance: safer and less disruption to traffic
• Address issues with using deep foundations

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• Use and performance of bridges supported by spread footings
• Selection of spread footings
• Bridge tolerable settlement (covered later).

Spread footing = Spread footings bearing on soils to support highway bridges

2007-2010 FHWA Surveys of State

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• States with extensive use
• States with no or limited use

Spread Footings

• n Soils

Spread Footings

• n Rock

Driven Piles Drilled Shafts

11.5 %

12.5 % 56.5 % 19.5 %

Distribution of State DOTs Use of Bridge Foundations

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States Spread Footings (%) Deep Foundations (%) Soil Rock Driven Piles Drilled Shafts

Northeast States

Connecticut 50 25 20 5 Vermont 40 10 45 5 Massachusetts 35 15 20 27 New Hampshire 30 30 30 10 New York 30 15 47 3 New Jersey 30 20 40 5

Southwest States

New Mexico 30 10 30 30 Nevada 25 3 18 54 Arizona 20 5

Northwest States

Idaho 20 10 60 10 Oregon 20 10 60 10

States with Extensive Use of Spread Footings

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No Use Limited Use (<10%) Midwest

Iowa, Missouri Illinois, Indiana, Wisconsin, Minnesota, Ohio

Northeast

West Virginia Maine, Virginia, Maryland

Southeast

All States

Southwest

Texas, Arkansas Colorado, Utah

Northwest

South Dakota, North Dakota Wyoming, Hawaii FHWA Conclusion: Use of spread footings when appropriate is not considered by many State DOTs

States with No or Limited Use of Spread Footings

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• Due to “perceived obstacles.”
• Not due to valid obstacles (i.e., scour)

These states are missing an opportunity to save time and money by not considering spread footings

States with No or Limited Use of Spread Footings. Why?

Photo credit: Derrick Dasenbrock,

• MnDOT. Used with permission

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Promote the use of spread footings on soils to support highway bridges when appropriate. Per AASHTO/FHWA consider spread footings bearing on:

• Competent natural soils
• Improved natural soils
• Engineered granular fills (embankment)
• Engineered MSE fills (walls, embankments)

FHWA Goals

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Reference to Achieve FHWA Goals

https://www.fhwa.dot.gov/resourcecenter/teams/geohydraulics/spreadfootings.pdf

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• 1. Identify perceived obstacles in using spread footings
• 2. Develop:
• Recommendations to address perceived obstacles
• Guidance to implement the recommendations

Approach Used in the FHWA Reference

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• 1. Limited knowledge of good performance and

successful use and selection of spread footings

• 2. Use of very conservative settlement analysis
• Concern of excessive bridge settlement that are

costly and difficult to repair

9 Obstacles States Main Obstacles

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1. Deploy AASHTO/FHWA technical resources

• 2. Review FHWA surveys of State DOTs for use,

performance, and selection of spread footing

• 3. Consider spread footing on granular/MSE fills and with

semi-integral and integral abutments.

• 4. Consider load tests and instrumentation programs

5. Deploy adequate subsurface investigation, construction, and quality control procedures.

8 Recommendations to Address Perceived Obstacles

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• 6. Deploy a rational procedure for settlement analysis of

bridges supported on spread footings bearing on soils

• 7. Develop a rational procedure to determine the LRFD design

bearing resistances for spread footings

• 8. Based on previous recommendations, develop LRFD

Guidance for:

• Selection of spread footings, and
• Design of spread footings

8 Recommendations to address Perceived Obstacles

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• 1. Use and Performance of bridges supported by spread footings

2. Selection of spread footings 3. Bridge tolerable Settlement (Covered later)

• I. Use, Performance, and Selection of Spread Footings

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State Soils (%) Rocks (%) Performance

Connecticut 50 25 Good performance Vermont 40 10 Good performance Massachusetts 35 15 Good performance New Hampshire 30 20 Good performance New York 30 15 Good performance New Jersey 30 20 Good performance Delaware 13 4 Good performance Pennsylvania 10-20 45-55 Good performance Rhodes Island 10 Good performance Maine 5 31 Good performance Virginia 5 30 Good performance Maryland 2-4 Good performance West Virginia 20 No use

Use and Performance of Bridges Supported by Spread Footings Northeast States

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State Soils (%) Rocks (%)

Use and Performance

Michigan 10 5 Hundreds of bridges with spread footings were constructed (70% before 1980, reduced to 50% by 1990, and currently 10%). Their overall performance is adequate and as bridges with deep foundations. Illinois 5 10 Very limited use with MSE walls. To support piers with very hard tills and dense sand). No performance or movement problems with these bridges. Wisconsin 5 10 Roughly 75 bridges supported on stiff natural soils in the last 10 years. Very limited use with MSE walls. Mix use of abutment piling and spread footing piers. These bridges are performing well or as good as bridges with piles” Indiana 1 5 Recently allowed spread footers for median piers in the accelerate I-465 project. Considered with glacial tills, IGM’s and engineered fills and in process to allow them over MSE walls. There has been no visible evidence of excessive settlement.” Minnesota 7 2 Recently used spread footing in simple span bridges (at abutments only) on dense sand and gravel at a rate of around 4 bridges per year. “Bridges appeared to be in fine shape and perform well.” Ohio 5 1 Since January, 1998 built 244 structures on spread footing on MSE walls and rocks. Currently use of spread footings on MSE walls is not permitted, and allowed with dense sand and in few cases with very stiff clays. Problems are not observed Missouri ? 5 No use/not allowed Iowa

Use and Performance of Bridges Supported by Spread Footings Midwest States

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State Soils (%) Rocks (%) Use and Performance

New Mexico 30 10 Extensive use of spread footings on MSE walls (30 out of the 55 bridges in the I-25/I-40 interchange). Also, used on abutments on embankments, to support piers, for single and multi span “Performed well, better than deep foundations as there is no bridge bump.” Nevada 25 3 With all types of bridges. Not allowed on MSE fills. “No known Issues, they are performing well” Arizona 20 5 Success with spread footings on MSE walls. Performance is not reported, but expected to be OK California 5% (30% -50% in South California Significant savings. With any type of bridge. “ Performed very well, no indications of poor performance” Utah 8 5 Mostly single span bridges. Not allowed on MSE fills. “Performed well” Colorado 3 bridges Two bridges on MSE walls, 3rd bridge on 2:1 approach embankment. Costly with MSE walls! “All three bridges are performing well” Texas No use Kansas

• No use

Use and Performance of Bridges Supported by Spread Footings Southwest States

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State Soils (%) Rocks (%) Performance

Idaho 20 10 “Not aware of any performance issues with spread footings” Oregon 20 10 “Not aware of any performance issues with spread footings” Washington 10 25 “Not aware of any performance issues with spread footings Nebraska 10 “Not aware of any performance issues with spread footings” Montana 10 5 “Not aware of any performance issues with spread footings” Wyoming 5 17 “Not aware of any performance issues with spread footings” Alaska 5 “Not aware of any performance issues with spread footings” Hawaii 7 2 “Not aware of any performance issues with spread footings” South Dakota 5 No use North Dakota

• No use

Use and Performance of Bridges Supported by Spread Footings Northwest States

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• Minnesota DOT. In recent projects:
• Settlement < 1”
• Use of spread footing is increasing (DB, CMGC)
• Ohio DOT. 1990-2006, 54 bridges:
• All bridges are in good conditions
• Spread footing are viable option for supporting bridges
• Colorado DOT. Founders/Meadows Bridge (1998):
• Excellent performance with no bridge bump problem

Reported Performance from Instrumented Bridges

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• Use of spread footings by states varies significantly (0 to 50%)
• Significant use in the Northeast
• Some states do not consider spread footings even when they

are appropriate

• Good performance and economical use reported by all

states that used spread footings

Conclusion: many states are missing an opportunity to save time and money by not considering spread footings when appropriate

Summary: Use and Performance of Spread Footings to Support Bridges

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• States considered (see FHWA Reference for more details):
• Type of foundation soils
• Favorable and unfavorable conditions
• Bridges

supported

• n

spread footings bearing

• n

recommended soils and fills have been safely and economically constructed by State DOTs

Summary: Selection of Spread Footings on Soils to Support Bridges

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• 1. Service Limit State for Settlement
• 2. Bridge Tolerable Settlement
• 3. Foundation and Bridge Settlements
• 4. Summary and Recommendations
• IIII. Rational Settlement Analysis of Bridges

Supported on Spread Footings Bearing on Soils

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• Bridge foundation settlement (SF). Due to loads

transferred to the foundation during

• a. Placement of bridge substructure (i.e., piers)
• b. Placement of bridge superstructure (i.e. deck, girders)
• c. After construction due to traffic loads
• Bridge settlement at foundation locations, SB ≤ SF
• Foundation settlements during stages b and c
• Bridge settlement at foundation locations that

impacts bridge performance, SBP ≤ SB

• 1. Service Limit State for Settlement

Three Types of Settlements

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Bridge Settlements: Uniform, Differential, Angular Distortion

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• Bridge differential settlement =

larger of bridge settlement at both ends of the span

Computed settlement profile Assumed settlement profile

• Bridge tolerable settlement (SBT) = Bridge tolerable

differential settlement

Conservative Settlement Analysis

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SBP ≤ SBT

• SBP: Bridge settlement that impacts bridge performance
• SBT: Bridge Tolerable settlement

Note: Spread footing performance is ensured with addressing just the strength and extreme event limit states

Service Limit State for Settlement of Bridge (not Foundation)

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• Preliminary Design. Develop acceptable range of

tolerable settlements based on:

• A. Settlement measurements of bridges performed well during their

design lives

• B. Successful Practices of State DOTs
• Final Design: Develop bridge specific tolerable settlement
• 2. Bridge Tolerable Settlements (SBT)

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Reference Recommendations Conditions FHWA (2006a, 1985) Angular distortion of

• 0.004: simple span
• 0.008: continuous

span 56 simple span, 119 continuous span bridges.

• Span length > 50 ‘
• Not for rigid structures
• No integral abutments

FHWA (1982)

• Settlement: 1-3”

28 bridges FHWA (1987)

• Settlement: 1”

21 bridges FHWA (2010b) Settlements

• 1” for 69 bridges
• 1”-2” for 7 bridges

87 bridges

• A. Settlement Measurements of Bridges

Performed Well During their Service Life

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• 1” Settlement: most states
• Settlement > than 1”: Maine and Massachusetts (2”),

California (1”-2”), Utah (1.5”)

• FHWA (1982), differential settlement:
• < 1” for continuous bridges
• < 1.5”-2” for simple span bridges
• B. Successful Practices of State DOTs

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• Structural tolerance of the bridge: Perform structural analysis of

the bridge during various construction stages under various bridge settlements to evaluate

• Bridge: type, materials, size, design life, importance,

aesthetic, and past experience

• Tolerance of structures associated with bridge

Final Design: Bridge Specific Tolerable Settlement

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• Foundation settlement is the summation of elastic or

immediate and consolidation settlements

• Hough and Schmertmann methods for immediate settlements
• FHWA (2006a): settlement of cohesive soils and structural fill
• AASHTO: live loads may be omitted from consolidation

settlement of clays

• 3. Foundation and Bridge Settlements

Foundation Settlements (SF)

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Bridge Settlement that Impacts Bridge Performance (SBP)

Don’ t consider foundation settlement that occurs:

• Before placement of bridge superstructure.
• During placement of bridge superstructure but can be

accommodated or corrected with no impact on bridge performance (FHWA 987, 2010):

• Foundation settlement occurs prior to bridge deck may not impact

bridge performance. For example,

• Settlements due placement of girders.

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Bridge Settlement that Impacts Bridge Performance (SBP)

• FHWA (987, 2010):
• 60% to 75% of SF occurs before placement of bridge superstructure
• SBP is 25% to 50% of SF
• Roles for Project Engineers:
• Geotechnical Engineer: compute SF at various stages
• Structural Engineer: finalize SBP ≤ SB ≤ SF
• Do not consider bridge settlements that can be accommodated

by the bridge or corrected during construction with no impact on bridge performance

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Stage Foundation Compression Load by Structural Engineer Foundation Settlement (SF) by Geotechnical Engineer Bridge Settlement that Impact Bridge Performance (SBP) by Structual Enginneer 1 Q1 SF1 2 Q2 SF2 3 Q3 SF3 SF3 - SF2 4 Qse SFse SFe - SF2

Settlement Analysis by both Project Geotechnical and Structural Engineers

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• Presented more accurate/economical settlement

analysis than commonly considered in practice

• Much smaller computed settlements
• Larger tolerable settlement
• Service limit state for settlement is for bridges not

for footings

• Consider bridge settlement that impacts bridge performance

not footing settlement

• 4. Summary and Recommendations

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• Bridges with spread footings on soils can

perform well with respect to settlement.

• Concerns of bridge settlement should not limit

State DOTs from considering spread footings on soils to support highway bridges

Summary and Recommendations

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• Implement FHWA recommendations as suggested. Why?
• Address concerns/obstacles with using spread footings
• Help to develop more accurate and economical LRFD design

guidance for selection and design of spread footings.

Needed mostly by State DOTs with limited or no use of spread footings bearing on soils to support highway bridges

• III. Implementation

Benefits: Cost/time saving in highway construction projects

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• Implemented FHWA Recommendations
• Revised 2017 Geotechnical Manual
• More reliable settlement predictions to validate design
• CPT, DMT, Pressuremeter
• Performance Monitoring (survey targets, SAA)
• More used on DB, DBB, and CMGC projects
• Used when appropriate (not when scour is a concern).
• Large settlement allowed in some cases

Minnesota DOT: Increased Use of Spread Footings

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• Dr. Naser Abu-Hejleh, P.E.

FHWA Resource Center naser.abu-hejleh@dot.gov; (708) 283-3550

Questions/Comments Thank Your