subgrade: it’s just dirt. isn’t it? · factors influencing subgrade performance • soil type...
TRANSCRIPT
Subgrade: It’s Just Dirt. Isn’t it?
Daniel N. Fréchette, PhD, PE
Senior Associate Geotechnical Engineer,
Amec Foster Wheeler Environment & Infrastructure, Phoenix, AZ
Outline of Presentation
• Introduction
• Pavement Components
• Specifications
• Factors that Influence Subgrade Performance
• Mitigation Strategies for Poor Subgrade
• Case Study – I-17 Widening Project
• Summary
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Introduction
• What is Subgrade?
• MAG –
• ADOT –
• FHWA –
• DNF – ?
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Pavement Components
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Pavement Components
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Pavement Components
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Summary of Pavement Components
• Manufactured/Produced Components
• Portland Cement Concrete
• Asphaltic Concrete
• Base Course
• Naturally Occurring
• Subgrade
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Construction Specifications –Portland Cement Concrete
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Construction Specifications –Asphaltic Concrete
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Construction Specifications –Aggregate Base
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Construction Specifications –Subgrade
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Factors Influencing Subgrade Performance
• Soil Type
• Plasticity
• Percent fines (% < No. 200)
• Swell potential
• Collapse potential
• Environmental Effects/ Climatic Conditions
• Elevation
• Temperature
• Precipitation/Moisture
• Drainage
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Soil Type
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• Soils Map of Arizona
(USDA-NRCS)
Swell Potential Map – Phoenix Area
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Swell Potential Map – Tucson Area
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Collapsible Soils
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Environmental Effects/ Climatic Conditions
• Elevation
• Temperature
• Precipitation
• Seasonal Variation Factor
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Seasonal Variation Factor vs Elevation
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Seasonal Variation Factor vs Temperature
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Seasonal Variation Factor vs Precipitation
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Mitigation Strategies for Poor Subgrade
• Over-Excavation and Replace
• Over-Excavation and Recompaction
• Geogrid with Geotextile
• Lime or Cement Treated
• Alternative Engineering Solutions
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Case Study – I-17 Widening
• Project Location
• Project Soils
• Subgrade Characterization (R-Value Evaluation)
• Alternatives Evaluated
• Final Design
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Case Study – I-17 Widening
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Case Study – I-17 Widening
• Swell Potential Map – Phoenix Area
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Case Study – I-17 Widening Project Soils
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• Subgrade Criteria
• Percent Passing No. 200 Sieve
• Plasticity Index (PI)
Case Study – I-17 Widening
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• R-Value Data
Case Study – I-17 Widening Alternatives Evaluated
Alternatives Evaluated
• Over-Excavation and Replace
• Viable alternative
• Over-Excavation and Recompaction
• Not viable, soils not collapsible
• Geogrid with Geotextile
• Strain compatibility concerns
• Lime or Cement Treated
• ADOT did not want to pursue
• Alternative Engineering Solutions
• Evaluate impact of lower R-value on pavement thickness
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Case Study – I-17 Widening Alternatives Evaluated
Alternatives Evaluated
• Over-Excavation and Replace
• Viable alternative
• Over-Excavation and Recompaction
• Not viable, soils not collapsible
• Geogrid with Geotextile
• Strain compatibility concerns
• Lime or Cement Treated
• ADOT did not want to pursue
• Alternative Engineering Solutions
• Evaluate impact of lower R-value on pavement thickness
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Summary
• What is Subgrade?
• MAG –
• ADOT –
• FHWA –
• DNF –
• Subgrade: Foundation for the pavement
• Largest variability, Know the least about
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