subgrade: it’s just dirt. isn’t it? · factors influencing subgrade performance • soil type...

Subgrade: It’s Just Dirt. Isn’t it?

Daniel N. Fréchette, PhD, PE

Senior Associate Geotechnical Engineer,

Amec Foster Wheeler Environment & Infrastructure, Phoenix, AZ

Daniel.Frechette@amecfw.com

Outline of Presentation

• Introduction

• Pavement Components

• Specifications

• Factors that Influence Subgrade Performance

• Mitigation Strategies for Poor Subgrade

• Case Study – I-17 Widening Project

• Summary

Sustainable Asphalt Pavements Workshop2 March 22, 2017

Introduction

• What is Subgrade?

• MAG –

• ADOT –

• FHWA –

• DNF – ?

Sustainable Asphalt Pavements Workshop3 March 22, 2017

Pavement Components

4 Sustainable Asphalt Pavements Workshop March 22, 2017

Pavement Components

5 Sustainable Asphalt Pavements Workshop March 22, 2017

Pavement Components

6 Sustainable Asphalt Pavements Workshop March 22, 2017

Summary of Pavement Components

• Manufactured/Produced Components

• Portland Cement Concrete

• Asphaltic Concrete

• Base Course

• Naturally Occurring

• Subgrade

7 Sustainable Asphalt Pavements Workshop March 22, 2017

Construction Specifications –Portland Cement Concrete

Sustainable Asphalt Pavements Workshop8 March 22, 2017

Construction Specifications –Asphaltic Concrete

Sustainable Asphalt Pavements Workshop9 March 22, 2017

Construction Specifications –Aggregate Base

Sustainable Asphalt Pavements Workshop10 March 22, 2017

Construction Specifications –Subgrade

Sustainable Asphalt Pavements Workshop11 March 22, 2017

Factors Influencing Subgrade Performance

• Soil Type

• Plasticity

• Percent fines (% < No. 200)

• Swell potential

• Collapse potential

• Environmental Effects/ Climatic Conditions

• Elevation

• Temperature

• Precipitation/Moisture

• Drainage

12 Sustainable Asphalt Pavements Workshop March 22, 2017

Soil Type

13 Sustainable Asphalt Pavements Workshop March 22, 2017

• Soils Map of Arizona

(USDA-NRCS)

Swell Potential Map – Phoenix Area

14 Sustainable Asphalt Pavements Workshop March 22, 2017

Swell Potential Map – Tucson Area

15 Sustainable Asphalt Pavements Workshop March 22, 2017

Collapsible Soils

16 Sustainable Asphalt Pavements Workshop March 22, 2017

Environmental Effects/ Climatic Conditions

• Elevation

• Temperature

• Precipitation

• Seasonal Variation Factor

17 Sustainable Asphalt Pavements Workshop March 22, 2017

Seasonal Variation Factor vs Elevation

18 Sustainable Asphalt Pavements Workshop March 22, 2017

Seasonal Variation Factor vs Temperature

19 Sustainable Asphalt Pavements Workshop March 22, 2017

Seasonal Variation Factor vs Precipitation

20 Sustainable Asphalt Pavements Workshop March 22, 2017

Mitigation Strategies for Poor Subgrade

• Over-Excavation and Replace

• Over-Excavation and Recompaction

• Geogrid with Geotextile

• Lime or Cement Treated

• Alternative Engineering Solutions

21 Sustainable Asphalt Pavements Workshop March 22, 2017

Case Study – I-17 Widening

• Project Location

• Project Soils

• Subgrade Characterization (R-Value Evaluation)

• Alternatives Evaluated

• Final Design

22 Sustainable Asphalt Pavements Workshop March 22, 2017

Case Study – I-17 Widening

23 Sustainable Asphalt Pavements Workshop March 22, 2017

Case Study – I-17 Widening

• Swell Potential Map – Phoenix Area

24 Sustainable Asphalt Pavements Workshop March 22, 2017

Case Study – I-17 Widening Project Soils

25 Sustainable Asphalt Pavements Workshop March 22, 2017

• Subgrade Criteria

• Percent Passing No. 200 Sieve

• Plasticity Index (PI)

Case Study – I-17 Widening

March 22, 2017Sustainable Asphalt Pavements Workshop26

• 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

27 Sustainable Asphalt Pavements Workshop March 22, 2017

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

28 Sustainable Asphalt Pavements Workshop March 22, 2017

Summary

• What is Subgrade?

• MAG –

• ADOT –

• FHWA –

• DNF –

• Subgrade: Foundation for the pavement

• Largest variability, Know the least about

Sustainable Asphalt Pavements Workshop29 March 22, 2017

30