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Modeling Unbound Base And Subgrade Layers For Pavement Performance Prediction

Cheryl Allen Richter

Federal Highway AdministrationOffice of Infrastructure Research & Development

Turner-Fairbank Highway Research Center McLean, VA

Modeling Unbound Base And Subgrade Layers For Pavement Performance Prediction

Cheryl Allen Richter

Federal Highway AdministrationOffice of Infrastructure Research & Development

Turner-Fairbank Highway Research Center McLean, VA

Research that is Essential, Indispensable, and Connected to our Customers

Research that is Essential, Indispensable, and Connected to our Customers

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Modeling Unbound Base and Subgrade Layers for Pavement Performance Prediction

Modeling Unbound Base and Subgrade Layers for Pavement Performance Prediction

• A little history• What we know• The challenges before us

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Soil & Aggregate in AASHTO Design – FlexibleSoil & Aggregate in AASHTO Design – Flexible

2007

1993

1986

1972

Soil

Supp

ort V

alue

a Ef

fect

ive

Road

bed

Soil

Resi

lient

Mod

ulus

a as

f(E)

Effe

ctiv

e Ro

adbe

d So

il Re

silie

nt M

odul

usa

as f(

E)

Stre

ss D

epen

dent

Re

silie

nt M

odul

us**

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Soil & Aggregate in AASHTO Design – RigidSoil & Aggregate in AASHTO Design – Rigid

1986

1993

1996

2007

1972

Mod

ulus

of S

ubgr

ade

Reac

tion

Effe

ctiv

e M

odul

us o

f Su

bgra

de R

eact

ion

Effe

ctiv

e M

odul

us o

f Su

bgra

de R

eact

ion

Resi

lient

Mod

ulus

**

(con

verte

d to

k)

Effe

ctiv

e M

odul

us o

f Su

bgra

de R

eact

ion

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Unbound Base and Subgrade Layers: What We KnowUnbound Base and Subgrade Layers: What We KnowStructural Properties (Mr or E) vary with…

• Material factors• Density• Gradation• Particle angularity• Surface texture• Plasticity• …

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Unbound Base and Subgrade Layers: What We KnowUnbound Base and Subgrade Layers: What We KnowStructural Properties (Mr or E) vary with…

• Environmental conditions• Moisture• Temperature (frost)

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0

50

100

150

200

250

300

350

400

450

8/1/1994 2/17/1995 9/5/1995 3/23/1996

Mod

ulus

, MPa

A-1-a BaseA-4 Upper SubgradeA-4 Lower Subgrade

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Unbound Base and Subgrade Layers: What We KnowUnbound Base and Subgrade Layers: What We Know

Structural Properties (Mr or E) vary with…

• Stress state • Confinement (overburden)• Load-induced

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So what do we do about modeling?So what do we do about modeling?

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ME PDG Approach - Stress (Level 1)ME PDG Approach - Stress (Level 1)

32

11

k

a

oct

k

aar PP

PkM ⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛=

τθ

232

231

221

321

)()()(31 σσσσσστ

σσσθ

−+−+−=

++=

oct

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So what do we do about modeling?ME PDG Approach – Moisture (All Levels)

So what do we do about modeling?ME PDG Approach – Moisture (All Levels)

[ ][ ])/log(max

)/log(min

)(*ln1log

roptr

roptr

optmropt

r

MMb

MMa

SSkabEXP

abaMM

=

=

⎥⎦⎤

⎢⎣⎡ −+

−+

−+=

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The Challenges Before UsThe Challenges Before Us

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E = k1Pa (θ/Pa)k2(τ/Pa+1)k3E = k1Pa (θ/Pa)k2(τ/Pa+1)k3

EXPECTATIONk2 > 0 k3 < 0

REALITYk2 < 0k3 > 0

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The Challenges Before UsApplied Stresses

The Challenges Before UsApplied Stresses

Lab FieldMeasuredIndependent of MrCompressiveIndependent Apply to Sample

ComputedNOT Independent of ESometimes TensileNOT Independent Point-Specific

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0

400

-100 200Radial Stress

Ver

tical

Str

ess

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Octhadral Shear Stress

Bul

k St

ress

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Implications for ModelingImplications for Modeling

32

11

k

a

oct

k

aar PP

PkM ⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛=

τθ

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The Challenges Before UsThe Challenges Before UsImprovements in:• Pavement Structural Models• Material Constitutive Models• Testing & Data Interpretation Procedures

(FWD & Backcalculation?)

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The Challenges Before UsThe Challenges Before Us

TURNER-FAIRBANK HIGHWAY RESEARCH CENTERTURNER-FAIRBANK HIGHWAY RESEARCH CENTER

Soil & Aggregate in AASHTO DesignSoil & Aggregate in AASHTO Design

1986

1993

1996

2007

1972

20??

Mod

ulus

of S

ubgr

ade

Reac

tion

Effe

ctiv

e Ro

adbe

d Re

silie

nt M

odul

us o

r M

odul

us o

f Sub

grad

e Re

actio

n, a

as f

(Mr)

Stre

ss D

epen

dent

Re

slie

nt M

odul

us *

* (c

onve

rted

to k

for P

CC)

"Uni

vers

al" f

ound

atio

n

para

met

er(s

) as f

(stre

ss,

moi

stur

e…)

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Thank you!Thank you!