Modeling Unbound Base And Subgrade Layers For Pavement ...

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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


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


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**


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


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• …


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)


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


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


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


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


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

=

=

⎥⎦⎤

⎢⎣⎡ −+

−+

−+=


The Challenges Before UsThe Challenges Before Us


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

EXPECTATIONk2 > 0 k3 < 0

REALITYk2 < 0k3 > 0


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


0

400

-100 200Radial Stress

Ver

tical

Str

ess


Octhadral Shear Stress

Bul

k St

ress


Implications for ModelingImplications for Modeling

32

11

k

a

oct

k

aar PP

PkM ⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛=

τθ


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

(FWD & Backcalculation?)


The Challenges Before UsThe Challenges Before Us


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…)


Thank you!Thank you!

Modeling Unbound Base And Subgrade Layers For Pavement ...

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