Modeling Unbound Base And Subgrade Layers For Pavement ...

TURNER-FAIRBANK HIGHWAY RESEARCH CENTER
Modeling Unbound Base And Subgrade Layers For
Pavement Performance Prediction
Cheryl Allen Richter
Federal Highway Administration
Office of Infrastructure Research & Development
Turner-Fairbank Highway Research Center
McLean, VA
Research that is Essential, Indispensable,
and Connected to our Customers

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Modeling Unbound Base and Subgrade Layers
for Pavement Performance Prediction
• A little history
• What we know
• The challenges before us

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TURNER-FAIRBANK HIGHWAY RESEARCH CENTER
Soil & Aggregate in AASHTO Design – Flexible
1972 Soil Support Value
a
1986
1993
2007
Effective Roadbed Soil
Resilient Modulus
a as f(E)
Effective Roadbed Soil
Resilient Modulus
a as f(E)
Stress Dependent
Resilient Modulus**

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TURNER-FAIRBANK HIGHWAY RESEARCH CENTER
Soil & Aggregate in AASHTO Design – Rigid
1972 Modulus of Subgrade
Reaction
1986
1993
1996
2007
Effective Modulus of
Subgrade Reaction
Effective Modulus of
Subgrade Reaction
Effective Modulus of
Subgrade Reaction
Resilient Modulus**
(converted to k)

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Unbound Base and Subgrade Layers: What We Know
Structural Properties (M r or E) vary with…
• Material factors
• Density
• Gradation
• Particle angularity
• Surface texture
• Plasticity
•…

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Unbound Base and Subgrade Layers: What We Know
Structural Properties (M r or E) vary with…
• Environmental conditions
• Moisture
• Temperature (frost)

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450
400
350
Modulus, MPa
300
250
200
150
100 A-1-a Base
50
A-4 Upper Subgrade
0
A-4 Lower Subgrade
8/1/1994 2/17/1995 9/5/1995 3/23/1996

TURNER-FAIRBANK HIGHWAY RESEARCH CENTER
Unbound Base and Subgrade Layers: What We Know
Structural Properties (M r or E) vary with…
• Stress state
• Confinement (overburden)
• Load-induced

TURNER-FAIRBANK HIGHWAY RESEARCH CENTER
So what do we do about modeling

TURNER-FAIRBANK HIGHWAY RESEARCH CENTER
ME PDG Approach - Stress (Level 1)
M
r
=
k
1
P
a
⎛
⎜
⎝
θ
P
a
⎞
⎟
⎠
k
2
⎛
⎜
⎝
τ
oct
P
a
+
⎞
1⎟
⎠
k
3
τ
oct
=
1
3
( σ
1
θ =
− σ
2
)
σ
2
1
+
+ σ
( σ
1
2
−
+
σ
σ
3
3
)
2
+
( σ
2
−
σ
3
)
2

TURNER-FAIRBANK HIGHWAY RESEARCH CENTER
So what do we do about modeling
ME PDG Approach – Moisture (All Levels)
log
M
M
r
ropt
=
a
a
+
=
1 + EXP
min
⎡
⎢
ln
⎣
[ log( M / M )]
r
b
− b
a
− a
+
k
ropt
m
* ( S
−
S
opt
⎤
)
⎥
⎦
b
=
max
[ log( M / M )]
r
ropt

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The Challenges Before Us

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E = k 1 P a (θ/P a ) k2 (τ/P a +1) k3
EXPECTATION
k 2 > 0
k 3 < 0
REALITY
k 2 < 0
k 3 > 0

TURNER-FAIRBANK HIGHWAY RESEARCH CENTER
The Challenges Before Us
Applied Stresses
Lab
Measured
Independent of M r
Compressive
Independent
Apply to Sample
Field
Computed
NOT Independent of E
Sometimes Tensile
NOT Independent
Point-Specific

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400
Vertical Stress
0
-100 200
Radial Stress

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Bulk Stress
Octhadral Shear Stress

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Implications for Modeling
M
r
=
k
1
P
a
⎛
⎜
⎝
θ
P
a
⎞
⎟
⎠
k
2
⎛
⎜
⎝
τ
oct
P
a
+
⎞
1⎟
⎠
k
3

TURNER-FAIRBANK HIGHWAY RESEARCH CENTER
The Challenges Before Us
Improvements in:
• Pavement Structural Models
• Material Constitutive Models
• Testing & Data Interpretation Procedures
(FWD & Backcalculation)

TURNER-FAIRBANK HIGHWAY RESEARCH CENTER
The Challenges Before Us

TURNER-FAIRBANK HIGHWAY RESEARCH CENTER
TURNER-FAIRBANK HIGHWAY RESEARCH CENTER
Soil & Aggregate in AASHTO Design
1972
1986
1993
1996
2007
20
Modulus of Subgrade
Reaction
Effective Roadbed
Resilient Modulus or
Modulus of Subgrade
Reaction, a as f(Mr)
Stress Dependent
Reslient Modulus **
(converted to k for PCC)
"Universal" foundation
parameter(s) as f(stress,
moisture…)

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Thank you!