Geoscience Reference
In-Depth Information
13.25 Typical relationship of modulus with subgrade CBR
•
This is the resilient modulus value (dynamic modulus of elasticity), which is
significantly higher than the foundation (secant) modulus.
•
The CBR Test is carried out at a high strain level and low strain rate while sub-
grades under pavements experience a relatively low strain level and higher stress
rates.
•
Design Modulus
=
Equivalent Modulus, which is dependent on materials above
and below.
Table 13.25
CBR/modulus subgrade relationships.
Reference
Relationship
Comments
E
(MPa) based on
CBR
=
2% CBR
=
5% CBR
=
10%
Heukelom and
E
∼
10 CBR
Most common relationship
20
50
N/A
Klomp (1998)
(actually 10.35
(Range of 20 to 5 for upper
CBR)
to lower bound). CBR
<
10%
Croney and
E
6.6 CBR
Zone defined by E
10 CBR
13
33
66
=
=
Croney (1991)
(from repeat load to E
=
20 CBR using wave
test data -
velocity tests - low strain
significant strain)
NAASRA (1950) E
16.2 CBR
0
.
7
For CBR
5%
26
50
81
=
<
E
22.4 CBR
0
.
5
For CBR
5%
=
>
Powell, Potter,
E
17.6 CBR
0
.
64
A lower bound relationship
27
49
77
=
Mayhew and
(TRRL Study)
Nunn (1984)
For CBR
12%
<
Angell (1988)
E
19 CBR
0
.
68
For CBR
15%
30
57
91
=
<
•
For weathered rock subgrade E
=
2,000MPa (typically)
•
For competent unweathered rock subgrade E
=
7,000MPa (typically)
13.26 Typical relationship of modulus with base course CBR
•
A laboratory CBR value can be achieved in the field only with a suitable underlying
subgrade.
Table 13.26
CBR/modulus base relationships.
Reference
Relationship
Comments
E
(MPa) based on
CBR
20%
CBR
50%
CBR
80%
=
=
=
AASHTO (1993)
E
=
36 CBR
0
.
5
For CBR
>
10%
88
109
134
NAASRA (1950)
E
=
22.4 CBR
0
.
5
For CBR
>
5%
100
142
200
Queensland Main
E
=
21.2 CBR
0
.
64
For CBR
>
15%
144
225
350
Roads (1988) Maximum of 350 MPa
Minimum Subgrade Modulus for Base CBR modulus to apply
3.5%
7.5%
15%
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