Resilient Modulus for New Hampshire
Subgrade Soils for Use in
Mechanistic AASHTO Design
VINCENT C. JANOO, JOHN J. BAYER JR., GLENN D. DURELL, AND CHARLES E. SMITH JR.
SN = structural number of pavement,
INTRODUCTION
pt = terminal serviceability index.
The American Association of State Highway
Later, eq 1 was modified to account for
and Transportation Officials (AASHTO) pavement
subgrade soil types other than the type A-6
design procedure is an empirical design method
found at the AASHO Test Road. A soil support
based on the results from the AASHO road tests.
term (Si) was added to eq 1 and is shown in
The AASHO (the precursor of AASHTO) road tests
eq 2:
were conducted near Ottawa, Illinois, around 1958
to 1960. The road tests were conducted on asphalt
concrete pavements, portland cement concrete
log Wt18 = 9.36 log(SN + 1) - 0.20
pavements, and bridge decks. A total of 10
lanes were tested under controlled loading ranging
[
]
log (4.2 - pt ) / (4.2 - 1.5)
from 9-kN (2,000-lbf) single axle loads to 215-kN
+
[
]
0.40 + 1094 / (SN + 1)
(48,000-lbf) tandem axle loads. A total of 1,114,000
5.19
axle loads were applied during the road test.
The design is based on the functional proper-
1
+ 0.372(Si - 3.0) .
+ log
(2)
ties of the pavement structure, such as cracking,
R
rutting, and roughness. This change in the func-
Si for the AASHO subgrade soil was set at 3.0,
tional properties is indexed by the present service-
and Si can range between 3 and 10, with 10 repre-
ability index (PSI). The PSI ranges from 0 to 5, with
senting a crushed rock type subgrade. R is a re-
5 designating excellent conditions. On highway
gional factor introduced to account for climates
pavement, when the PSI has reached 2.5, major
other than Ottawa, Illinois.
rehabilitation is required. The original design, eq
With the introduction of the AASHTO 1996 de-
1, is a relationship between 80-kN (18-kip) axle
sign guide (AASHTO 1996a), the soil support
loads to thickness of the pavement layers and the
value was replaced by the effective resilient modu-
subgrade at the AASHO road tests:
lus Mr of the subgrade soil:
log Wt18 = 9.36 log(SN + 1) - 0.20
log Wt18 = 9.36 log(SN + 1) - 0.20 +
log[(4.2 - pt ) / (4.2 - 1.5)]
[
]
log (4.2 - pt ) / (4.2 - 1.5)
+
[
]
(1)
5.19
0.40 + 1094 / (SN + 1)
+
[
]
0.40 + 1094 / (SN + 1)
5.19
where Wt18 = 80-kN total load application at end
+ 2.32 log Mr - 8.07 .
(3)
of time t,
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