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Subgrade Physical Properties
Subgrade materials are typically characterized by (1) their
resistance to deformation underload, in other words, their
stiffness or (2) their bearing capacity, in other words,
theirstrength. In general, the more resistant to deformation a
subgrade is, the more load it cansupport before reaching a critical
deformation value. lthough there are other factorsinvolved when
evaluating subgrade materials (such as swell in the case of certain
clays),stiffness is the most common characterization. !here are
three basic subgradestiffness"strength characterizations commonly
used in the #.S.$
California bearing ratio (CBR) . simple test that compares the
bearing capacityof a material with that of a well%graded crushed
stone (thus, a high &ualitycrushed stone material should have a
' 1**+). ' is basically a measureof strength. It is primarily
intended for, but not limited to, evaluating thestrength of
cohesive materials having ma imum particle sizes less than
*.-inches ( S/!0, 2***). It was developed by the 'alifornia ivision
of /ighways
around 1 3* and was subse&uently adopted by numerous states,
counties, #.S.federal agencies and internationally. 4ost agency and
commercial geotechnicallaboratories in the #.S. are e&uipped to
perform ' tests.
Resistance value (R-Value) . test that e presses a material5s
resistance todeformation as a function of the ratio of transmitted
lateral pressure to appliedvertical pressure. It is essentially a
modified tria ial compression test. 4aterialstested are assigned an
%value. !he testing apparatus used in the %value test iscalled a
stabilometer and is identical to the one used in /veem /4 mi design
.!he %6alue is basically a measure of stiffness.
Resilient modulus (M R ) . test used to estimate elastic modulus
(a material5sstress%strain relationship). !he resilient modulus
test applies a repeated a ial
cyclic stress of fi ed magnitude, load duration and cyclic
duration to a cylindricaltest specimen. 7hile the specimen is
sub8ected to this dynamic cyclic stress, it isalso sub8ected to a
static confining stress provided by a tria ial pressurechamber. It
is essentially a cyclic version of a tria ial compression test9 the
cyclicload application is thought to more accurately simulate
actual traffic loading.
esilient modulus is basically a measure of stiffness.
Table 1: Typical CBR and Modulus of Elasticity Values for
Various Materials
Material
( USC given !ereappropriate"
CBR Elastic Modulus
(psi"
iamond%
1-*,***,***
Steel% 3*,***,***
luminum% 1*,***,***
7ood%
1 % 2,***,***
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'rushed Stone(:7, :;, :4) 2* % 1** 2*,*** % ,***
#$%$ Pavement Note on Subgrade Stiffness&Strengt! Tests
ll three types of subgrade stiffness"strength tests are used in
7ashington State to somedegree. 7S 0! uses the resilient modulus
when possible, while many geotechnical firmstypically use ' .
lthough not common any more, 7S 0! still has an %6alue procedure.It
is possible to convert one value to another but these conversions
are based on empiricallyderived e&uations and may not be
appropriate for the specific conditions being considered.Use t!e
belo conversion e'uations it! e tre)e caution . ?or instance, in 7S
0!!est 4ethod @11 Adesign %6aluesB are determined at an e udation
pressure of
