EXECUTIVE SUMMARY
Resilient modulus tests were conducted on five subgrade soils considered to be
representative of the subgrade soils found in the state of New Hampshire. Tests
were conducted at the optimum density and moisture content. The optimum den-
sity and moisture content for the five soils were provided by the New Hampshire
Department of Transportation (NHDOT). Although the focus was on resilient modu-
lus tests, a limited number of hydrostatic and triaxial compression tests were con-
ducted.
The tests were conducted to determine the effective resilient modulus of the
soils for use in the AASHTO design procedure. Resilient modulus tests were con-
ducted at several temperatures to reflect freezing and thawing. Temperature was
selected as the primary variable because it is the easiest to measure in the field. The
tests were conducted using the AASHTO TP 46 test protocol, except when the
samples were frozen. When frozen, the CRREL testing protocols was used. Although
tests were done at different confining pressures and deviator stress, the average
values were used to determine the effective modulus. This is justifiable, as the
AASHTO design method requires a single resilient modulus value. These values
can be used with most mechanistic design methods as they use linear elastic prop-
erties. However, the nonlinear information is available in this report for future non-
linear analysis. A limited number of radial strain measurements were made, and
Poisson's ratio was calculated. Many of these values were outside the conventional
range of Poisson's ratio for elastic materials. This is to be expected since subgrade
soils are not linear homogenous material but a conglomeration of aggregates or
particles.
Samples were prepared using a kneading compactor. A series of tests were con-
ducted to determine the correct kneading pressure and the number of tamps to
provide a uniform density (as a function of depth) for the sample at the optimum
moisture content.
The computer program FROST was used to determine the temperature at top of
the subgrade for typical interstate and rural pavements. Temperatures for both the
Concord and Lebanon, New Hampshire, areas were used in the analysis. It was
found that the subgrade temperatures were similar at Concord and Lebanon. The
results presented here are for Concord but can be used in most of the state. The
exception may be in high areas at higher elevations. The monthly resilient modu-
lus selected was based on the subgrade temperatures, not on the mean air tempera-
tures. The effective resilient modulus for the five soils for each month of the year
are presented in the recommendation/conclusion sections. The results presented
in this report are for optimum density moisture conditions. Care must be taken
with its use at other densities or moisture conditions.
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