Technical information

2-9
thickness in Section N1 was not constant at 5.0 in. (127 mm), but varied between 6.7 to
4.5 in. (170 to 114 mm).
The as-built air void contents were also surveyed during construction. On
average, all three lifts in Section N1 were compacted to an air void content of 7.0%.
However, in Section N2 only the top lift was compacted to 7.0% voids while the two
bottom lifts were compacted to an air void content of 6.0%. The asphalt content also
varied slightly relative to the design values. Additional details regarding the HMA
construction can be found in Powell and Brown (2004).
2.3 MECHANICAL TESTING
2.3.1 Resilient Modulus of Subgrade and Base Materials
The resilient modulus of the upper subgrade soil was tested according to the AASHTO
T307 protocol. Tests were done either on recompacted material or on undisturbed
specimens obtained from the field after subgrade construction. The test conditions
included three levels of moisture content (denoted by
ω
): 7.2%, 9.7% and 20.1%; three
levels of applied confining pressure: 2, 4 and 6 psi (13.8, 27.6 and 41.4 kPa); and five
levels of applied cyclic axial stress: 2, 4, 6, 8 and 10 psi (13.8, 27.6, 41.4 and 68.9 kPa).
The average dry density of the tested samples was 114.4 pcf (1834 kg/m³) which
represents a relative compaction degree of about 96% (recall that the compaction degree
in the field was about 100%). The raw test results are presented in Table 2.3.1. These
were obtained directly from the laboratory reports.
Timm and Priest (2006) provided three regression equations, each fitting the test
results for a single level of moisture content; the mathematical expression they used was
as follows:
3
2
)()(
31
b
b
cR
SSbM ⋅= ...................................................................................... (2.3.1)
where
R
M is the resilient modulus (in psi),
1
b ,
2
b and
3
b are regression constants,
c
S
is the peak applied uniaxial cyclic stress (in psi), and
3
S is the applied confining
pressure (in psi).