Humboldt H-4140 GeoGauge - Manual (Page 28)

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The following represents a effort by Koch Performance Roads to quantify the
performance of approximately 50 miles of lime stabilized, silty clay, ~ 12” thick
subgrade which was part of the Koch managed reconstruction of Virginia (VA)
Route 288. As with similar jobs managed by Koch (e.g., NM 44), the traditional
7 day waiting period between subgrade installation and resumption of
construction was inconsistent with Koch’s aggressive construction schedule. To
reduce the waiting period, Koch needed a QC method that could quantitatively
estimate when the subgrade was strong enough to support construction and
estimate subgrade ultimate strength. The method would have to determine
these factors from measurements made within the first few days after installation.
Limited measurements made on a similar lime stabilized subgrade of New
Mexico 44 in 2000 indicated that evaluating a material modulus vs. time might
provide the needed method. This was accomplished by calculating a modulus
from measurements of lift stiffness made using the Humboldt GeoGauge.
To confirm this, a comprehensive evaluation of subgrade lift stiffness was
conducted on VA 288. The construction specification used on NM 44 was also
used on VA 288. The NM 44 data established that a modulus calculated from
lift stiffness could be used as the estimate of strength. If the VA 288 subgrade
performance was consistent with the subgrade of NM 44, then stiffness based
QC of the lime stabilized subgrade is indeed viable if not a practical and form
the basis for a specification.
From the sum of the data provided by Koch, it appears that the effective
modulus of the lime stabilized subgrade increases with time at a logarithmic rate
to a good degree of correlation (see Figure 1). This data also shows that of the
approximately 1,000 measurements made on the subgrade, approximately 95%
fall within +/- 36% of the average modulus. Based on FHWA, Office of Pavement
Design guidelines, this variability in subgrade modulus could support a 15+ year
pavement life. The modulus and rate of change of modulus with time for the NM
44 lime stabilized subgrade from over two years prior falls within the data range
for VA 288. Also, core testing by Koch was consistent with the modulus data.
Where modulus data fell with in +/- 36% of the average, the corresponding
strength data was acceptable.
DATA ANALYSIS SUMMARY:
Lime Stabilized Subgrade
Virginia Route 288
February, 2003
PREPARED FOR:
Koch Performance Roads, Inc.
4111 East 37th Street North
Wichita, Kasas 67220