United States Department of Transportation
i
Subgrade characterization for highway pavement design.
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2000-12-01
[PDF-3.70 MB]
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Edition:Final; Jan. 26, 1999-Dec. 31, 2000.
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Abstract:Subgrade soil characterization expressed in terms of Resilient Modulus (MR) has become crucial for pavement design. For a new design, MR
values are generally obtained by conducting repeated triaxial tests on reconstituted/undisturbed cylindrical specimens. Because of the
complexities encountered with the test, in-situ tests would be desirable, if reliable correlation can be established. In evaluating existing
pavements for rehabilitation selection, subgrade characterization is even more complex. The main focus of this study is to determine subgrade
MR employing a Dynamic Cone Penetrometer (DCP), especially the automated version. In support of the study, side-by-side Falling Weight
Deflectometer tests are also conducted.
Twelve as-built test sections reflecting typical subgrade soil materials of Mississippi are selected and tested with DCP and FWD before and after
pavement construction. Undisturbed samples are extracted using a Shelby tube, and tested in repeated triaxial machine for MR. Other routine
laboratory tests are conducted to determine physical properties of the soil. In analyzing the data, the soils tested are categorized into two groups,
fine-and coarse-grain soils.
DCP results (DCP index, penetration/blow) from tests conducted directly in the prepared subgrade are employed to develop regression models
for laboratory MR prediction. The predictability of the model is substantiated by repeating DCP tests at an independent site. Models for in-situ
modulus prediction are also developed in the study. Deflection measurements facilitated the calculation of in-situ modulus, for which three
programs were used: MODULUS 5, FWDSOIL and UMPED. The MODULUS 5 –backcalculated subgrade modulus shows good agreement
with the laboratory MR. The FWDSOIL backcalculation program predicts subgrade moduli which are slightly lower than the laboratory MR.
With emplacement of pavement structure (lime treated subgrade, lime fly ash subbase, and several inches of asphalt concrete) atop the subgrade,
the subgrade backcalculated moduli are enhanced, coarse-grain soil showing a larger increase than the fine-grain soil. This latter result, namely
the enhancement of subgrade moduli, is substantiated employing the data compiled from 20 LTPP pavement sections in Mississippi. In order to
analyze the automated DCP results, a software designated Dynamic Cone Penetrometer ANaylsis (DCPAN), has been developed. With the
regression equation incorporated in the software, real time laboratory as well as backcalculated subgrade modulus calculations are plausible in
the field.
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