The quality of base and subgrade construction has conventionally been evaluated using specifications based on density and moisture content. Such specifications for highway base and subgrade require the use of a nuclear density and/or moisture gauge that poses potential health hazards to the operator and requires expensive certification and monitoring. Moreover, density and moisture do not relate to pavement design input parameters or performance. The fundamental material properties such as elastic and resilient moduli that are key inputs in the new mechanistic empirical-based design cannot be obtained from density and moisture content measurements.
The primary objective of this study was to investigate the suitability of the lightweight deflectometer (LWD) to measure in-situ pavement layer moduli. The LWD, along with two other devices, the GeoGauge and dynamic cone penetrometer (DCP), were used to measure and monitor subgrade and base layer moduli during construction. Three existing gravel roads were also tested.
A high spatial variability was found for the stiffness modulus values measured by all three devices. There were no significant correlations among the results with the devices. Although no unique relationship between mean LWD moduli and either GeoGauge or DCP moduli was found, a good correlation was found when the 85th percentile stiffness values were compared. The effect of dry density was not evident, but moisture content showed a significant influence on the measured stiffness with all three devices, especially the LWD. A limited laboratory investigation indicated that the high modulus value for the LWD may be attributable to soil suction or a pore pressure development from transient loading of the LWD on a fine-grained soil.
The LWD is not recommended for use for construction quality control until further research has been conducted to determine the causes of the high spatial variability and the effect of moisture on the LWD-measured modulus. The study further recommends that additional well-controlled laboratory testing be performed to evaluate the effect of moisture on LWD-modulus measurements and that field studies be conducted to verify the findings.
The advantage of the LWD is the lower operating cost and lower health risk compared to the conventional nuclear density and moisture content devices. In addition, the LWD can directly measure the modulus properties that are the basis for the new MEPDG pavement design.