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This paper describes the interpretation of intelligent compaction (IC) data from two layered soil test beds using center of gravity (CG) roller-measured soil stiffness. Conventional edge-mounted (EM) roller-measured soil stiffness values are interpreted from vertical accelerations measured by discrete, single-position accelerometers. However, these EM accelerometers are located at variable distances from the drum CG; the resulting vertical accelerations are thus affected by the rotation of the roller drum about the drum CG in the direction of roller travel. This leads to undesirable measurement artifacts, specifically multiple possible soil stiffness values for one soil location and an artificial dependence of the soil stiffness values on the direction of roller travel. In this study, left and right EM acceleration data from a vibratory roller are used to compute vertical accelerations at the CG of the roller drum. These vertical accelerations are used to compute CG stiffness values, which are not subject to the measurement artifacts associated with EM stiffness values. The resulting CG stiffness values are used to interpret IC data from two test beds with multiple 15-30 cm thick base-subbase-subgrade lifts. CG soil stiffness increases with the addition of subbase and base lifts, showing a desired sensitivity to changes in soil materials. CG stiffness also increases with the addition of multiple base lifts, showing a desired sensitivity to an increase in the overall thickness of the base material. This study demonstrates the efficacy of this unambiguous measure of soil stiffness for practical usage in IC of layered earthwork systems. [ABSTRACT FROM AUTHOR]