Dynamic deflection equipment presents a very practical and cost-effective approach for nondestructive testing and evaluation of pavement-subgrade systems. The in situ Young's moduli are calculated by the inverse application of layered elastic theory to the measured dynamic deflection data. This paper discusses some of the fallacies associated with this iterative approach. The calculated moduli are user dependent and nonunique if the user inputs the initial set of moduli to start iteratives. A proven methodology to ensure uniqueness of the moduli is the generation of seed moduli as the function of measured deflections and radial distances of the sensors. The FPEDD1 (for pavements with asphalt concrete surfacings) and RPEDD1 (for rigid pavements) computer programs incorporate this methodology. The predictive equations used to generate seed moduli are based on numerous layered theory solutions and are therefore applicable to any region and soil condition.
This paper describes the approaches used in these programs to estimate nonlinear moduli for granular layers and subgrade soil, and considers rock layers. Examples are presented for in situ material properties from dynamic deflection data measured on a variety of pavementsubgrade systems. It is shown that an appropriate analysis technique yields comparable pavement moduli from the dynamic deflection data measured by vibratory or impulse loading equipment.