Forwardcalculation of Pavement Moduli with Load-Deflection Data

A new approach has been developed for determining layered elastic moduli from in situ load-deflection data. This approach is called forwardcalculation, and it differs from backcalculation in that modulus values are calculated directly from the load-deflection data by using closed-form formulas rather than through iteration. The closed-form equations are used for the subgrade and the bound surface course for both flexible and rigid pavement systems. Intermediate-layer moduli are estimated through commonly used modular ratios between adjacent layers. The entire pre-1998 Long-Term Pavement Performance (LTPP) set of backcalculated results was screened with forwardcalculated moduli. Because both back-and forwardcalculation techniques use identical falling weight deflectometer load-deflection data as input, it was expected that the moduli derived from each approach should be reasonably close to each other. This result was not always the case, however. Forwardcalculated values were, in most cases, more stable on a section-by-section basis than the backcalculated values in the LTPP database. Similarly, the forwardcalculated values usually appeared more reasonable. The exception to this finding was the portion of the database based on slab-on-dense-liquid or slab-on-elastic-solid theory, in which the correspondence between the two approaches was excellent, and both approaches were stable.