A new approach has been developed to detect the presence of a rigid layer at a shallow depth below the pavement using falling weightdeflectometer (fwd) deflection data. A parameter, slope, has been derived from the sixth and seventh sensor deflection values by applying boussinesq's linear elastic idealization to pavement systems under quasi-concentrated fwd load. Typical values of slope have been determined that indicate the presence of a rigid bottom at a shallow depth. The depth of such a rigid layer was determined by matching the outer sensor deflections corresponding to an estimated subgrade modulus with an elastic layer analysis program and a gradient-based optimization routine. The detection and depth determination procedure were verified using fwd measurements, cone penetration results and drilling records on 13 in-service pavements in arizona, and existing results from manual backcalculation analysis. Satisfactory agreement was observed between the results of this study and results from manual backcalculation analysis. The rigid layer detection and depth determination procedures have been coded in a computer program, arizona deflection analysis method. This paper appears in transportation research record no. 1293, Backcalculation of pavement moduli 1991 .
Abstract