ADVANCED BACKCALCULATION USING A NONLINEAR LEAST SQUARES OPTIMIZATION TECHNIQUE

In recent years the analysis of pavement structures has relied increasingly on characterizing material properties (such as resilient modulus) by use of nondestructive deflection testing and backcalculation procedures. An important element common to all backcalculation procedures--the technique used to achieve a "convergence" of the measured and calculated deflection basins--will be described. A convergence method based on the use of nonlinear least squares is described. The method was adapted to a layered elastic program (CHEVRON N-layer). This convergence approach improves moduli estimates over prior procedures; however, the most important element is the ability to efficiently backcalculate not only layer moduli but also layer thicknesses. This ability is illustrated by using hypothetical two- and three-layer pavement sections and by using real data for a three-layer section.