Influence of Seed Layer Moduli on Finite Element Method–Based Modulus Backcalculation Results

Abstract

The determination of pavement layer moduli from falling weight deflectometer test data is known as backcalculation analysis. Generally, backcalculation analysis is unstable–-greatly influenced by several causes of error. They may be categorized as modeling error in the forward analysis, deflection measurement error, or numerical computation error due to instability in the backcalculation procedure, for example. Because of these problems, the seed values selected for layer moduli greatly influence backcalculation results. To reduce the effects of measurement error, truncated singular-value decomposition is used for regularization. Variable scaling, often used in optimization algorithms, is implemented to improve numerical accuracy. A Ritz vector reduction method is used to solve a large system of dynamic equations in dynamic backcalculation efficiently, and various other means are introduced to decrease computational time. Recent updates of Dynamic Back Analysis for Layer Moduli software, first developed in 1993 and whose solver is based on axi-symmetric finite element method, are presented, as are examples of airfield pavement applications. Results are compared with results from Back Analysis for Layer Moduli static backcalculation software, whose solver was developed using multilayered linear elastic theory. Experience indicates that a dynamic backcalculation is superior to static back-calculation. The results from the two methods are presented and compared.