SYMPOSIA PAPER Published: 01 January 1989

General Procedure for Backcalculating Layer Moduli


An efficient microcomputer program for backcalculating moduli from nondestructive testing results is described. Its use is illustrated by analyzing a typical section where several deflection bowls were measured at the same location.

Sources of errors in the testing and backcalculating are presented and discussed. The error source is taken into consideration in the formulation of the objective function, the relative squared error between measured and computed deflections.

The Hooke-Jeeves' pattern search algorithm was selected for minimizing the objective function. The direct computation of the deflection at each move of the pattern search was replaced by the three-point Lagrange interpolation, using a data base generated ahead of time. The pattern search and interpolation were included in a microcomputer program named MODULUS, capable of performing and backcalculation in 1 to 2 min per deflection bowl. It is highly efficient where several deflection bowls are measured along the pavement section or at the same location.

The analysis of the typical section shows clearly that the measurement errors can be quite large. Averaging deflection measurements at each sensor cancels out some of the random errors and generally leads to acceptable values of the backcalculated moduli. The program MODULUS is especially suited toward these cases where multiple deflection basins are available for the same pavement geometry.

Author Information

Uzan, J
Technion, Israel
Lytton, RL
Texas Transportation Institute, Texas A&M University, College Station, TX
Germann, FP
Texas Transportation Institute, Texas A&M University, College Station, TX
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Developed by Committee: D04
Pages: 217–228
DOI: 10.1520/STP19809S
ISBN-EB: 978-0-8031-5087-4
ISBN-13: 978-0-8031-1260-5