Backcalculation

Backcalculation Backcalculation is a mechanistic evaluation of pavement surface deflection basins generated by various pavement deflection devices. Backcalculation takes a measured surface deflection and attempts to match it (to within some tolerable error) with a calculated surface deflection generated from an identical pavement structure using assumed layer stiffness (moduli). The assumed layer moduli in the calculated model are adjusted until they produce a surface deflection that closely matches the measured one. The combination of assumed layer stiffness that results in this match is then assumed to be near the actual in situ moduli for the various pavement layers. The backcalculation process is usually iterative and normally done with computer software. Typical flowchart A basic flowchart that represents the fundamental elements in all known backcalculation program is shown as Figure 1 Briefly, these elements include: • Measured deflections. Includes the measured pavement surface deflections and associated distances from the load. • Layer thicknesses and loads. Includes all layer thickness and load levels for a specific test location. • Seed moduli. The seed moduli are the initial moduli used in the computer program to calculate surface deflections. These moduli are usually estimated from user experience or various equations. • Deflection calculation. Layered elastic computer programs are generally used to calculate a deflection basin. • Error check. This element simply compares the measured and calculated basin. There are various error measures which can be used to make such comparisons ( more on this in a subsequent paragraph in this section). • Search for new moduli. Various methods have been employed within the various backcalculation programs to converge on a set of layer moduli which produces an acceptable error between the measured and calculated deflection basins. • Controls on the range of moduli. In some backcalculation programs, a renge (minimum and maximum) of moduli are selected or calculated to prevent program convergence to unreasonable moduli levels (either high or low)