The structural adequacy of pavements is routinely evaluated by analyzing measured deflections collected with nondestructive testing devices such as the falling weight deflectometer (fwd). The fwd is an impulse loading device that closely simulates moving wheel loads in both magnitude and duration. The fwd is typically equipped with a circular loading plate and several deflection sensors positioned at discrete locations from the center of the loading plate. Virtually all available backcalculation programs that use linear elastic theoryassume a circular loaded area and uniform stress distribution underthe loaded area. Any deviation from these assumptions will introduce an error. To investigate the magnitude of this error, the multielastic layer computer program bisar was used to calculate surface deflections for a limited factorial of pavement layer thicknesses and stiffnesses. Three contact stress distributions were considered: uniform (full contact), partial edge distribution (rutting), and partial circumferential distribution (weak pavement). The calculated deflections from each loading case were used to backcalculate layer moduli with the elsdef computer program. Results from the three cases were analyzed and the relative errors computed. The analysis indicated that if full contact is assumed when in reality it did not occur, significant errors in the backcalculated moduli values of the pavements analyzed may result. This paper appears in transportation research record no. 1293, Backcalculation of pavement moduli 1991 .
Abstract