Analysis of Pressure Distribution under Falling Weight Deflectometer Loading

An analysis of the pressure distribution beneath the loading plate of a falling weight deflectometer is presented. The analysis is conducted in two stages: (1) Rigid plate deformation shapes without the rubber pad are obtained for uniform and parabolic pressure distributions; and (2) a rubber pad is inserted between the loading plate and the pavement, and the resulting pressure distribution is evaluated for the extreme cases of rigid and soft pavements. It is shown that a uniform pressure distribution can be achieved only for the case when, under the same uniform pressure, the deformed shapes of the loading plate and the pavement surface are identical. Without the rubber pad, the pressure distribution can be highly nonuniform, with a pressure concentration near the center or the rim of the loading plate. When the rubber pad is inserted between the loading plate and the pavement, the pressure distribution is close to uniformity, within about 10% between the center and the rim of the plate. Therefore, with the rubber pad inserted, the assumption of uniform pressure distribution, which is commonly made in back-calculation analysis, is a viable approximation.