COMPARING MEASURED AND THEORETICAL DEPTH DEFLECTIONS UNDER A FALLING WEIGHT DEFLECTOMETER USING A MULTIDEPTH DEFLECTOMETER

Installation and use of the multidepth deflectometer (MDD) for monitoring pavement response are described. The MDD measures depth deflections in pavements. MDDs are installed in specially drilled holes and up to six modules may be placed in a single hole. This device measures the relative deflection of each layer with respect to an anchor point located approximately 7 ft below the surface. Three sections at the Texas Transportation Institute Research Annex have been instrumented with MDDs. These sections have the same materials but varying layer thicknesses. An effort was made to measure the movement of the anchor. Measuring the anchor movement permits calculation of the absolute depth deflection at each MDD sensor. Surface and depth deflections were measured under FWD loadings. The MODULUS computer program was used to backcalculate the layer moduli from surface deflections. These moduli values along with the layer thickness information were entered into the BISAR layered elastic program to predict the theoretical deflections at depths corresponding to the MDD sensor locations and at the anchor. The analysis was conducted for an infinite subgrade and for a 20-ft depth to bedrock. A comparison of measured versus calculated deflections revealed that a better match was obtained between the two with the bedrock at 20-ft depth.