Subgrade Dynamic Backcalculation Based on PFWD

In order to evaluate the bearing capacity and the compaction performance of subgrade in a reasonable way, according to the time history results of pressure and displacement tested with the PFWD (Portable Falling Weight Deflectometer), the extraction methods of time history data were put forward to use for the backcalculation. Combined with the analytic solutions of dynamic response of vertical displacement on the half-space surface under the impact loading of semi-sine wave in the circular rigid vertical distribution for the dynamic half-space theory, the mathematical models of subgrade dynamic backcalculation were established based on the least square criterion, then the corresponding backcalculation flow and method were proposed in accordance with the modified Gauss-Newton algorithm. Firstly the pressure time history was fitted as a semi-sine wave to obtain the two backcalculation parameters of impact load, i.e. the angular velocity and the nonresponsive period. Secondly the displacement time history was fitted to obtain the two subgrade dynamic parameters, i.e. the dynamic modulus and the dynamic density. Lastly, through comparing the dynamic backcalculation to the static backcalculation from the tested data, the results show that the dynamic modulus is substantially equal to the static modulus, and there are the well correlations between the dynamic density and the dry density and the moisture content measured with the sand replacement method. Therefore, the PFWD can simultaneously test the design index (modulus) and the construction index (compactness) for the subgrade to achieve the both unified evaluation.