The Right Question to the Right Answer

ABSTRACT: Highways are designed as a multi-layered half-space including asphalt/concrete road surface, granular aggregate base, cohesive sub-base, and granitic bedrock. The FWD (falling weight deflectometer) is a dynamic field test to back-calculate the layer thicknesses and moduli, to ensure design and performance. This investigation is the first attempt to solve this problem using the correct theory (elastodynamics), correct method (regression), correct boundary conditions (mixed), and correct response (velocities). The finite difference numerical model F2D (Fast Lagrangian Analysis of Continuum) was used to backcalculate (regression) these results, with a recently added computer language called Python to obtain the correct problem of the correct solution. Using the measured dynamic results from a FWD test performed on a test section of MnROAD, the layer thicknesses and moduli were backcalculated using F2D/Python with greater accuracy than with using previously used incorrect numerical methods. 1. INTRODUCTION The falling weight deflectometer (FWD) is a device designed to simulate deflection of a pavement surface caused by a fast-moving truck. The FWD generates a load pulse (dynamic surface wave) by dropping a circular weight on a layered half-space (asphalt or concrete road system). This load pulse is transmitted to the pavement through a 300-millimeter (mm) diameter circular load plate. The load pulse generated by the FWD momentarily deforms the elastic pavement under the load plate into a dish or bowl shape (ASTM, 2015). Envisioned from a side view, the shape of the deformed pavement surface is an axisymmetric deflection basin, as shown in Figure 1, with the nine geophone locations given in Table 1 using accelerometers. Based on the force imparted to the pavement and the shape of the deflection basin, it is possible to estimate the stiffness and thickness of the pavement layers by using various computational methods, including the computer language Python integrated into Itasca (Itasca, 2024). A typical FWD test result would include vertical velocity histories at the surface geophone locations, as shown in Figure 2.