Pavement evaluation

Airport pavements bearing capacity assessment using Heavy Weight Deflectometer (HWD)

Evolved from the French “déflectomètre à boulet”, the Heavy Weight Deflectometer (HWD) is today seen worldwide as the most appropriate device to assess the bearing capacity of airport pavements. Its principle consists in applying a transient impulsive load simulating the effect of a rolling aircraft wheel, onto a stationary loading plate positioned on the pavement, and studying the surface deflections induced by this dynamic loading. The latter are continuously monitored during the test by means of geophones.

These deflection measurements are used to determine the structural properties of the pavement, by means of a “backcalculation” numerical process which consists in:

• 1- choosing a mechanical model for the pavement,
• 2- identifying the model’s parameters to be set so as to fit theoretical computed deflections to the experimental data set. Then, forward calculations can be performed to estimate the bearing capacity or the remaining life of the pavement structure.

Usual processing methods for flexible pavements assessment are based on static multilayered elastic models. The structural properties to be backcalculated are the stiffnesses of the different layers. The backcalculations are performed from pseudo-static deflection bowls reconstituted from the deflection peak values measured by each geophone. As emphasized by several authors, these methods have shown limitations. Indeed, they use only part of available information (peak values), and the static modelling is far from the reality of the test.

Hence the necessity for STAC to develop an advanced method for HWD data analysis.

This work has been achieved as part of the PhD thesis of Michaël Broutin, Head of Pavement Testing at the Studies and Research Division of the Airport Infrastructure Department. It was defended on 11th June, 2010, at the LCPC Paris and is entitled “Assessment of flexible airfield pavements using Heavy Weight Deflectometers; Development of a FEM dynamical time-domain analysis for the backcalculation of structural properties.”

The objective of the thesis was to develop an advanced method for flexible pavements assessment using HWD tests data. It achieves a better representation of the observed physical phenomena during dynamic loading and allows taking into account the whole available information.

A time-domain FEM modelling has been developed, where the applied dynamical load, inertia of materials and structural damping are modelled. It allows the computation of ensuing time-related deflections. An automated convergence algorithm has been developed for numerical resolution of the backcalculation procedure.

A full-scale validation of both backcalculation method and strains determination was conducted on the STAC’s pavement test facility. It consisted in test surveys run on a reference instrumented pavement. The validation relied on the comparison between backcalculated and laboratory-determined material properties, and on the comparison between expected strains and measured strains.

A numerical tool has been developed which allows automating the finite elements mesh creation and both backcalculation and forward calculation phases: the PREDIWARE (Pavement Rational Evaluation using Deflections Induced by Falling Weights, for Airfield and Road Engineers) software.

A STAC technical guidance provides all airport technical managers with a methodology for pavement testing using HWD. It gathers recommendations relative to operational survey and data analysis, based on the advanced dynamic modeling developed in the PhD thesis.