Comparison between HWD Backcalculated Subgrade Dynamic Moduli and In Situ Static Bearing Capacity Tests

摘要

Heavy weight deflectometer (HWD) backcalculated moduli for subgrade materials appear from experience to be always overestimated in comparison with commonly expected values used in design methods. This can be explained by the dynamical loading applied to the pavement whereas the tests used during the construction phase for the characterization of those materials in the frame of a new pavement design are static plate tests. In order to better understand the relationships between the apparent bearing capacity of the subgrade under respectively static and dynamic solicitation, the French Civil Aviation technical center (STAC) decided to build a specific test facility, in partnership with the French Road and Research Center (CER). It consists in an airfield flexible pavement comprised of a subgrade, a UGA subbase, and bituminous materials. It presents two test areas, one with a subgrade presenting regular water content, and the other with a water-saturated subgrade. Static and dynamic bearing capacity tests were performed during the construction for each constitutive layer. This paper focuses on the subgrade moduli backcalculated from HWD tests. Two backcalculation methods are compared: a full backcalculation where all layer moduli are backcalculated under HWD tests performed at the surface of the bituminous materials, and a step by step process using the HWD test performed on each layer, the modulus backcalculated for the underlying layer being fixed and injected in the next one-modulus backcalculation process, until the top of the pavement is reached. Both results are compared with the bearing capacities obtained during construction, for both subgrades.