Impact of groundwater level on the mechanical response of a flexible pavement structure: a case study at the Torpsbruk test section along county road 126 using Falling Weight Deflectometer

Excess moisture presence in pavement unbound layers can result in lower structural stiffness and reduce the service life of road systems. A field study was conducted along the county road 126 near Torpsbruk to investigate the effect of the moisture content on the bearing capacity of a flexible pavement structure and its unbound layers stiffness. The subsurface drainage system of the test road was clogged during a three-month period in summer 2010, raising the groundwater level under pavement structure and therefore increasing the moisture content of the pavement unbound layers. During this period, the groundwater level and the volumetric moisture content of the pavement was continuously monitored using the instrumentations at test site and the structural response of the pavement was evaluated by conducting frequent Falling Weight Deflectometer (FWD) tests with multilevel loads. Using the FWD data, the unbound layers stiffness were back-calculated and their sensitivity to the moisture content and load level were studied. The field measurements showed that the groundwater level can significantly affect the moisture condition of the unbound layers and therefore their stiffness. Both the unbound granular layer and the subgrade showed stress dependent behavior to multilevel FWD loads.