Full-Scale Field Study of a Geogrid-Reinforced Unpaved Road System
Abstract
Geogrids are widely used to improve the performance of unpaved roads constructed over weak subgrade. However, the behavior of geogrids under traffic loads and their reinforcing mechanisms, as well as the resulting benefits to the roadway performance are not well understood. A full-scale field study was conducted to quantify the effectiveness of geogrids in unpaved roads, evaluate their reinforcing mechanism, and identify which geogrid properties are most directly related to performance improvement. Ten full-scale unpaved road test sections were constructed with varying relevant parameters including geogrid aperture shape, geogrid tensile modulus, and thickness of the base course layer. Five of the test sections were constructed with a 200-mm nominal base course thickness and five were constructed with a 250-mm nominal base thickness. For each base course thickness, four test sections were reinforced with different geogrids while one test section was unreinforced in order to evaluate their performance. The geogrids were placed at the subgrade-base course interface on top of a non-woven geotextile separator. Trafficking was applied using a single-axle dump truck. Measurements of rut depth and surface deformation were taken to evaluate the respective performance of each test section. Additionally, the test sections were instrumented for measuring road response under traffic loading. Dynamic and permanent geogrid strains were measured using foil strain gauges attached to the geogrid ribs. Earth pressure cells were installed at the top of the subgrade layer to measure dynamic and permanent vertical stresses transferred to the subgrade soil. The measured performance indicated that the geogrids effectively reduced surface rutting and reduced the dynamic vertical stresses transferred to the top of subgrade. Traffic benefit ratio of up to 3.6 and reductions in base layer thickness between 6 and 25% were achieved, with higher benefit values observed for stiffer geogrid and smaller base layer thickness. Improvement in performance was related to the tensile strength at 2% and junction strength in the cross-machine direction. The geogrid strain data demonstrated that the geogrid was not under constant tension across the road. The field testing results clearly confirmed the validity of shear-resisting interface or lateral restraint mechanisms.