Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

M. Hesham El Naggar

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.

Summary for Lay Audience

Geogrids are polymer materials used to improve the performance of unpaved roads constructed over weak soils. However, the behavior of geogrids under traffic loads and their reinforcing mechanisms are not well understood. A full-scale field study was conducted to quantify the effectiveness of geogrids in improving unpaved roads performance, evaluate their reinforcing mechanism, and identify which geogrid properties are most directly relate to performance improvement. To achieve these objectives, ten full-scale unpaved road test sections were constructed. Test variables included geogrid type, geogrid strength, and thickness of the fill layer. Two fill thicknesses were evaluated, each implemented in four reinforced test sections and one unreinforced test section. The geogrids were placed at the bottom of the fill layer on top of a fabric separator. Trafficking was provided by a dump truck. Measurements of 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. Strains were measured in the geogrids. Vertical stresses were measured at the top of the subgrade layer to quantify the stresses transferred to the subgrade soil. Analysis of the measured performance data indicated that the geogrids effectively reduced surface deformation and reduced the vertical stresses transferred to the top of the subgrade. The geogrids resulted in extension of the road service life and reductions in fill layer thickness. The geogrid benefits were more pronounced when a stronger geogrid was used and were reduced by increasing the fill layer thickness. The geogrid strain data demonstrated that the geogrid was not under constant tension across the road. The results of the field testing clearly supported the existence of the mechanism of lateral restraint.

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