ON THE BEHAVIOUR OF GEOGRIDS IN STABILISAT ION APPLICATIONS

INTRODUCTION Geogrids are the most popular type of geosynthetics used in the road construction industry for mechanical stabilisation and reinforcement purposes. Geogrids are co mmonly used over weak subgrade soils to provide a working platform for construction equipment. Often referred to as “subgrade restraint” design, such an application places a geogrid at the subgrade/aggregate cover interface to increase the bearing capacity or the support of construction equipment over a soft subgrade. Since the aggregate cover opt ion requires large thicknesses for low subgrade strengths, the subgrade restraint use of geogrid reinforce ment can therefore be quite beneficial by offering a reduced aggregate thickness alternative. Geogrids can perform as tensile reinforcement for aggrega te base courses in flexible or asphalt pavements. Addin g a geogrid layer can mechanically stabilise aggregate part icles and increase bearing capacity of a pavement struct ure by forcing the potential bearing capacity surface to develop al ng alternate, higher shear strength surfaces. The late ral restraint and/or membrane tension effects may also cont ribute to load carrying capacity as the wheel loads att emp to cause rutting in the pavement foundation layers, i.e., u nbo nd aggregate base/subbase and subgrade soil. Through the interlock between the geogrids and aggregate, ge ogrids are assumed to have higher friction and confining stresses than the smoother surfaced geotextile s. This is in part due to the additional bearing stresse s created in the geogrid apertures as soil and aggregate particle s provide the interlock in these openings. When placed in a granular base course, geogrids may restrain the lateral s preading of the granular base layer, and through interlockin g, may develop a relatively “stiffer” layer surrounding the geogrid. Granular “base reinforcement” of geogrids could be crucial to ensuring their successful and beneficial applic ation in low to moderate volume roads having thin hot-mi x asphalt (HMA) surfaces and subgrade California Bearing Ra tios (CBRs) between 3 to 8 percent. In addition to potentially reducing shear deformation in aggregates, the control of aggregate movement, especially in the upper part of the layer adjacent to the HMA, may also reduce HMA f atigue. Hence, a geogrid interlayer system can typically be used to reduce the overall thickness of a pavement system f or a target design life or extend the design life of the pavement. This paper highlights subgrade restraint and base reinfor cement applications of geogrids in road infrastructure. T h paper also describes recent research efforts in numerical modeling and field studies which helped identify and quantify the geogrid reinforcement mechanism in unbound aggregate la yers of pavement systems. The primary geogrid reinforcement mechanism of improved aggregate interlock and its importance in preventing aggregate lateral movements is demonstrated in this paper by the use of an inno vative image-aided Discrete Element Modeling (DEM) approach for different rectangular and triangular geogrids in a shear box DEM model. Finally, a recent field val idated mechanistic model developed at the University of Illinois f r geogrid base reinforced flexible pavements is given a s an example to demonstrate how the concept of compaction induced residual stresses can be included in finite element analysis as an initial condition and how such an approach c n effectively represent the contribution of geogrid reinforcement in the form of stiffened zone in mechani stic pavement analysis.