Evaluation of mechanisms governing the stabilization of unbound aggregate bases by geogrids in flexible pavements

The benefits of using geosynthetics to stabilize the unbound aggregate layers in flexible pavements, in terms of improvement to pavement life or potential decrease in layer thickness, have been well documented. Early research focused on developing ratios such as the traffic benefit ratio (TBR) or the layer coefficient ratio (LCR) to quantify (empirically) the benefits of including geogrids, and for their use in the design of flexible pavements with those geogrids. However, the lack of a mechanistic understanding of geogrid-stabilization and the empirical nature of this data limited their use to the geogrids and pavement materials that were used to develop them. With the proliferation of different geogrid products and pavement materials, the scope of the early research has become limited and attempts to correlate the properties of the geogrids and pavement materials to the improvement in performance of the pavement have not been successful. This study aims to further the understanding of the mechanisms involved in geogrid-stabilization and to identify the mechanistically relevant properties that contribute to this stabilization. Accelerated pavement tests are conducted, using the model mobile load simulator (MLS11), on reduced-scale pavement test sections with and without geogrids for stabilization under controlled, laboratory conditions. The performance of the pavement sections is evaluated by monitoring the deformation of the surface, the internal particle displacements (within the base), and the dynamic increase in stress (within the pavement structure) for increasing traffic volume. The deformation of the surface is obtained by profiling the surface of the pavement at regular traffic intervals using a laser profilometer, designed and built in-house for this study. A unique cost-effective displacement measuring technique is developed and implemented to obtain the horizontal displacement data of particles in the base. The data from the dense array of the particle tracking sensors is used to generate the horizontal displacement field, horizontal normal strain field and shear transfer efficiency plots in the base. The vertical stress distribution is obtained from the earth pressure cells installed within the pavement structure that are monitored for dynamic stress responses with applied traffic. The inclusion of the stabilizing geogrid resulted in reduced rut development, reduced particle displacements within the base and a wider distribution of the applied load for similar traffic volumes in the control and stabilized sections. The improvement in pavement life due to the stabilization of the base by the geogrid is quantified as the traffic benefit ratio (TBR). The TBRs are determined for seven different geogrids from the accelerated pavement tests, and correlated with the most commonly used in-isolation properties of the geogrid (geometric and tensile properties), and interaction properties of the geogrid-base aggregate composite. The TBRs are found to be best correlated to the coefficients of soil-geosynthetic composite stiffness (KSGC) obtained from the soil-geosynthetic interaction tests.Civil, Architectural, and Environmental Engineerin