Identification of optimum aggregate gradation for transportation applications of multiaxial geogrids

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Peralta, Andres Felipe
Frost, J. David
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The stabilizing effect of multiaxial geogrids on aggregate materials is largely influenced by the morphological properties of the aggregate, such as its size, angularity, and surface roughness, and the overall aggregate gradation. In order to evidence this phenomenon at the microscale level, a Discrete Element Method (DEM) model, which considered these properties was developed. The morphological properties of different aggregate specimens across the State of Georgia were quantified, by utilizing the University of Illinois Aggregate Image analyzer (UIAIA). The indices obtained from this procedure, such as the Flat-and-Elongated (FE) ratio, the Angularity Index (AI), and the Surface Texture (ST) Index were used to model aggregate particles as clumps using the Particle Flow Code (PFC) 3D software. DEM models for three different types of multiaxial geogrids (TX130S, TX140, and TX190L) were developed, by implementing the PFC3D parallel-bond contact method. These models were calibrated against physical data obtained from single rib tensile tests of multiple rib specimens from each geogrid type. The stabilizing effect of multiaxial geogrids was assessed by performing simulations of cyclic load tests on binary mixtures with a particle size ratio of 2.1. Two binary mixtures, identified as binary mixture – 50% and binary mixture – 70%, were used in this study. The binary mixture – 50% had a Dmax equal to 30 mm and a Dmin equal to 14.3 mm, while the binary mixture – 70% had a Dmax equal to 40 mm and a Dmin equal to 20 mm. From the results obtained from the simulations, it was observed that the stabilized binary mixture – 50% exhibited smaller surface deformations due to cyclic loading, in comparison to the unsterilized case; however, it was also observed that both geogrid stabilized and non-stabilized binary mixture – 70% exhibited significant surface deformations, and the magnitude of these deformations was the same for both binary mixture – 70% specimens with and without a multiaxial geogrid layer. This DEM model was effective in providing an insight into the behavior of this composite system, since it permitted the examination of particle interlocking, and the development of tensile and compressive force chains within the multiaxial geogrid model. From these results, it was possible to visually determine that optimal interlocking existed between the binary mixture – 50% and the multiaxial geogrid model. It was also noticed that minor interlocking was developed between the binary mixture – 70% and the multiaxial geogrid model, which explained its poor performance and the development of the same amount of surface deformation between the specimens with and without a multiaxial geogrid layer. Finally, it was evidenced that the optimum interaction between the binary mixture – 50% and the multiaxial geogrid layer hindered the surface deformation by 33% with respect to the non-stabilized case. Furthermore, no stabilizing benefit was observed for the case where the binary mixture – 70% was used.
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