Advanced Testing and Characterization of Iowa Soils and Geomaterials

(2023) Advanced Testing and Characterization of Iowa Soils and Geomaterials. Transportation, Department of

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Abstract

Accurate modeling of stress-strain characteristics of geomaterials plays a significant role in determining the achievable design life of roadways (e.g., granular roads and paved roads). Currently, the geomechanical characteristics of these materials are obtained from standard laboratory tests such as California Bearing Ratio (CBR) and standard resilient modulus (MR). However, standard MR tests commonly conducted in the laboratory do not always apply the most damaging field loading conditions for predicting MR and permanent deformation (rutting) responses of granular roadways and pavement base/subbase/subgrade layers. This is the main problem that causes significant performance problems for roadways. The geomaterials used in granular road surfaces and pavement foundation layers exhibit cross-anisotropic behavior indicating that deformation characteristics of such materials depend on the direction of the applied loads. In the real field conditions, loads applied via moving wheels on the roadway systems are imposed to not only the vertical direction, but all three directions (both horizontal and vertical directions). Therefore, it is crucial for advanced material characterization test equipment to be built and used to determine the anisotropic (all directions) behavior of geomaterials. In addition, it's worth noting that the stiffness and plastic deformation of these geomaterials are also significantly affected by the freeze-thaw (F-T) cycles. This combination of directional dependency and sensitivity to F-T effects underscores the complex nature of their behavior under various conditions. In this study, various geomaterials (granular aggregates and subgrade soils) collected from different regions of Iowa were tested in the laboratory through the advanced testing equipment (which was designed and built as part of this project) to determine and quantify the cross-anisotropic behavior of these materials in addition to the effect of F-T on the deformation characteristics. The findings revealed that the tested geomaterials exhibited cross-anisotropy regardless of factors like gradation, material origin, and applied stress levels. Notably, granular materials demonstrated higher cross-anisotropy, where the horizontal resilient modulus (MR) was only a fraction of the vertical MR, while fine-grained materials displayed the lowest cross-anisotropy; in certain cases, their horizontal stiffness even surpassed the vertical stiffness, depending on applied stress history. Anisotropy ratios (the ratio of horizontal MR to vertical MR) were calculated for all cross-anisotropic tests. Moreover, the MR was observed to be influenced by stress history during simulated laboratory testing mimicking moving vehicle loads. For permanent deformation (PD), similar to stiffness behavior, all tested materials exhibited cross-anisotropy, with the highest deformations recorded horizontally for aggregate materials subgrade soils. Nevertheless, the discrepancy in deformation levels between different directions for the subgrade soil was relatively lower than that of the granular aggregate materials. Furthermore, investigations into F-T effects indicated that an increase in the fines content of granular materials, coupled with an escalation in applied F-T cycles, led to stiffness degradation and increased PDs.