Critical State-Based Model for Simulated Weathering Effects on Compacted Crushed Shales

Shale, the most abundant sedimentary rock globally, is frequently employed as compacted crushed shale in roadway embankments. However, these shales are problematic due to rapid deterioration under cyclic wetting and drying conditions. Initially exhibiting hard rock-like properties with high frictional resistance, compacted shale deteriorates into a softer, fine-grained soil with reduced shear strength and increased deformability. An innovative strategy to mitigate these adverse effects involves intentionally accelerating weathering by pre-wetting and breaking the shale into a well-graded material before compaction. This approach allows compaction to greater dry unit weights, enhancing mechanical properties and reducing long-term deterioration. However, existing constitutive models inadequately describe weathered compacted shale behavior. This study develops a new constitutive model based on critical state theory, utilizing isotopically consolidated undrained shear (CIU) triaxial tests on five shale samples to understand shear and deformation characteristics as weathering progresses. The NorSand constitutive model, grounded in critical state theory suitable for coarse-grained materials, serves as the foundational framework. However, its original formulation poorly represented the observed plastic and volumetric strains in weathered shale. Therefore, new expressions for plastic potential and yield functions were introduced, reflecting the experimental outcomes. The developed predictive framework, a modified NorSand model, accurately simulates the mechanical behavior of weathered compacted shale under undrained loading. This advancement offers broader implications, facilitating reliable predictions of deformational characteristics across various low-dilatancy frictional materials from fine to coarse grains.