Physical Modeling on Coupled Impact of Seismic Event and Wet-dry Environment on Mechanical and Crack Propagation Characteristics of Untreated and Treated Cohesive Soils

Abstract

This study examines the intricate interactions between wetting-drying (W-D) cycles, initial compaction states, seismic events, polypropylene fiber (PF) and wheat straw fiber (WSF) on the desiccation cracking and mechanical behavior of cohesive soil (CL soil). Using CL soil (both treated and untreated) with a defined chemical composition and plasticity, an extensive experimental program was meticulously designed. Untreated specimens were remolded at various initial compaction states, including varying initial dry unit weights (γd0), and treated specimens were remolded with PF and WSF of various percentages: 0.2%, 0.4%, 0.6%, and 0.8%. These specimens then underwent multiple WD cycles, with their progress systematically documented through cinematography. The desiccation cracking behavior and mechanical response were evaluated after each W-D cycle and subsequent seismic event. Results indicated that desiccation cracking in CL soil begins after the first W-D cycle, intensifies significantly after the second cycle. As the number of W-D cycles increases, the values of desiccation crack parameters rise due to the steady increase in tensile stresses during drying. Higher initial dry unit weights (γd0) reduce the extent of desiccation cracking, evidenced by lower surface crack ratios (Rsc), crack line density (Dcl), and total crack lengths (Ltc) attributed to the denser packing of soil particles which enhances tensile strength and structural integrity. After applying seismic events, a decrease in Rsc and Dcl was noted, while Ltc increased. This phenomenon is attributed to the redistribution of stresses and structural realignment under seismic loading, which closes some surface cracks but exacerbates internal stress concentrations, driving the extension of subsurface cracks. However, the inclusion of PF and WSF effectively reduced the crack parameters, enhancing the soil's crack resistance. The fibers' ability to enhance tensile strength and distribute stresses more evenly within the soil matrix was key in mitigating desiccation cracking. W-D cycles generally increased the cone index (CI) of the soil, indicating improved compaction and strength due to cyclic moisture-induced consolidation. PF and WSF inclusion further enhanced these properties, with higher fiber content leading to greater increases in CI values. This enhancement is attributed to the fibers' ability to reinforce the soil matrix, providing additional tensile strength andxvi resistance to mechanical deformation. Applied seismic event led to a decrease in CI values across all specimens, reflecting the disruptive effect of dynamic loading on soil structure. Despite this reduction, PF and WSF treated soils maintained higher CI values compared to untreated soils, demonstrating the resilience provided by fiber reinforcement. The deformation behavior of CL soil was significantly influenced by W-D cycles, applied seismic event, PF and WSF treatment. PF and WSF treated soils exhibited substantially lower deformation values compared to untreated soils, with higher fiber content providing greater reductions in deformation. The fibers' ability to enhance tensile strength and interlock soil particles was key in mitigating volumetric changes under moisture variations.