Collapsible soils are located in various parts of the world. These soils are characterized by their low values of dry unit weight and natural water content. Collapse and large induced settlements at the saturation state damage the structures built on them. Therefore, measuring the collapse potential of these soils is essential for safe engineering works. This study aims to investigate the collapse index and collapse potential of clayey soil stabilized with calcium carbide residue (CCR). For this purpose, seven different contents of CCR, five curing periods, three different water contents, and two relative compactions were used. The results of tests showed that the CCR contents, relative compaction, and water content during sample preparation were the most key factors in collapsibility measurements. It was observed that CCR contents greatly reduced collapse index and collapse potential of soil and changed the degree of collapse from moderately severe to slight and non-collapsible one. Furthermore, increasing the relative compaction reduces the pore space between the soil particles, leading the denser structure. The denser the soil, the lower the initial void ratios, hence, there is less collapse upon wetting. Finally, the stabilized samples prepared with 2% less than optimum water content have a higher degree of collapse than those with optimum water content and 2% more than optimum water content. The results of this study corroborate the effectiveness of CCR as a by-product material to improve collapsible soils.

Featured Application This research paper encourages repurposing waste material for ground improvement. The results of this study contribute towards a greater understanding of the strength and durability performance of treated soils under normal, fluctuating, and adverse moisture conditions.Abstract Expansive soil underlying structures pose a significant risk to the integrity of superstructures. Chemical soil stabilization can be used to strengthen soils due to the cost and impracticality of mechanical approaches. Waste materials such as recycled gypsum and rice husk ash have been considered alternatives because of their sustainable and economic advantages. A combination of these additives was used to address the high absorption of gypsum and the lack of cohesion of the pozzolan. The study assessed the short-term and long-term performance of expansive soil treated with recycled gypsum and rice husk ash under normal and fluctuating moisture conditions. Direct shear tests indicated ductile and compressive soil behavior with improved shear strength. A good approximation of stress-strain response was made with a modified hyperbolic model for treated soils that exhibited strain hardening and compressive volumetric strain. Durability and water immersion tests were performed for samples after varying curing periods and cycles of capillary soaking to assess the behavior when exposed to varied environmental conditions. Samples under the modified durability test experienced significant strength loss, with decreasing compressive strength as curing durations increased. Specimens in the modified water immersion test experienced significant strength loss; however, it was determined that curing durations did not contribute to the change in the strength of the sample. Expansion index tests also determined that the treatment effectively mitigated expansivity and collapsibility in all samples. Despite improvement in shear strength and expansion potential, further investigation is needed to enhance the durability of soil treated with gypsum and rice husk ash.