# Shear Strength and Durability of Expansive Soil Treated with Recycled Gypsum and Rice Husk Ash
###### 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. recycled gypsum; rice husk ash; expansive soil; shear strength; modified hyperbolic model; wet-dry cycle; durability; water immersion; collapse index; strength loss Article  
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projects, yet they can be found to be highly prevalent throughout the country. [PERSON] and [PERSON] [7] cited shrink-swell behavior in soils as incorrectly identified as settlement. Furthermore, they explained that expansive soils in the Philippines are formed due to the flooding of low-lying areas containing volcanic ash deposits. Research on expansive volcanic soils has been mainly focused on their viability as landfill liners. [PERSON] et al. [8] revealed that smectite in Manila indicates moderate to high plasticity indices and low hydraulic conductivity values ideal for landfill lining. Research by [PERSON] and [PERSON] [9] on compacted clay linings was conducted in response to the imminent operation of a sanitary landfill in Kauswagan, Lanao del Norte, and nearby municipalities. Local soil samples intended as landfill lining material were identified as expansive soils. The distress caused by shrink-swell soil behavior impacts overlying structures, causing costly damage to life and property. Swelling action has led to cracking and breakup on pavements, roads, building foundations, slab-on-grade members, channel and reservoir linings, irrigation systems, water lines, and sewer lines [3; 10; 11]. [PERSON] and [PERSON] [12] also stated that damage done by expansive soils spread over vast areas rather than concentrating on a small locality. Furthermore, expansive soils can lie in inclined areas on embankments, cuttings, and channel slopes, leading to a greater risk of landslides [4]. To address these concerns, engineers often resort to implementing soil stabilization techniques. Soil stabilization and improvement include the modification of specific properties in a mass of soil to produce improved parameters, bringing forth improved engineering performance [13]. [PERSON] et al. [14] emphasize that the progress in soil performance should be relevant to its intended use. Some parameters considered and improved in this practice are shear strength, compressibility, density, and hydraulic conductivity.