Recycled gypsum and rice husk ash as additives in the stabilization of expansive soil

• Main Author: Tan, Jomari F.
• Format: text
• Language: English
• Published: Animo Repository 2018
• Subjects: Swelling soils; Soil stabilization; Gypsum as soil amendment; Rice hulls; Geotechnical Engineering
• Online Access: https://animorepository.dlsu.edu.ph/etd_masteral/6397; https://animorepository.dlsu.edu.ph/cgi/viewcontent.cgi?article=13469&context=etd_masteral
• Institution: De La Salle University

The presence of expansive soils in strata with overlying structures pose great risk to the integrity of the built superstructure. While commonly addressed by means of mechanical stabilization, the practice is deemed costly and impractical especially for small projects. The research aims to identify an economic and environmentally-safe measure in alleviating detrimental consequences brought forth by expansive soils through chemical stabilization. Various amounts of expansive specimen shall be replaced with a set proportions of recycled gypsum. Another set of samples will also be complemented with an additional 10% rice husk ash sourced from a biomass power plant. Evaluation of the effectivity of the admixtures was conducted via determination of index properties and pertinent soil parameters. ASTM D4609 specified laboratory tests in gauging the effect of additives on soil stability. Atterberg limits were only reduced with the introduction of RHA in gypsum-stabilized specimen. Maximum reduction was at 17.76%. Compaction characteristics were improved among gypsum-only and gypsum-RHA treated samples. Maximum dry density increased by as much as 1.918 kPa, which was corresponded with 25.98% decrease in optimum moisture content. Swelling potential was measured through expansion index as specified by ASTM D4829. This value was evaluated based on provisions in Section 303.5 of the National Structural Code of the Philippines. Results identified only 15% gypsum + 10% RHA specimens are considered non-expansive after treatment. Compressive strengths were enhanced with addition of gypsum as lone additive, with peak mean strength of 1.128 MPa at 15% gypsum. Direct shear tests at saturated condition indicated ductile and compressive soil behavior with shear strength of 49.20 kPa for 15% gypsum and friction angle of 41.68° for 5% gypsum + 10% RHA. Good approximation of stress-strain response was made with modified hyperbolic model due to strain hardening and compression in the specimen. Considering criteria from ASTM D4609 and NSCP Section 303.5, 15% gypsum was selected as the optimum additive proportion as it attained the highest compressive strength values among all specimen and registered a low expansion potential, while exhibiting improvements in compaction characteristics.