This study compares micro cement and fly ash injection for improving clayey sand soil (SC) beneath a model strip foundation. A total of 54 laboratory model tests were conducted using injection pressures of 100, 200, and 300 kPa and curing periods of 7, 14, and 28 days. Performance was evaluated from load-settlement response, model test capacity, settlement at a common comparison stress of 100 kPa, unconfined compressive strength, grout consumption, cost, and a screening environmental inventory. The untreated soil had a model test capacity of 185 kPa. After 28 days, micro cement increased this capacity by 162% to 219% at pressures of 200 to 300 kPa, whereas fly ash produced gains of 146% to 189%. At the same pressures and curing age, settlement was reduced by 65% to 72% with micro cement and by 56% to 63% with fly ash. At 200 kPa and 28 days, fly ash achieved approximately 94% of the micro cement model test capacity and 90% of its unconfined compressive strength. The 28-day material-cost index was 3.41 times more favorable for fly ash ($0.044 versus $0.150 per kPa of capacity gain), while the screening inventory indicated an approximately 77% lower binder-related carbon burden. The highest measured responses occurred at 300 kPa; however, grout spread reached the chamber boundaries, so these values may include confinement effects. For the tested soil and model geometry, 200 kPa provided the most defensible balance among performance, grout consumption, cost, and experimental reliability. The results are laboratory-specific and require field trial sections and verification testing before design application.
