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The coronavirus pandemic (COVID-19) has led to a surge in disposable mask waste, posing an urgent environmental challenge. This study explores a sustainable solution by using mask chips as reinforcement material in geotechnical engineering, simultaneously improving ground strength and recycling waste. The mechanical behavior of Fujian sand reinforced with mask chips under plane strain biaxial shear condition was examined. Biaxial shear tests were performed on dense pure sand and sand samples containing three mask chip sizes (20 mm × 5 mm, 10 mm × 10 mm, and 12 mm × 3 mm) at a 0.5% mass ratio, under effective lateral pressures of 50 and 100 kPa. The test results show that incorporating mask chips improved strength and delayed stress peak, with the 20 mm × 5 mm chips demonstrating the best performance. Within the tested lateral pressure range, the sample dilates along the minor principal strain direction during shearing. The volumetric strain of mask–sand mixtures exhibits more significant contraction before the dilation compared with pure sand. Moreover, two distinct shear band shapes were observed through imaging technique where the thickness of the shear band decreases as the lateral pressure rises. Additionally, the Roscoe solution was found to provide the closest approximation of the inclination angle of the shear band in mask-reinforced sand. [ABSTRACT FROM AUTHOR]