Mechanical behaviour and SEM-based microstructural interpretation of Grewia optiva fiber-reinforced expansive soil

Introduction Expansive black cotton (BC) soils exhibit poor strength and significant volumetric instability, creating challenges for infrastructure development. This study evaluated Bhimal ( Grewia optiva ) natural fiber as a sustainable discrete reinforcement for improving the geotechnical response of BC soil. Methods Fiber contents of 0.25, 0.50, 0.75, 1.00 and 1.25% and fiber lengths of 15, 20 and 25 mm were investigated using Modified Proctor, direct shear and unconfined compression tests. Two-way analysis of variance (ANOVA) and regression analysis were used to evaluate the effects of fiber content and length, while scanning electron microscopy (SEM) provided qualitative support for soil–fiber interaction mechanisms. Results The selected optimum configuration, M13 (0.50% fiber, 25 mm), increased maximum dry density from 1.53 to 1.69 g/cm 3 (10.46%) and optimum moisture content from 11.00 to 15.50% (40.91%). UCS increased from 55.6 to 68.0 kPa (22.30%) and cohesion from 51.0 to 60.5 kPa (18.63%). ANOVA showed significant effects of fiber parameters, with fiber length exerting the stronger influence. SEM observations provided localized qualitative evidence of soil–fiber interlocking, particle attachment, crack bridging and pull-out mechanisms. Discussion The classical UCS ≈ 2c relationship showed weak agreement with the reinforced-soil data ( R 2 = 0.088), indicating that this simplified cohesive-soil correlation is not directly applicable to Bhimal fiber-reinforced expansive soil. The results support Bhimal fiber as a promising sustainable reinforcement while highlighting the need to account for altered stress-transfer and failure mechanisms in fiber-reinforced soils.