Hydro-Mechanical Performance and Microstructural Evolution of Biopolymer-Modified Granite Residual Soil

This study comparatively investigates the efficacy of two natural, plant- and microbe-derived polysaccharides—xanthan gum (XG) and guar gum (GG)—in enhancing the water stability and shear strength of granite residual soil (GRS). GRS specimens treated with varying dosages of XG and GG were cured for 14 days and subsequently evaluated through direct shear and static-water disintegration tests. Concurrently, scanning electron microscopy (SEM) and low-field nuclear magnetic resonance (LF-NMR) were employed to elucidate the underlying microstructural and pore-scale mechanisms. Direct shear test results indicate that the peak shear strength reached 295.9 kPa (2.0% GG) and 221.0 kPa (1.5% XG), representing increases of 58.2% and 35.7%, respectively. Quantitatively, GG and XG treatments yielded maximum internal friction angle improvements of 52.96% and 39.37%, with peak cohesion increases of 55.27% and 35.7%, respectively. During static-water immersion, the untreated GRS suffered complete disintegration within 200 s. In contrast, the 2.0% GG- and XG-treated specimens preserved overall structural integrity for 24 h. SEM observations revealed that XG and GG reconstruct the soil fabric by forming encapsulating films and interparticle bridging structures. Finally, LF-NMR analysis provided definitive quantitative proof of a “pore refinement” effect, where biopolymer treatment shifted the primary T2 peaks from 4.64 ms to 3.51 ms. Notably, at a 2.0% dosage, dramatic NMR signal surges (up to 747.5 a.u. for XG and 704.3 a.u. for GG) revealed that excessive biopolymers tend to form localized ‘gel lumps’ rather than uniform films. These blobs weaken the biting force between soil particles, thereby accounting for the observed degradation in shear strength.