Effects of Xanthan gum, biochar, and vegetation on hydro-mechanical behaviour of expansive soil

Expansive soil presents major engineering challenges due to its water sensitivity. This study proposes a novel bio-based strategy using Xanthan gum (X), biochar (B), and vegetation (R) to stabilise weakly expansive soil. The study aims to investigate the synergistic effects of these modifiers on the hydro-mechanical properties of soil, and to provide a nature-based solution for infrastructure resilience and slope revegetation. Different dosages of Xanthan gum (0.5% and 1%) and fixed-content biochar (5%) were adopted. A series of lab tests of vegetated and bare soil groups were prepared for comparison. Test results indicated that 0.5% Xanthan gum combined with biochar most effectively promoted plant growth. The R0.5XB group reduced the no-loading swelling ratio to 18% of that of bare soil and increased the shrinkage limit by 40%. The increase in shrinkage limit effectively restrains volumetric shrinkage of soil and improves its moisture stability. Xanthan gum primarily reduced saturated hydraulic conductivity by one order of magnitude and decreased disintegration, while plant roots enhanced shear strength parameters, although this improvement weakens at higher Xanthan gum concentrations. The synergy interactions among these components regulate soil behaviour by complementary and competitive mechanisms.