Multi-scale mechanical behaviour of XG–EICP reinforced calcareous silt under wet-dry cycles

Calcareous silt is susceptible to degradation under climatic variations, particularly wetting–drying (W-D) cycles. This study investigates the synergistic effects of xanthan gum (XG) and enzyme-induced carbonate precipitation (EICP) on the shear strength of calcareous silt under varying moisture contents and W-D cycles. Five XG dosages (0%–1.2%), two moisture contents (2% and 10%), and four W-D cycle numbers (0, 4, 8, and 12) were considered, with direct shear tests and discrete element method (DEM) simulations conducted. At the optimum dosage of 0.9% XG, the XG–EICP composite increased peak shear strength by 190%–273% and cohesion by 3.7 times relative to plain soil, whereas EICP-only treatment yielded limited cohesion gain. Raising moisture content from 2% to 10% substantially reduced cohesion across all treatments, yet the 0.9% XG–EICP composite retained the highest cohesion. After 12 W-D cycles, XG–EICP soil showed only 21.2% cohesion reduction, versus 32.1% for XG-only treatment. DEM simulations revealed that the composite produced the narrowest shear band (51.3% reduction) and the densest force-chain network, with the highest average contact force increment (51.9%). These findings confirm that XG–EICP composites offer a durable, low-carbon stabilisation strategy for calcareous silt slopes.