Study on Plugging Scheme and Effect of Coarse Sand Cushion after Drainage Consolidation Treatment of Dike Soft Foundation

To address the anti-seepage risks associated with retained sand cushions in dike soft foundation treatment adopting the drainage consolidation method, this study takes the Huayang River Flood Storage and Detention Area dike project as the research object. Combining numerical simulation, laboratory tests, and gradation analysis, this study explores the impacts of sand cushion layouts pattern on the anti-seepage stability of dikes and compares the plugging performances of cement mortar and Microbially Induced Calcium Carbonate Precipitation (MICP) technology. Numerical simulation results demonstrate that front (upstream)-arranged or continuous sand cushions exceed design thresholds in terms of hydraulic gradient and seepage discharge, thus necessitating mandatory plugging measures. In contrast, downstream arrangement of sand cushions forms an optimized "front blocking and rear drainage" structure, where all indicators meet design requirements; meanwhile without sand cushion condition may lead to slope instability caused by blocked horizontal pore water drainage. Laboratory tests results show that cement mortar fails to satisfy anti-seepage demands due to poor particle-pore compatibility and filtration effects, which impede its effective penetration into the sand layer. In contrast, MICP technology realizes full sand layer grouting by virtue of the high fluidity of bacterial solution and cementing solutions, with  the cementitious product generation rate of 4.47% and the sand layer permeability coefficient reduced to 4×10⁻⁵ cm/s. After plugging, all  performance indicators comply with design standards. This research clarifies the necessity of sand cushion plugging in dike engineering, verifies MICP’s superiority, and provides reliable technical support for optimization of the drainage consolidation method in dike soft foundation treatment.