Optimization of Bacterial-to-Cementation Solution Ratio for MICP-Treated Sand: Effects on Compressibility and Slope Erosion Resistance.

In engineering applications such as filling and slope protection, natural river sand suffers from high compressibility and poor erosion resistance. Microbially induced calcium carbonate precipitation (MICP) can mitigate these issues by sand solidification, but the optimal volumetric ratio of bacterial solution to cementation solution (rv) for natural river sand remains unclear. This study used natural river sand (0.063-1.6 mm), Bacillus subtilis, and a cementation solution (2 M urea + 2 M CaCl2, 1:1). Eight rv values from 2:5 to 3:1 were tested. Compressibility was evaluated via one-dimensional consolidation tests, and erosion resistance via a slope model. Results show a non-linear "U-shaped" relationship between rv and compression index (Cc). The optimal rv = 3:2 yields the lowest Cc (0.044). Higher or lower ratios increase Cc to ≥0.064. Microscopy reveals that at rv = 3:2, a dense, continuous CaCO3 network fills pores, whereas excess bacteria cause sparse cementation and too few cause local agglomeration. The optimal ratio reduces erosion modulus by 55.0-57.5% compared to untreated slopes. This work provides a quantitative, eco-friendly optimization strategy for MICP-treated natural river sand, balancing mechanical performance with ecological adaptability (pH within vegetation tolerance).