Investigation of the Microstructure and Hydraulic Response of MICP-Improved River Sand

Biomineralization through microbially induced calcium carbonate precipitation (MICP) has been investigated extensively in porous media. The precipitation of biomineralized products often induces complex dynamic pore structures, which affect the fluid flow characteristics in porous media. This study investigates the interconnection between the porosity and the hydraulic conductivity of biocemented sand and the particle cementation patterns. A staged injection strategy was employed using cylindrical molds for biotreatments on river sand. Two-dimensional microscopic analysis and three-dimensional volume reconstruction were performed on the sand after 24 treatment cycles, utilizing stacked images and micro-CT scanning to reproduce the spatial distribution of mineralized precipitation and extract cementation patterns at the pore scale. The variation in pore structure and hydraulic characteristics of the sand before and after biotreatment were evaluated through porosity and permeability tests. The findings showed that CaCO3 occupied the pores between particles, causing a significant decrease in pore coordination number and connection pathways within the pore network. A feedback phenomenon exists between the decrease in connected pore networks and its effect on local hydrodynamics, resulting in fluid-concentrated flowing into a small number of channels that remain connected, facilitating the formation of preferential flow. The overall frequency distribution of pore sizes before and after biotreatment shifts from dispersed to concentrated (from approximately 450 µm before treatment to 350 µm after treatment). Based on mutual validation findings of microscopic image stack and three-dimensional volume reconstruction (with a relative error of <2.49%), three cementation patterns were identified as G-C-G, G-C, and G-G, with the corresponding pore-filling rates decreasing sequentially. The G-C-G exhibited the most effective cementation, with an effective calcium carbonate ratio of 68.5%, whereas G-C had the poorest effective calcium carbonate ratio of 2.4%. Furthermore, through analysis of the representative elemental volume, the feasibility and reliability of cementation patterns extracted from the microscale were verified, providing guidance for macroscale hydraulic feature analysis.