Insights Into the Mechanisms Controlling the Residual Strength of Bio-cemented Sands

Microbially induced carbonate precipitation (MICP) is an emerging technique for enhancing the mechanical properties of granular soils. Although several experimental studies have reported increased shear strength in MICP-treated soils at both peak and residual states, other findings have shown reductions in residual strength compared to untreated soils. This study uses the discrete element method (DEM) to investigate the mechanisms governing the residual strength of bio-cemented sands. The results indicate that residual strength may decrease when carbonate precipitates in the form of grain-bridging patterns. In that case, the introduction of carbonates alters the contact network and may induce metastable configurations, particularly when the bonds are weak or non-cohesive. These configurations are prone to strain localisation upon shearing, leading to the development of shear bands and a reduction in residual strength. Conversely, higher cohesive strength enhances microstructural stability, offsetting the weakening effects of localisation. The residual strength of bio-cemented sands is therefore governed by two competing mechanisms, namely bond-induced stabilisation and instability-driven localisation.