Investigation on the Microstructure, Unconfined Compressive Strength, and Thermal Conductivity of Compacted CDG Soil by MICP Treatment during Curing

As an eco-friendly treatment method, the microbially induced calcite precipitation (MICP) approach has been widely adopted in sand, but its use on compacted fine-grained soils remains scarce. In this study, compacted completely decomposed granite (CDG) soils at both the dry and wet side of the optimum water content (dry- and wet-of-optimum) were treated by the MICP approach, using a bacterial suspension and chemical reagents at fixed concentrations. After various curing periods, the microstructure (by scanning electron microscopy testing), the produced calcium carbonate content, the unconfined compressive strength (UCS), and the thermal conductivity were analyzed through a series of laboratory tests. The testing results indicate that the sample at dry-of-optimum exhibits a flocculated microstructure, while a dispersed microstructure is shown for the sample at wet-of-optimum. The calcium carbonate content increases with the curing period until reaching a stable value at around 6 days’ curing. After 6 days’ curing, the MICP process is relatively static, without significant amounts of calcium carbonate produced. With this treatment, the UCS of the sample is improved, where higher efficiency was observed for the sample at wet-of-optimum conditions. The variation of UCS for the MICP-treated samples with respect to the curing period follows the same trend as that of calcium carbonate content. At dry-of-optimum, the air-solid phases control the heat transfer in the MICP-treated samples with a discontinuous water phase, leading to insignificant effects of the MICP treatment on the thermal conductivity. In contrast, the water-solid phases dominate the heat transfer in the MICP-treated samples at wet-of-optimum, resulting in variation of the thermal conductivity with the curing period similar to that for the calcium carbonate content.