Durability Evolution Mechanism of MICP-Treated Calcareous Sand Exposed to Seawater Dry–Wet Cycles

To investigate the long-term performance evolution of microbial-induced carbonate precipitation (MICP) technology for treating calcareous sand, a 150 day dry–wet (DW) cycle test was conducted considering the actual occurrence of calcareous sand in a seawater environment. Based on the changes in physical and mechanical properties and microstructure of MICP-treated calcareous sand samples under different DW cycles, the macro- and microcharacteristics and evolution mechanism were analyzed. Under the action of the seawater DW cycle, the permeability coefficient and porosity of calcareous sand sample first decrease, then increase, and gradually stabilize. Compared with the initial state without DW cycles, the permeability coefficient increased by 54.85%, and the porosity of the sample increased from 8.90% to 11.41%. The unconfined compressive strength and initial tangent modulus of the samples exhibited a trend of first increasing, then decreasing, and finally stabilizing. The main failure mode was tensile shear failure. As the number of DW cycles increased, the shear failure phenomenon of longitudinal tensile cracks in the samples gradually decreased, indicating that their brittleness gradually weakened and ductility gradually increased. During the seawater DW cycle process, on the one hand, the infiltration and seepage of seawater led to the dissolution and detachment of cementitious materials, and the cemented characteristics between calcareous sand particles gradually became less dense and looser, and the contact mode between particles gradually changed from surface contact to point contact. The increase, expansion, and connectivity of interparticle pores as well as the increase of seawater infiltration channels resulted in frequent changes in the stress state at the contact points due to the alternating DW cycles. As a result, the pores inside the samples gradually became more interconnected, leading to a decrease in their physical and mechanical properties. The relevant research ideas and results can provide valuable reference for the long-term performance evaluation of MICP technology.