Experimental study on the reinforcement performance of unloading-damaged mudstone by microbial grouting

To address the structural deterioration and water-induced swelling of mudstone caused by unloading damage during foundation excavation, MICP and MICP-volcanic ash grouting reinforcement tests were conducted. Mudstone specimens with different unloading damage degrees were prepared through triaxial unloading tests. Combined with physical-mechanical tests and microstructural analyses, the swelling deformation, mechanical properties, and microscopic reinforcement mechanisms of unloading-damaged mudstone under different reinforcement methods were systematically investigated. The results show that unloading damage significantly reduces the strength of mudstone and enhances its swelling sensitivity. Both MICP and MICP-volcanic ash treatments can effectively suppress swelling deformation and improve strength. Specifically, MICP increased the residual strength by 7.41%-13.98% and the peak strength by 2.49%-6.34%, while MICP-volcanic ash increased them by 13.45%-20.61% and 5.36%-9.45%, respectively. Microstructural analysis shows that calcium carbonate mineralization products induced by MICP improve the structural integrity of mudstone by filling pores, sealing cracks, and enhancing interparticle cementation. The introduction of volcanic ash further promotes microbial retention, mineralization deposition, and densification of the pore-fracture structure, resulting in a better reinforcement effect than MICP alone. The results can provide a reference for the green reinforcement and stability improvement of unloading-damaged red-bed mudstone foundations.