Strengthening Mechanism of Fiber-Reinforced Cement–Microbially Improved Red-Bed Mudstone Filler

Red-bed mudstone fill is generally treated with cementitious materials. In this study, basalt fibers were added to the cement–MICP stabilization system to improve the ductility of the treated fill, and a series of laboratory tests was performed to examine how fiber content affects the unconfined compressive strength (UCS), shear strength, and impermeability of the improved fill and to identify the underlying improvement mechanism. The results indicate that (1) fiber inclusion considerably increased both the UCS and shear strength of the cement–MICP-improved red mudstone fill: at a fiber content of 0.2%, the UCS reached 1.76 MPa and the shear strength ranged from 122.91 to 290.92 kPa (under normal stresses of 100–400 kPa), corresponding to gains of 396.21% and 70.25–127.09% relative to the untreated soil, respectively; (2) the deformation resistance of the fill was markedly strengthened by fiber addition, as reflected by the cohesion and internal friction angle at a fiber content of 0.2%, which rose by 136.54% and 62.36%, respectively, compared with the untreated soil; (3) fiber inclusion improved the impermeability of the cement–MICP-treated fill, lowering the permeability coefficient by 46.93–57.14% relative to the untreated soil; and (4) the improvement mechanism relies on the synergy among fibers, microorganisms, and cement: the fibers build a network skeleton between particles and serve as attachment sites; the microorganisms produce calcium carbonate that fills pores, bonds particles, and reinforces anchorage; and cement hydration provides an alkaline environment whose products, together with the calcium carbonate, fill pores and reinforce anchorage. Through this three-way synergy, the integrity and mechanical properties of the improved fill are substantially enhanced.