Phosphate cement-based materials are fast-hardening cement materials, which have been applied to the rapid repair of concrete structures. However, the excessive setting rate could lead to initial cracks in the cement-based matrix. Therefore, a proper retarder is required to reduce the setting rate, thus improving the strength of structures. In this study, a magnesium chloride retarder was selected, and its influence on the setting time, slump flow, and the mechanical strengths (flexural strength, compressive strength, and bond strength) of phosphate cement paste curing for 3 h~28 d was investigated. Scanning electron microscopy, X-ray diffraction, and thermal analysis were used to analyze the mechanism of the properties of phosphate cement paste. Results showed that the setting time increased exponentially with the mass ratio of magnesium chloride by the total mass of magnesium oxide. Meanwhile, the slump flow increased linearly with the increasing dosage of magnesium chloride, and the drying shrinkage rate exhibited a quadratic function with the curing age. The addition of magnesium chloride decreased the mechanical strengths of phosphate cement paste at earlier curing age (lower than 3 d) and effectively improved the mechanical strengths at a later curing age (equal to or higher than 3 d). Moreover, magnesium chloride could also decrease the drying shrinkage rate. It can be obtained from the microcosmic researching results that magnesium chloride can inhibit the hydration of phosphate cement and reduce cracks induced by drying shrinkage at later curing age (higher than 3 d).

To reveal the mechanical response mechanism of bolted connections during the jacking construction of prefabricated utility tunnels, this study combines field monitoring and numerical simulation based on the utility tunnel project in Xiong’an New Area. The evolution characteristics and spatial distribution patterns of bolt stress are systematically investigated. The results indicate that during the jacking and unloading stages, bolt connection stress exhibits a V-shaped spatial distribution with opposite evolutionary trends. This reflects both the weakening effect of transient construction loads on the connection system and the self-recovery characteristics after unloading. Segments at the launching stage experience the most complex loading conditions, with bolt stress displaying a nonlinear W-shaped gradient distribution. The mechanical mechanism transitions from being jacking-load dominated to stratum-load dominated as construction progresses. The distribution of bolt stress shows pronounced non-uniformity, with higher values at the upper edge and lower values at the lower edge. Compared to the lower edge, bolt stress at the upper edge of segments 1–5 attenuates by an average of 25.52–37.63%. This non-uniformity results from the coupled effects of ground friction, which dissipates jacking stress at the bottom while inducing forward tilting of the upper structure, thereby causing greater compression and rebound of the waterproof rubber strip at the upper edge. The non-uniformity coefficient decreases linearly with construction progress, reflecting a transition from an unstable to a uniformly stressed state. The cumulative effect of jacking load exhibits stagewise attenuation along the utility tunnel. In segments 1–2, significant strain concentration occurs, with peak compressive and tensile strains. In segments 3–4, direct jacking influence weakens, but delayed strain deterioration is observed, with maximum compressive strain reaching only 69.72% of that in the initial stage. By segment 5, the jacking effect attenuates to background levels. The relationship between bolt stress and jacking spacing follows an exponential decay function. Bolt stress during the loading stage is approximately 76.99–89.07% of that during the unloading stage. Deformation at the roof is significantly greater than that at the floor and sidewalls, consistent with bolt stress distribution patterns.

Microbially induced carbonate precipitation (MICP) is a promising biogeotechnical technology, but its large-scale application is constrained by the high cost and energy consumption of bacterial cultivation and harvesting. This paper presents a metal ion-induced flocculation strategy for the preparation and storage of high-density Sporosarcina pasteurii seed cultures. The flocculation performance and biological effects of Ca²⁺ and Fe³⁺ were systematically compared. Ca²⁺ achieved efficient flocculation through electrical double-layer compression while maintaining near-neutral pH, thereby preserving urease activity and bacterial viability. Even at an inoculation ratio of 1:10,000, the recovered bacteria exhibited rapid regrowth and high urease activity. In contrast, although Fe³⁺ induced rapid flocculation at low concentrations, the acidic conditions severely impaired bacterial viability and regrowth. Cryopreservation with glycerol at −20 °C maintained bacterial activity for at least 90 days and markedly outperformed storage at 4 °C. The recovered flocculated bacteria exhibited CaCO₃ precipitation performance comparable to that of freshly cultured suspensions. This study provides a simple, economical, and scalable strategy for the preparation and long-term storage of MICP seed cultures, facilitating the practical implementation of large-scale biogeotechnical applications.