Excavation in foundation pits can result in serious issues for nearby tunnel structures like deformation, differential settlement, and seepage damage, which profoundly impact project timelines and potentially endanger life and property safety. Therefore, it is imperative to investigate these impacts before and after construction and to facilitate timely adjustments of construction measures and reinforcement where possible. In this study, a foundation pit construction project near a rail transit line is employed as a case to comprehensive study the impact of on-site deep foundation pit excavation on adjacent rail transit structures by numerical simulation. A three-dimensional finite-element model of the foundation pit based on site geological characteristics and construction procedures is established to study the excavation and maintenance processes. Through analysis of key parameters including soil deformation, displacement, shear force, and bending moment of the tunnel structures, the designed protective structure is found to have effectively mitigated soil deformation, ensuring the stability of the foundation pit. As excavation progresses, lateral soil deformation and vertical uplift gradually increase but remain within specified control values. During various excavation stages, the maximum displacement of the tunnel structure gradually increases, with the increase rates of maximum settlement being 29.09%, 20.51%, and 6.45%, respectively. This indicates a gradual enhancement of the stability of the tunnel structure. Additionally, excavation of the foundation pit has a significant impact on the bending moment distribution of the tunnel structure but does not affect the axial force and shear force of the tunnel structure. The findings of this study offer crucial scientific insights for evaluating the safety and stability of construction near tunnel structures.

To study the combined effects of freeze–thaw cycles and chemical corrosion on deterioration mechanism of triaxial mechanical properties of sandstone in cold regions, triaxial compression tests were carried out on homologous sandstone specimens after corrosion action of acidic, alkaline or neutral solution and cyclic freeze–thaw action, by taking the pH value of the hydrochemical solution and the number of freeze–thaw cycles as control parameters. The failure mode of triaxial compression is shear-slip failure, and the deviatoric stress–strain curves can be divided into four stages of compaction, linear elastic, yield, and failure. The stress–strain curves and deterioration law of triaxial mechanical properties were analyzed. With the increase of freeze–thaw cycles, the peak stress, peak strain and modulus all decrease, the deterioration degree of the rock specimens increases, and the yield plateau and plastic characteristics in the yield stage become weaker. The strong acid solution shows the largest degree of damage, and the peak stress, peak strain and modulus decreased significantly. The precipitates produced by the reaction between the strong alkaline solution and the specimens adhere to the specimen surface, pores, and fissures, thereby inhibiting further damage and deterioration. Confining pressure is an essential factor affecting the rock mechanical properties. As confining pressure increases, the strength and deformation-resistant ability of specimens increase, and the yield stage on the stress–strain curve is more obvious. The sensitivity of acidic solution to the deterioration of mechanical properties of sandstone is obviously greater than that of alkaline solution. These results provide a theoretical basis for the construction of geotechnical engineering structures in cold regions.

With diminishing natural aggregate resources and increasing environmental protection efforts, the use of recycled fine aggregate is a more sustainable approach, although challenges persist in achieving comparable mechanical properties. Exploration into the incorporation of steel fibers with recycled aggregate has led to the development of steel-fiber-reinforced recycled aggregate concrete. This study investigates the shrinkage performance and compressive constitutive relationship of steel fiber recycled concrete with different steel fibers and recycled aggregate dosages. Initially, based on different replacement rates of recycled coarse aggregate and different volume contents of steel fiber, experimental results demonstrate that as the replacement rate of recycled coarse aggregate increases, shrinkage also increases, while the addition of steel fiber can mitigate this effect. An empirical shrinkage model for steel fiber recycled concrete under natural curing conditions is also proposed. Subsequently, based on the uniaxial compression test, findings indicate that with an increasing replacement rate of recycled fine aggregate, the peak stress and elastic modulus of concrete decrease, accompanied by an increase in peak strain, and the addition of steel fiber limits concrete crack development and enhances its brittleness while the peak stress and strain of recycled fine aggregate concrete are enhanced. However, the steel fiber volume percentage has a negligible effect on the elastic modulus. A constitutive relationship for concrete considering the effects of recycled fine aggregate and steel fiber is also proposed. This finding provides foundational support for the influence patterns of steel fiber dosage and recycled aggregate ratio on the mechanical properties of steel fiber recycled concrete.

Calcareous sand is a widely used foundation material in marine engineering. Particle breakage can lead to an increase in its compressibility, affecting the safety and stability of structures. The compression characteristics and particle breakage effects of calcareous sand were analyzed through 33 sets of confined compression tests using calcareous sand specimens. The initial void ratios of the specimens were set between 1.0 and 1.3, and three gradations were selected: 1–2 mm, 0.5–1 mm and 0.25–0.5 mm. Moreover, the loading–unloading–reloading tests were conducted. The initial test was loaded to a maximum of 2 MPa and began to unload to 0.4 MPa and then to load to 6 MPa. The test results indicated that the deformation of calcareous sand during compression was mainly plastic deformation, which was irreversible or irrecoverable. With the increase in the vertical pressure, the compression curves of the specimens under different initial void ratios gradually converged to a common straight line. With the increase in the loading, the existence of particle size less than 0.074 mm in the specimen showed a decreasing trend, and all types of particle breakage indices showed an increasing trend. The particle size of the specimen was larger, the value of the breakage index was larger. Comparison of multiple breakage indices proposed by researchers revealed that the breakage indices increased with the increase in the vertical pressure. Some breakage indices cannot adequately represent the wholeness of the gradation curves and are not easy to be used as breakage index. Generally, the relative breakage index of Hardin B_r was smaller than the relative breakage index of Einav B_e, and there was a close relationship between the two. The test results can provide theoretical guidance for the construction of calcareous sand islands and reefs.

In urban metro construction, shield tunneling often needs to pass through building and bridge pile foundations, potentially affecting the stability of existing structures. Therefore, accurately assessing the impact of shield tunneling on bridges and buildings is crucial. This study presents a comprehensive prediction method combining numerical simulation and empirical formulas, taking the underpass project of the Shijiazhuang–Wuhan High-Speed Railway Bridge by Zhengzhou Metro Line 5 as a case study. Three-dimensional numerical model calculations were performed using finite element software to analyze the displacement and stress changes of buildings and tunnel structures at different construction stages, revealing the deformation patterns of buildings adjacent to the tunnel during shield tunneling. In particular, the ground settlement caused by twin-tunnel excavation was compared with Peck’s empirical formula to verify the reliability of the numerical simulation. The results show that twin-tunnel excavation exacerbates the horizontal displacement, uplift, and settlement of the ground, with maximum deformation rates increasing by 7.10%, 20%, and 11.4%, respectively. Comparing the ground deformation results of Peck’s empirical formulas with numerical calculations revealed similar trends in the settlement curves, with a maximum deviation of 6.67%. It can be concluded that using Peck’s empirical formula to calculate ground deformation characteristics complements the limitations of numerical simulations, making the assessment results more reliable. The findings of this study demonstrate that integrating numerical simulation with empirical formulas significantly enhances the reliability of deformation predictions in complex tunneling scenarios. This research not only offers a comprehensive safety assessment method for shield tunneling construction but also provides valuable guidance for the design and construction of similar projects, serving as a theoretical reference for future engineering endeavors.