Particle breakage has a crucial impact on the dilatancy and critical state of calcareous sand. This study performed several triaxial experiments on calcareous sand samples to describe the stress–dilatancy relationship of calcareous sand. The experimental results revealed that critical friction angle always is lower than the mobilized friction angle for calcareous sand. The shear resistance correlates with dilatancy and particle breakage. In addition, particle breakage highly affected the critical state line in the conducted tests. By introducing the concept of breakage parameters, this study established a modified dilatancy equation considering particle breakage under the theoretical framework of state parameters. The experimental results of this study exhibited a good correlation with the model results employing unique parameters.

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.

Studies show that the shear strength of angular-shaped calcareous sand is mainly affected by dilatancy, interparticle locking, and particle breakage. In the present study, a series of consolidated-drained triaxial compression tests on calcareous sands are carried out to investigate the strength impacts of dilatancy, interparticle locking, and particle breakage. It is found that as the stress level increases, the friction angle difference reflecting the effect of the particle breakage increases. In contrast, the friction angle difference reflecting the interparticle locking and dilatancy effect decreases gradually. Moreover, the critical stress ratio of the high-angled calcareous sands decreases, which may be attributed to particle breakage. The obtained results demonstrate that when the stress level exceeds a certain value, the critical stress ratio remains constant. Then the correlation between the particle breakage, interparticle locking, and dilatancy effects in the shear strength is studied. Finally, based on the performed analysis on energy dissipation in calcareous sands, a dilatancy model is proposed that covers the particle breakage effect. The performed analyses demonstrate that the proposed model can accurately explain the experimental data.

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.