Zhaoping Li

Artificial ground freezing (AGF) is widely used for ground reinforcement and groundwater control in urban underground construction, but its application to irregular subway station passages is challenging when the freezing zone is constrained by existing underground structures and nearby buildings. In this study, the planned X-1 passage at Gaojiayuan Station of Beijing Metro Line 14 was adopted as an engineering design scenario. The passage was still at the engineering design stage and had not yet entered construction. An asymmetric freezing pipe layout was proposed to balance frozen wall development and frost heave control under the asymmetric surrounding constraints. Freezing pipes near the existing subway structure were arranged to compensate for the prescribed thermal disturbance, whereas those near the residential buildings were shifted inward to limit unnecessary outward freezing. A three-dimensional sequential thermo-mechanical numerical model was established to evaluate temperature and displacement responses during active freezing. After 60 days, a continuous frozen wall was formed, and the volume-averaged temperature within the designed 2.0 m thick frozen wall domain reached −10.56 °C, below the prescribed limit of −5 °C. Compared with a conventional symmetric layout, the maximum ground surface uplift, building foundation displacement, and existing subway structure displacement were reduced by approximately 2.5%, 3.1%, and 8.3%, respectively. Sensitivity analysis showed that these relative displacement reductions remained stable when the equivalent frost heave coefficients were varied by ±20%. Among the investigated cases, a 200 mm inward offset provided a reasonable comparative balance between frozen wall development and deformation mitigation under the adopted model assumptions. For the investigated design-stage case, the results indicate that a constraint-oriented asymmetric freezing-pipe layout can redistribute the freezing influence while maintaining the prescribed frozen-wall requirements.

In open-cut assembled subway tunnels, foundation pit enclosure piles are typically cast in place. However, this conventional approach limits the functionality of the piles to serving as retaining structures during excavation, resulting in resource inefficiency and the underutilization of prefabrication techniques. To address this issue, a fully prefabricated pile-wall composite scheme is proposed for cut-and-cover tunnels to optimize the retaining effect of the piles and leverage the benefits of prefabricated technology. In this scheme, the retaining pile and lining are both prefabricated. The pile is the temporary retaining structure during pit excavation and a part of the sidewall. This scheme was researched and applied in Jinan, China. Field monitoring and numerical simulation were used to investigate the load transfer within the fully prefabricated pile-wall composite structure (PPWS) and its mechanical response, respectively. The results show: (1) The development of lateral earth pressures on the PPWS experienced three stages. The lateral earth pressure distribution indicates that the PPWS can fully activate the retaining effect of precast piles. (2) Following the backfilling of the joints, the horizontal displacement at the bottom of the precast pile reduced by 0.39 mm. Numerical simulation results indicate the effectiveness of precast pile restraint in PPWS. (3) The PPWS exhibited uniform deformation transition at the joints. The joints play a crucial role in coordinating deformation between the precast piles and sidewalls, utilizing the restraining effect of the precast piles.

Traditional support structures cannot meet the complex conditions of different excavation depths and areas in underground transportation hubs. On the basis of fully considering the spatial position relationship of foundation pit groups, this article proposes a multilevel retaining system that meets the requirements of multilevel foundation pit excavation. The evolution law of the support structure during the excavation process of the inner pit was explored using on-site monitoring and numerical simulation methods. The results indicate that the excavation of the inner pit reduces the passive earth pressure, and the deformation of the outer support structure can be effectively suppressed by setting a retaining structure or a bottom slab in the bench zone. The excavation of the inner pit causes significant vertical deformation of the support structure adjacent to the foundation pit, while the impact on the structure far away from the foundation pit is relatively small. According to the contact force chain and soil pressure between the two rows of support structure, the soil in this area is divided into a “relaxation zone” and a “compression zone.” The evolution mechanism of earth pressure in the case of mutual-effect failure between two rows of piles is revealed. This paper addresses the deformation properties of multilevel support structures as well as the mechanism of earth pressure evolution between structures.

To solve the impact on the pre-existing main structure due to the need for early traffic restoration of urban roads within the cross-hub and the difficulty of co-ordinating a construction plan among groups of pits, a construction method combining TD and BU was proposed. By establishing a three-dimensional finite difference method (FDM) of parallel pit excavation considering the small strain constitutive model of the strata, the influence of four different excavation schemes on the pre-existing main structs is studied. The results of the study show that: (1) the asynchronous excavation of the pit on both sides of the cover excavation area causes additional deformation of the steel pipe structs; sequential excavation produces the greatest additional deformation of the steel tube columns, followed by staggered excavation and the least simultaneous excavation; (2) the middle 1 slab enhances the overall stiffness of the main structure in the cover excavation area, reducing the additional deformation caused by the unsynchronized excavation of the pit, with the maximum horizontal deformation of 5.8 mm, a reduction of 61%; (3) the greater the depth of excavation and the closer the distance from the pit, the more obvious the deformation of the steel pipe column; and (4) the function relationship between δ_hm/H and the relative stiffness coefficient R_d was obtained by fitting, and the maximum controlled mis-step spacing of the foundation excavation on both sides was 4.3 m when the middle 1 floor slab was cast.