Experimental study on the mechanical interaction in the “tunnel before station” method

The “tunnel before station ” (TBS) construction method is increasingly being adopted in urban core areas featuring upper-soft and lower-hard strata to resolve scheduling conflicts in metro projects. However, the interaction mechanism between shield tunneling and subsequent mined station excavation remains poorly understood, thus posing significant risks to ground stability and structural integrity. Based on the Guangzhou Metro Memorial Hall Station project, this study clarifies how the timing of shield passage, either before or after the construction of the arch support of the station, affects the mechanical behavior of the surrounding rock and supporting structures. The goal is to provide a theoretical and experimental basis for optimizing construction sequences in similar complex geological conditions.

Herein, a combined approach of physical model tests and numerical simulations was employed. A geomechanical model test system was designed based on standard similarity relationships, utilizing stratified similar materials with varying mix proportions for accurate simulation of the mechanical characteristics of the upper-soft and lower-hard composite strata. Two representative working conditions were established: Condition 1, in which shield tunneling occurred after arch support installation, and Condition 2, in which shield tunneling was conducted before arch support formation. Comprehensive monitoring was conducted to assess surface settlement, subsurface displacement, and stresses in shield segments and the arch support.

The following results were obtained: ① Surface settlement during the pile–beam–arch station construction followed a typical unimodal Peck curve. Shield tunneling conducted after arch support formation (Condition 1) resulted in a maximum settlement of 42.1 mm, which was 5.9% lower than the 44.6 mm observed under Condition 2. This indicated that a completed arch support notably minimizes surrounding rock disturbance and improves settlement control. ② The stress on shield segments was lower under Condition 1, with a maximum principal stress of 1.31 MPa, compared with 1.63 MPa under Condition 2, indicating a 24% increase in the latter. Condition 2 also showed larger stress concentration areas and tensile regions on the inner surface of segments, indicating a less favorable mechanical state. The pre-existing arch support under Condition 1 enabled better load release and redistribution, thereby reducing demands on the segments. ③ The arch support above the main station excavation experienced the highest structural stress, reaching a maximum compressive stress of 6.7 MPa under Condition 1, remarkably higher than the 4.7 MPa under Condition 2. This confirmed the role of arch support as the primary load-bearing component. Subsequent shield tunneling had a limited impact on the stress state of the arch support in both conditions. Validation revealed a high degree of agreement between model tests and numerical simulations, with errors in key metrics remaining below 6%, further confirming the reliability of the findings.

The results from both physical model tests and numerical simulations indicated that the timing of shield tunneling in relation to arch support construction is crucial for understanding the interaction mechanics in TBS projects within upper-soft and lower-hard strata. Prioritizing the completion of the arch support before shield passage (Condition 1) offers better settlement control and reduces stress on shield segments, making it the recommended sequence for enhanced safety and performance. If shield tunneling must occur before arch support (Condition 2), additional measures such as ground improvement or segment reinforcement should be considered to mitigate the identified risks of increased settlement and adverse structural loading. The developed model test system and analytical methodology provide a reliable and adaptable framework for investigating similar complex tunneling-station interaction challenges in underground engineering.