Theoretical and Experimental Analysis of Stress Variations in Precast Concrete Segment Linings during Double-Shield TBM Excavation of Subway Tunnels

During double-shield tunnel boring machine (DS-TBM) excavation of urban subway tunnels, backfilling with pea gravel and grouting is usually performed behind segment wall linings. However, insufficient pea gravel backfill and delayed grouting compromise segment stability, leading to issues such as dislocation, damage, and groundwater leakage. This study developed a simplified cantilever-type Timoshenko beam model to analyze segment stresses during DS-TBM construction. Numerical solutions for the deflection and internal forces of the tunnel structure were obtained using the finite-difference method. An experimental evaluation was performed to assess stress variations in segments during construction. Theoretical results were then compared with field data indicating that the model slightly underestimated the observed segment settlements. Subsequent optimization of the model suggests adjustments to account for plastic-stage tunnel deformation and construction-related disturbances. The optimal correction coefficient values were determined as follows: kc=0.85 (longitudinal stress), kl=0.91 (axial stress), and ks=1.0 (radial stress). Field tests identified three stress variation stages in segments: (1) inadequate pea gravel backfill, (2) full pea gravel backfill (pre-grouting), and (3) post-grouting with the second stage contributing most to settlement. Consequently, practical recommendations include improving the pea gravel backfill quality and adopting advanced and stepwise grouting techniques to control segment settlements. The primary contribution of this study lies in its focus on stress and stability control of segmental tunnels during the construction of DS-TBM. This research offers detailed recommendations and strategies aimed at enhancing segment stability, emphasizing the importance of engineering construction and site management. In addition, it provides valuable insights to advance the technology associated with DS-TBM tunnel construction.