Stability of a Connecting Tunnel in a Shaft–Tunnel System under High Hydraulic Gradient and Staged Excavation: Implication from Numerical Modelling

Feng Gao¹, Guotao Meng², Yuepeng Sun^3,*, Xianglin Huang¹, Heyi Yang¹, Nuwen Xu^3,4,*
1 Power China Sinohydro Bureau 7 Co., Ltd., Chengdu, China
2 HydroChina Itasca Research and Development Center, Hangzhou, China
3 State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, Chengdu, China
4 State Key Laboratory for Fine Exploration and Intelligent Development of Coal Resources, China University of Mining and Technology (Beijing), Beijing, China
* Corresponding Author: Yuepeng Sun. Email: email ; Nuwen Xu. Email: email

Computer Modeling in Engineering & Sciences https://doi.org/10.32604/cmes.2026.082323

Received 13 March 2026; Accepted 27 May 2026; Published online 04 August 2026

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Abstract

During flood seasons in hydropower expansion projects, reservoir level rise may hydraulically connect the excavation pit and shaft to the reservoir. The resulting high-head boundary can impose a strong hydraulic gradient across the unexcavated blocking section, while staged bench excavation further redistributes stresses. This study proposes a three-dimensional hydro-mechanical coupled numerical framework based on Fast Lagrangian Analysis of Continua in 3 Dimensions (FLAC3D), which explicitly accounts for pore water pressure evolution, asymmetric hydraulic boundary conditions, and staged bench excavation disturbance. The framework enables systematic evaluation of rock-plug stability under different retained lengths using plastic-zone connectivity, local strength reserve, and strength-reduction convergence criteria. The results show that hydraulic connection increases pore pressure in the surrounding rock of the connecting tunnel by approximately 0.1–0.3 MPa. When the upper bench advances to Chainage 0+76 m (18 m remaining), the strength reserve of the central rock-wall element exceeds 2.0; and the global strength reduction factor of safety is about 3.0, indicating an adequate safety margin. When the upper bench advances to Chainage 0+82 m (12 m remaining), the plastic zone and the low-strength-reserve band tend to become continuous, and the analysis becomes globally non-convergent at K = 3.0, implying markedly increased instability risk. Plastic-zone connectivity, point safety factor degradation (strength reserve), and strength-reduction convergence provide a consistent integrated criterion. A remaining rock-plug length of at least 18 m is recommended (i.e., not beyond Chainage 0+76 m). The findings provide practical guidance for construction control and rock-plug length design in shaft–tunnel connections under high-head hydraulic conditions.