Instability of the middle partition in deep shafts of tunnels in water-rich strata: A case study of Tianshan Shengli Tunnel No.1-1 shaft

The middle partition is crucial structure in the ventilation shaft of a transportation tunnel, and it is also a weak point in the overall stability of a deep tunnel shaft. However, there is a lack of research reports on the instability of middle partition in deep shafts in water-rich strata. Based on the No. 1-1 shaft of the Tianshan Shengli Tunnel, currently the world's longest operational expressway tunnel. Theoretical analysis and on-site measurement methods were adopted to conduct research on the instability and failure of middle partition in shafts. The results of both methods with numerical calculations were compared. It was indicated that the main cause of the instability of the middle partition is unbalanced water level on both sides. The larger the tunnel shaft diameter, the more unfavourable the bearing stability of the middle partition. The midspan displacements and bending moments of the middle partition increase with increasing water level difference load, and the values increase with higher axial loads. Furthermore, the midspan displacements and bending moments of the middle partition decrease with the increase of section parameters and increase with the increase of tunnel shaft diameter. However, they are not significantly affected by the change of concrete strength grade and reinforcement diameter. The water level difference between the one side of the middle partition led to a three-phase trend of "rapid growth, slow growth, decline" in middle partition displacement, and they all had the characteristics of the closer to the water level. The theoretical solution is better matched with the results of on-site measurements and numerical calculations, which can reflect the instability and failure of the middle partition. On-site measurements also confirmed the cause of the instability and failure of the middle partition. The axial load threshold for instability and failure is 0.03 MPa, and the water level difference load threshold is 10 kN/m. Research results can provide a reference for the design and construction of middle partition in tunnel shaft in water-rich stratas.