Abstract: To solve the problem of safety bearing load for the curved middle wall during construction of the doublearch tunnel, the mechanical evolution law and bias mechanism of the curved middle wall during the construction of a composite doublearch tunnel in Zhejiang province, China, are analyzed using numerical calculation and field monitoring methods. Furthermore, the influence laws of the curved middle wall thickness and the main tunnel excavation distance on the stress and bias effect of the curved middle wall are investigated from the design and construction perspectives. The results reveal the following: (1) Excavation of the top heading of the main tunnel has a significant influence on the stress distribution of the curved middle wall. The stress increase accounts for roughly half of the total stress value. The curved middle wall bears the majority of the vertical stress, and the smallest crosssection is the least favorable position. (2) Unsymmetrical excavation of the multiarch tunnel causes the bias load on the curved middle wall, leading to deflection to the side of the first tunnel. The transition from the first tunnel excavation to the later tunnel excavation has a significant deflection effect. (3) Thickness of the curved middle wall has a significant influence on its stress distribution and deflection effect. The reduction in thickness of the middle wall reduces the structure′s bearing capacity. When the thickness is 1.3 m, the stress in the curved middle wall increases dramatically, potentially resulting in collapse instability. (4) The main tunnel excavation distance has a greater influence on the stress distribution of the curved middle wall, but has no effect on the final stress of the curved middle wall. The deflection moment of the curved middle wall increases as the excavation distance of the main tunnel increases, and the deflection effect lasts longer.

Key words: composite doublearch tunnel, curved middle wall, field monitoring, mechanical behavior, bias effect, curved middle wall thickness, excavation distance