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  Criteria for Describing the Arch Effect of Weak Surrounding Rock in Super-large-section Tunnels

  A custom-made wooden excavating device with pull-out and jacking functions achieved precise control of the excavating contour and simulation of the support. In the model tests, the progressive development of the arched collapse area in the weak surrounding rock was obtained with this device when excavating a super-large-section tunnel with the double-side pilot method. In previous studies the hoop stress peak point, the point with unchanged hoop stress compared with the unexcavated state, the radial stress peak point, or the maximum tensile strain criterion was used to determine the inner boundary of the pressure arch (i.e., the outer boundary of the collapse area) in the surrounding rock. Unlike above criteria, this study suggested adopting the points with equal radial and hoop stresses directly above the pilot tunnel and middle partition wall as the control points of the inner boundary of the pressure arch, and the inner boundary of the pressure arch extended along the direction of the maximum compressive stress vector. This new criterion narrowed the gap of the results by numerical simulation and model tests and reflected the penetrating collapse problem (pressure arch cannot be formed) in the full-section excavation, which provided practical guidance for the lining design and optimization of super-large-section tunnels.

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