Abstract:

The existing primary support cannot bear all the design loads, structural safety risk induced by choosing the favorable structure of primary support inverted arch closure rather than the most unfavorable structure of inverted arch closure as final design target, and it is difficult to design the primary support by rock mechanics induced by lack of anchorage force of bolt and the large deformation of surrounding rock. As a result, a calculation model based on loadstructure is put forward; the structural mechanical force method is adopted to calculate and analyze the design loads borne by primary support, corresponding construction threestep method, the proportionality coefficient of load that should be borne by steel frame and shotcrete, and bearing capacity of steel frame and shotcrete, respectively; and the requirements for shotcrete strength and anchoring force of anchors during construction are clarified. The calculation results show that: (1) The distributed anchor is uniformly distributed except the central angle of 60°; the primary support of tunnel with steel frame + distributed bolt + shotcrete can fully bear all the design loads; and the excavation of inverted arch will not threaten the support stability. (2) The anchoring force is determined by using bolt as support of chain link. (3) The 3hour strength of bolt and shotcrete has great influence on the bearing capacity of supporting structure.

Key words: tunnel, primary support, single load bearing, structural mechanics, twohinged arch, load combination ratio, early strength, anchorage force, inverted arch excavation