Abstract: To prove more intuitively the fact that the primary support of tunnel has independent bearing capacity, the principle of direct elastic resistance method is put forward. In the direct elastic resistance method, two functions of arch (curved beam) and elastic foundation arch (curved beam on elastic foundation) are used to reflect the internal force and displacement of the support structure of arch from surrounding rock and the support structure of sidewall pressing against the twosection structure of surrounding rock, respectively, which can greatly simplify the calculation process and reflect the stress state of tunnel support structure more realistically. Combined with the chain rod support arch and hinged support arch models, three supporting stress states of actual construction, i.e. no distributed bolt influence, no tangential force with distributed bolt influence, and tangential force with distributed bolt influence, are simulated. The possible stress state of tunnel support construction is summarized comprehensively, and the theoretical fact that the primary support of tunnel has independent bearing capacity is fully proven. It is emphasized that "flexible support first and then rigid support, stress release first and then resistance" belongs to concept; the main reason for the primary support deformation is the settlement due to insufficient foundation bearing capacity, and the anchoring force of bolt and early strength of shotcrete severely affect the independent bearing capacity of the primary support. Finally, according to the various possible stress states of the tunnel support structure, the improvement suggestions on the detail design of the primary support structure of the tunnel are postulated, which include the connection between the anchor rod and steel frame, connection between steel frames, the treatment of the bottom foot of steel frames,footlocking pipe pile, and the combined structure of the shapedsteel frame and the grid steel frame. The theory that the primary support of tunnel has independent bearing capacity is proven by practice.

Key words: tunnel primary support, assumed resistance, direct elastic resistance, arch (circular curved beam), elastic foundation arch (circular curved beam on elastic foundation), link support, hinged support