Abstract: For up bamboo-truncating tunnel portal without obvious wall-style structure, there is no existent design method and structure checked mode in present code. In this paper, aiming at bored method and open-cut method for portal section, 3D FEM numerical analysis method is adopted to study the mechanical behaviors and design method of tunnel portal section. The research results show that: Choosing 42 m as the length of portal section and designing as a whole could be reasonable; In the shallow burying section of the extension of 28 m, the surrounding rock pressure of lining crown increases with the increase of depth of tunnel; and pressure of the bored tunneling method tunnel is maximal, the calculation result following the method of the railway tunnel code is the second; and the pressure of the open-cut method is minimal; When taking open-cut method, in the longitudinal direction of the extension of 42 m of portal, the mechanical behaviors of lining crown shows some characters of stringer bending, part of crown is in bended-pulled state; the internal force shows a three-dimensional effect, and the internal force of the direction both transversal and longitudinal should be checked for structure; when taking bored method, the internal force checking is only considered for transversal direction; when the excavation condition allows, the bored method is suggested to do portal tunneling by the reason that this tunneling method can improve mechanical behaviors of tunnel portal structure effectively.

Key words: tunnel, up bamboo-truncating tunnel portal, structural stress, numerical simulation, design method