Abstract: According to the characteristics of long span underground engineering’s complex structure and high safety risk, the paper establishes a synthetically evaluation system by theoretical analysis, numerical simulation and experience evaluation. Through a variety of plastic mechanical calculation method, the Caquot formula, Fenner formula, modified Fenner and Kastner formulas are applied to project practice; select one method is most suitable analyze way for large span structures in hard rock. Using Q system and engineering analogy, determine the supporting parameters of anchor bolt in large-span underground engineering. The length of anchor bolt depends directly on the excavation span in Q system, at same time we can get support parameters of shotcrete. By analyzing the same level span underground works, determined that cable length should longer than 40% of the underground work’s span. Through the three-dimensional discrete element to evaluate the stability of supporting structures. In numerical calculations, rock strength criterion can be defined by the strain-softening model of HB. Constitutive relations of structural surface can use ideal elastoplastic model in Mohr-Coulomb strength criterion. After applying three rows of prestressed-cables at the upper side wall, the of deformation of side wall is significantly reduced. At the same time, the number of potential instability surrounding rock mass and scope are effective significantly controlled. By stress analysis of supporting structure，it is show that the 60% of the anchor’s axial force is between 2.0-2.25 MN; the underground engineering has a good safety. Through the synthetically evaluation system including plastic area analysis, numerical simulation and experience analysis, the formation of three-in-one evaluation system is reached. This system can be used for large-span support structure’s rationality evaluation. Through the analysis, The supporting design scheme can effectively deal with the failure phenomenon of rock block stability risk. In local lowly safety parts, anchor and anchor-cable stress level is comparatively high, can reduce the anchor-cable’s pretension force and increase the number of random anchor.

Key words: large span, supporting structure, stability, comprehensive evaluation