Abstract: Cavern stabilizing mechanism, key geological parameters and their corresponding evaluation system, quantitative design method, construction techniques, and monitoring methods of a giantspan flat underground cavern are analyzed through literature investigation, theoretical analysis, and application verification. Conclusions drawn from the study are as follows: (1) A timely and active support system, which can warm up and recover the bearing capacity of surrounding rocks, is key to a giantspan flat underground cavern construction. (2) The size effect must be considered in the surrounding rock stability evaluation system of a giantspan flat underground cavern. The structural surface occurrence, geostress, span, heightspan ratio, and friction angle are also equally important for the cavern. (3) Spraying + anchoring + a flexible support system is feasible, and it is recommended to adopt a comprehensive design method combining numerical calculation, construction experience of similar projects, and monitoring measurement. (4) Reserved doublerock column support method is rational. (5) The stress and displacement fields of surrounding rocks and structures should be monitored using various methods. A comprehensive identification method that regards the convergence and settlements as key indices while considering internal surrounding rock displacement is proposed to evaluate the stability of surrounding rocks and structures during the construction and operation phases of giantspan flat underground caverns.

Key words: underground engineering, giantspan underground cavern, super flat, tunnel deconstruction and reconstruction field; surrounding rock stability, flexible support system, reserved doublerockcolumn support, monitoring measurement