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Abstract As restricted by project layout，geological and topographical conditions，the underground powerhouse of Three Gorges Project is arranged inside the mountain on right bank. The minimum thickness of rockmass over powerhouse is almost the same magnitude of powerhouse span. Obviously，it cannot meet the code?s requirement that overburden thickness of caverns should be not less than 2 times of the excavation span. For such shallowly buried underground cavern with large span and high sidewall，overburden thickness of rock mass is limited but initial geostress magnitude should be also taken into account. Therefore，the top arch stability is viewed as the primary issue that should be addressed. Based on rockmass structure and rock mass strength characteristics，and the concept that horizontal geostress controls surrounding rock stability，the bearing mechanism of surrounding rock at top arch area of underground cavern is studied. Definition of stable arch and its existence mechanical condition are presented. The method for determining stable arch of underground cavern is proposed as well. The feasibility of applying stable arch concept to determine overburden depth of underground cavern is analyzed. A methodology for stable arch design of overburden depth of shallowly buried underground cavern is then formed. It is revealed that surrounding rock at top arch area of underground powerhouse of Three Gorges Project possesses such stable arch formation conditions in terms of overburden depth and horizontal stress. Its minimum overburden thickness for stable arch formation is two-thirds of powerhouse span. The minimum and maximum values of horizontal lateral pressure coefficients for stable arch formation are 1.5 and 3.0，respectively. The proposed methodology was adopted in the arch design of underground powerhouse of Three Gorges Project. The observations of several years show that the surrounding rock at top arch area of underground powerhouse is stable，thus indicating that under given rock mass strength，rock mass structure and initial geostress conditions of Three Gorges Project，the overburden thickness determined by stable arch design methodology is appropriate and reliable. The surrounding rock stability and project safety can be both satisfied. The design of shallowly buried large scale caverns is therefore provided with reliable guidance.
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Received: 27 December 2012
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2013, Vol. 32 
