Upper Limit Analysis Of Collapse Shape Above Cavity Roof Based On The Generalized Hoek-Brown Criterion

The demand for underground openings is increasing,and cavities are frequently constructed for subways,railway tunnels or power houses in engineering practise,resulting in cavities needing to be built in close proximity to each other or close to the ground surface inevitably.In such cases the roof stability is of primary concern to engineers.Based on the generalized Hoek-Brown criterion and the upper bound theorem of limit analysis,the dimension of a rock bank which would actually collapse from the cavity roof for different overburden depths and various roof shapes is studied in this dissertation.The methods and results of the research can provide references for understanding the range of collapsing block,designing the schemes of support and optimizing shapes of cavity.Centered on the topics,the following works are carried out:1.Using the variational approach and the principle of virtual power,the equation of velocity discontinuity line(vdl)is derived according to a two-dimentional failure mode of surrounding rock mass above roof for deep or shallow cavern.Shapes of collapse are drawn and the effects of relevant parameters are analyzed.Comparison between a rectangular and a circular excavation profile for deep carvern is made.2.The instability conditions of roof in cavity are studied.The probability of progressive failure along the crown of cavity with generic profile and a critical depth expression for classifying deep and shallow circular caverns is discussed and proposed respectively.3.To acquire three-dimentional stability analysis of a deep cavern,a three-dimentional rotational failure mode of the cavity roof is built based on a two-dimentional failure mode.By calculating the internal energy dissipation and the external virtual power,an objective functional which contains equation of the surface of collapse block is obtained.With the help of the calculus of variations,the expression of velocity discontinuity is derived,and the influence rules of parameters,such as rock mass characteristics,supporting pressure and dimension of cavity roof,are analyzed.The collapsing block calculated by two-dimentional failure mode and three-dimentional failure mode for deep rectangular carvern is compared.4.The limit half-span of the ellipsoidal cavern being in a critical stable state are obtained,and a stability graph which relatives to support forces and roof dimension is proposed.