Stability And Support Optimization Analysis Of Complicated Large-Scale Caverns

For a country as China with per capital coverage much lower than the world’s average,the transportation relying on underground caverns can save much ground area for other uses.Constrained by other factors such as landform,some underground structures with close distance to others composes underground multi-cavern.The excavation of each cavern will affect the stability of the existing adjacent caverns significantly and the wall rock with poor stress state among the caverns is prone to shear failure for many disturbances caused by the excavations,which is the key to the success of the structure.Through numerical and theoretical analysises,the paper finishes the work as follows:(1)the bolt support.The interfacial shear stress and axial force distributions of anchor and prestressed anchor are analyzed theoretically.The influence of length of bolt,toroidal space and the value of pre-stressing force on the supporting effect are analyzed through numerical method with FLAC 30 software,conclusions derived as follows:In comparison to wall bolt,the effect of vault bolt is not obvious.The distal will not archor into the elastic zone if the length of archor is too short,while the carrying capacity of bolt is not made full use of if the length is too long,which leads to needless waste of material.In comparison to bolt without prestressed force,prestressed force can change the distribution of plastic zone of surrounding rock,contribute to reduction of displacement which is linearly proportional to the value of prestressed force,but contribute little to the safety factor of cavern.The prestressing force can increase the minimum principal stress,improve the stress state and benefite the stability of surrounding rock.(2)The strength reduction FEM.The theory of the strength reduction is discussed in detail and the current criterions to determine whether the cavern is about to collapse or not are summaried.Taking a two parallel tunnels project as an example,the rationalities of the criterions(the perforation of plastic zone,sudden change of displacement and divergence)are analyzed,conclusions derived as follows:In the process to gain the safety factor of multi-cavern,for the collapse concludes many failure modes and when the failure mode in secondary position occurs earlier than the dominant failure mode,the safety factor derived through the perforation of plastic zone will be erroneous from the reality.For the destructions of every cavern do not occur at the same time and the cavern collapsing earliest will lead to divergence,the factor of other caverns cannot be gained if divergence is taken as criterion.For the strength of bolt is determined by the least value of the ultimate tensile strength,the grip between bot and mortar and the ultimate shear strength between mortar and its surrounding rock,the three values should be reduced simultaneously when the safety factor is gained by strength reduction method if the cavern is supported by bolts.(3)Analysis of multi-cavern construction process.Through FLAC3D software to simulate construction process,the effect of each cavern excavation in the process on the stress state of surrounding rock and stabilities of the existing caverms,the failure mode and mechanism are analyzed and some corresponding support measures to the different failure modes are given,conclusions derived as follows:The vault vertical settlement caused by the excavation of the lower cavern and the vertical upward displacement of rock near inverted arch caused by the excavation of the higher cavern joining near the surrounding rock between the two caverns,eventually contributes to the shear failure.The plastic failure rock between caverns perforates and the plastic failure rock of the vault extendes to the ground surface or the vault plastic failure rock of two parralled caverns cross,leading to the integral instability of the multi-cavern.(4)Optimization of support measures to caverns.With constant length of bolt,toroidal space and layout,the toroidal space of bolt is optimized through global search method.After the optimization of the toroidal space of bolts,the thicknesses of the secondary lining of every cavern are optimized through GA method.