Analysis On Excavation Stability And Primary Support Stress Characteristics Of High Geostress Tunnel In Horizontal Gently Inclined Rock Strata

With the rapid development of China’s transportation network,the construction of tunnels in mountainous areas have to face a variety of complex and dangerous geological conditions,one of which is the rock mass with high geostress layered structure.Tunnel excavation in this kind of rock mass is prone to have some problems such as excavation instability,support structure failure and bias pressure,which cause great security risks to tunnel construction personnel.Combined with specific engineering examples,this paper studies the tunnel by the way of data investigation,theoretical analysis,on-site monitoring,indoor experiments and numerical simulations.The main contents and conclusions of this research are as follows:(1)Firstly,the in-situ lithology and stress environment were obtained by means of in-situ coring and in-situ stress testing.Furthermore,the stress-strain curves,basic mechanical parameters and failure modes of each rock are obtained through indoor compression tests.With the aid of acoustic emission system,the fracture process of rock samples is also analyzed.Finally,the 3D uniaxial discrete element model is established and verified by 3DEC,in which simulated results highly coincide with the actual indoor tests.(2)Considering the instability collapse problem,the excavation model of high geostress horizontal rock stratum is established and simulated by 3DEC,where the development process of surrounding rock instability is analyzed after tunnel excavation.Experimental results show that the vault collapse cavity is about 5m wide and 4m high,and the fracture depth of the tunnel face is 3～4m,and the instability form of surrounding rock are consistent with the engineering site.The instability mechanism is that the macro slip surface is formed under the action of tension bending shear failure and shear slip failure,and then the instability occurs under the gravity and other factors.Further analysis of surrounding rock instability factors found that small excavation dimension and large strength of advanced support measures have a greater impact on roof settlement.Finally,the overall support scheme is optimized,which has obvious control effects on surrounding rocks.(3)In this paper,the bias stress problem of the primary support structure under the condition of gently inclined strata is explored.The influences of rock stratum dip angle,rock stratum thickness,lateral pressure coefficient and bolt support on the deformation and stress of primary support structure are discussed,respectively.Results show that the interlayer bending deformation is greater than the interlayer sliding deformation.The greater of rock stratum dip angle,the more severe the bias pressure of the primary structure.The greater of rock stratum thickness,the weaker the force of the primary structure.Reasonable bolt support scheme can reduce the bias of the primary support structure.