Study On The Interaction Relationship Of Surrounding Rocks And Support In Xigeda Formation Strata

The Xigeda Formation strata is a hypabyssal rock formed in different fluvial and lacustrine sedimentary environments.It is widely distributed in the valleys of major rivers and their tributaries in the southwest of China,and is mainly composed of the silty sandstone and claystone.Variable structural characteristics and poor cementation determine the special mechanical properties of the Xigeda Formation.The main manifestations are softening and degradation in water,and the engineering strength is sharply dropped.A large number of tunnels of the newly completed Kunming Railway expansion and reconstruction project will pass through the Xigeda Formation.Construction of the tunnels is prone to lead to the cracking and spalling of shotcrete,deformation of steel frames,roof collapse,and other engineering disasters.The key to the problem is the interaction between surrounding rock and support in the process of tunnel excavation.By studying the dynamic changes in the response of surrounding rock and support mechanics,the steady state of the surrounding rock,and the carrying capacity of the supporting structure can provide a certain theoretical basis for the deformation control and design of surrounding rock in the Geda Formation tunnel.In view of the above problems,this article has conducted in-depth research using the methods of literature research,theoretical analysis and numerical simulation.The main research work and conclusions are as follows:(1)Based on the unified strength theory and ideal elasto-plastic mechanical behavior of surrounding rock,considering the influence of intermediate principal stress and dilatancy of surrounding rock,the analytical solution of stress and deformation of surrounding rock of circular tunnel is given.The study found that for the Xigeda shale-intercalated sandstone,the smaller the unified strength parameter b was,the smaller the radial displacement of the surrounding rock would be,and the larger the radial stress of the surrounding rock,the tangential stress at the inner wall of the tunnel,and the peak tangential stress of the surrounding rock would be.The radial displacement of the surrounding rock would increase with the increase of the dilatancy angle.The lower the strength of the surrounding rock was,the greater the degree of influence of the unified strength theoretical parameter b and the dilatancy angle,especially when the water content exceeds 20%.(2)Based on the constrained loss theory in the spatial effect of the tunnel face,taking into account the hardening properties of the shotcrete,a numerical method for the analysis of the interaction process between the tunnel surrounding rock and the support is proposed,and a series of parameter analysis is carried out.The research shows that under the influence of different theoretical parameters b of unified strength and water content,the interaction between surrounding rock and support are opposite before and after the critical point of support.From the perspective of deformation control,the force of the support structure increases with the increase of the dilatancy angle.When considering the hardening characteristics of shotcrete,the whole process of the force deformation of the steel frame combined supporting body can be divided into four stages.The supporting stiffness and supporting timing have a significant impact on the displacement of the surrounding rock and the stress of the supporting structure.The combined supporting structure of structural steel and grille is applicable to the tunnel of the Xigeda shale-intercalated sandstone formation,but the grid steel frame combined support system is more sensitive to the hardening parameters of the shotcrete and the tunnel excavation speed.(3)Considering the different types of steel frames combination support system,the numerical method was used to simulate the entire construction process of the Xigeda Formation tunnel.The research shows that under different water content and different types of steel frames,the evolution rules of stress of surrounding rock deformation is consistent with that of the supporting element.The control of the combined supporting structure of structural steel and grille to the plastic zone development of surrounding rock and the surrounding rock deformation is quite similar,and the stress of the structural steel support structure is greater.