Research On Damping Mechanism And Technology Of Steel Fiber Foamed Concrete Initial Support

The earthquake disaster of tunnel is serious,and the seismic and shock absorption technology of tunnel in high seismic intensity area has become a research focus.Adding damping layer is a common main anti-damping technology of tunnel,but adding damping layer leads to the increase of tunnel construction procedures and excavation section area,and the construction difficulty increases simultaneously.If the shock absorbing layer is combined with the initial support,the above shortcomings can be avoided.In this thesis,based on the National Natural Science Foundation of China（51978424）,the initial support of fiber optic foamed concrete serves as a tunnel shock absorbing layer,and adopts the research methods of theoretical analysis,laboratory test and numerical calculation to systematically carry out the research on the shock absorbing mechanism and performance of steel fiber foamed concrete initial support.It is of great significance to improve seismic safety.The main research contents and achievements are as follows:（1）Using the Fourier-Bessel function expansion method,a mechanical model of the interaction between surrounding rock,primary support and secondary lining under the interference of three wave modes is established,and the mechanics of tunnel support structure under dynamic load is carried out,and the shock absorption mechanism of primary support under different wave modes is revealed.Reducing the elastic modulus of the initial support can effectively reduce the dynamic stress concentration coefficient of the normal normal stress on the contact surface between surrounding rock,initial support and secondary lining and the dynamic stress concentration coefficient of the normal shear stress on the contact surface between surrounding rock and initial support.However,with the decrease of the elastic modulus of the initial support,The dynamic stress concentration coefficient of the normal shear stress on the contact surface between the primary support and the secondary lining increases at first and then decreases.The elastic modulus of the primary support with better damping effect ranges from 1.7GPa to 11.2GPa.The initial support thickness has little influence on the normal dynamic stress concentration coefficient of each contact surface.（2）By improving the material ratio,the three-factor and four-level orthogonal matching test block was prepared,and the porosity measurement,uniaxial compressive strength,splitting tensile strength and SEM tests were carried out and analyzed.The relationship between the compressive strength and the tensile strength,the volume ratio of steel fiber and the compressive strength of primary porosity and foam porosity were deduced.The optimal mix of initial support materials meeting the strength requirements of the code was determined,that is,the mixture with water-binder ratio of 0.35,density of 1 550 kg·m^-3,steel fiber volume ratio of 0.9%and cement silica fume mass ratio of 9:1.The elastic modulus of the initial support material under the optimal mix ratio was 1.9GPa,which met the requirements of the initial support shock absorption mechanism.（3）In order to accurately describe the nonlinear stress characteristics of steel-fiber foamed concrete,the elastic-plastic damage constitutive model of the initial supporting material of steel-fiber foamed concrete is constructed based on experiment and semi-empirical theory,and the numerical results are realized.Based on the different failure modes of tensile and compressive strength in uniaxial test of steel fiber foamed concrete,separation of tensile and compressive damage is advocated,and the damage evolution equation of differential form under uniaxial tension and compression is derived.The concept of"equivalent strain"is used to extend the uniaxial damage evolution equation to multi-axial stress state,and the material yield criterion and plastic potential function are given.Then the elastic-plastic damage constitutive relationship of the initial support of steel fiber foamed concrete is constructed.The calculation program of the elastic-plastic damage constitutive model was written in C++language to realize the program of the elastic-plastic damage constitutive model of steel fiber foamed concrete,and the numerical simulation of the mechanical properties of steel fiber foamed concrete was carried out,the model parameters were calibrated,and the validity of the model was verified.（4）Based on the elastoplastic damage constitutive model of the initial support of steel fiber foamed concrete,a three-dimensional composite lining tunnel model is constructed,and the damping performance of the initial support of steel fiber foamed concrete is studied.The results show that the steel fiber foamed concrete initial support structure has better damping effect than the conventional structure with damping layer,and its strength is high,which can reduce the construction process and excavation area,shorten the construction period,and has good research value and application prospect.The higher the level of surrounding rock,the higher the seismic intensity and the thicker the initial support thickness,the better the damping effect of the initial support of steel fiber foam concrete is,the damping rate is as high as 36.7%.According to the requirement of safety factor of standard tunnel structure,the recommended value of initial supporting thickness of steel fiber foamed concrete tunnel under different conditions is given.