Study On The Time-Dependent Reliability Of Secondary Lining Structure In Gypsum Rock Tunnel

Gypsum rock,which is widely distributed,is a kind of special rock often encountered in the geotechnical engineering project,with typical swelling and corrosive characteristics.In the tunnel life cycle,the expansion will increase the external load of the tunnel lining structure continually;and corrosion will decrease the strength of the lining structure itself.Under the influence of both inside and outside unfavorable effects,the reliability of tunnel lining structure will be gradually reduced,which will increase the difficulty of tunnel maintenance and bring potential security problems.Therefore,it is of great theoretical and practical necessity to study on the time-dependent reliability of secondary lining structure of gypsum tunnel.In this thesis,based on the existing project of Li-rang tunnel on the Liang-zhong highway,a dynamic simulation method of the whole process of the tunnel is proposed.A time-dependent reliability of the tunnel is discussed considering both inside and outside influences.The damage model of the secondary lining structure of the gypsum tunnel is analyzed,and the corresponding time-dependent reliability model is established.The reliability of the full life cycle of the Li-rang tunnel is calculated;a sensitivity analysis of the parameters is also conducted.The main research contents and innovations of the thesis are as follows:(1)The experiments on swelling characteristics of gypsum rock were carried out,and the time dependent constitutive model of gypsum rock was established.(2)Based on the theory of conventional reliability,combining with the new introduced time variables,the basic theory of time-dependent reliability of tunnel secondary lining structure was established.The failure model,corrosion failure model and comprehensive failure model of secondary lining structure of gypsum tunnel were proposed.The plastic zone volume ratio was used as the criterion of the failure of secondary lining structure of the tunnel.And then the time-dependent reliability models of the three failure models of the secondary lining structure of gypsum tunnel were established respectively.(3)Based on the damage mechanics and the equivalence principle of fracture,the equivalent model of the deterioration of the corrosion strength parameters and the complete corrosion depth of the secondary lining structure was established.In the time-dependent reliability model,the deterioration of corrosion strength parameters was replaced by the complete corrosion depth,which simplified the calculation.(4)Considering the large difference in the structural parameters of the secondary lining structure of the tunnel during the casting and vibrating forming process due to the gravitational effect and the nonuniformity of the vibrations,the secondary lining structure was divided into top,shoulder and bottom parts.The secondary lining structure strength parameter distribution model was established accordingly.(5)The dynamic simulation method of the whole process of tunnel construction was put forward.The dynamic simulation was carried out by the finite element method,using the expansion constitutive model including time variables in the stratum structure method of the continuum model.The basic flow of calculation and the method adjusting the mechanical parameters of the surrounding rock and the secondary lining structure were given.The dynamic simulation method could simulate the whole construction process of the New Austrian Tunneling Method.It could conform to the disciplinarian that at the intial stage,the initial support and surrounding rock resiste load together,and then the secondary lining structure resisted load too.Based on the dynamic simulation method,the stress and strain evolution of surrounding rock and lining structure during tunnel construction and service period could be simulated.The design parameters of initial support structure could be adjusted and optimized by convergence of surrounding rock.(6)Optimize the initial support parameters of the Li-rang tunnel.The optimization scheme were as follows:the amount of reserved deformation was 31cm,and the optimal time for the second lining was 36 days after excavation,and the length of the bolt was 5.5m.After the optimization scheme was adopted,the force and deformation of the surrounding rock and the initial supporting structure of the tunnel during the construction period were within the safe range.(7)The anti swelling lining structure of the gypsum rock tunnel was put forward.The anti swelling lining structure contained initial support,the EPS buffer layer and the secondary lining structure.The method of optimizeing the thinkness of the EPS buffer layer basied on the safety factor of the lining structure was established.The ability of the anti swelling lining structure was analyzed.The field experiment proved that the structure met the requirement of tunnel anti expansion.The 40cm thikness of EPS buffer layer was adopted as the optimum scheme of the anti expansion lining structure in Li-rang tunnel.(8)The dynamic simulation method of the whole process and the time-dependent reliability models of comprehensive failure,and the PDS module of ANSYS were used to calculate the time-dependent reliability of the whole life cycle of the secondary lining structure of the gypsum rock section in the Li-rang tunnel,that adopted the optimized the initial support and the anti swelling lining structure.And the sensitivity of the parameters was analyzed.The change of the parameter sensitivity and the reliability index were in accordance with the"bathtub curve" rule.The influence of the elastic modulus and thickness of the secondary lining structure on its reliability was significant.At the early stage of serviving period,the overall reliability index was above 4.2;after 30 years,it decreased to 4.1;when the serviving time reached 100 years,it declined to 2.39.