Research On Durability Of Shield Section Concrete Of The Metro Submarine Tunnel Under Complexity Environment

Submarine tunnel structures are subject to high level of soil-water pressure and multiple degradation factors during its service life,causing a series of flaws in the concrete,steel and joints of the shield.As the structure ages,the degradation in its bearing capacity,anti-deformation and anti-cracking capacity becomes prominent,creating menace to the safety and serviceability.Referring to the marine section in Dalian Metro Line 5 and considering the effects of corrosive marine environments and unfavorable geological condition,this dissertation has done experimental and theoriotical research on the mechanical properties and durability of the Steel Fiber Concrete(SFC)and Polypropylene Fiber Concrete(PFC),based on which the dynamic serviceability of the shield is predicted.Major contents include:1.Experimental study on the mechanical behavior of SFC and PFC shows that both concrete have small increment in compressive strength and Elastic modulus compared with Plain Concrete(PC).Both SFC and PFC have more scattered strength and elasticity with the maximum coefficient of variation to be 7% for strength and 2.5% for elastic modulus.Fiber concrete was found to have better ductility in the compressive constitutive laws experiment.2.Carbonization test of concrete under flexural loads shows that fiber concrete exhibits significantly smaller carbonization depth,and the value of SFC is greater than the PFC.All specimen have greater carbonization depth under the 60% flexural load than those under 30%,and the effect of load level is more prominent in PC.3.Chloride erosion test of fiber concrete after 28 day carbonization shows that the fiber concrete in the non-carbonization group has better anti-erosion capacity.As the carbonization age increases,coefficient of diffusion decreases for all specimens,which manifests better anti-erosion capacity of concrete after carbonization.4.Experiment on the mechanical properties of fiber concrete after carbonization and freeze-thaw cycles shows that fiber concrete has large relative elastic modulus than PC,and the difference grows with freeze-thaw cycles.Specimens that undergo 28-day carbonization shows less than 10% decrease in relative elastic modulus compared with PC,and shows faster degradation in late cycles which implies the initial carbonization cracks influence the freeze-thaw damage.5.Compressive constitutive law was modeled for fiber concrete based on the damage mechanics,and model parameters of damage threshold and strength possibility distribution were calibrated by experimental data.A time-dependent compressive strength and elastic modulus model was established based on the compressible packing model(CPM)and three spherical model(TSM)considering the reinforcement of fiber.6.Based on strip method,a time-dependent carbonization depth derivation was formulated for concrete under flexural load and model data was calibrated by experimental data.A damage model for concrete under freeze-thaw cycle which take the relative dynamic modulus as the parameter,was formulated based on damage mechanics,by considering both creep damage and fatigue damage during the process.7.Reliability analysis was conducted on the shield on two marine and tow land section in the case project,and results show a high initial reliability for the proposed scheme.Time-dependent reliability analysis of alternative fiber concrete scheme was done considering the better durability of fiber concrete and effects of carbonization,steel corrosion and stray current,and results show sightly lower reliability in 100 years for the original scheme while the fiber concrete could significantly improve the redundancy in safety.