Research On Mechanical Model Of Interface Bond Degradation And Evolution Mechanism Of Support System Caused By Section Steel Frame Corrosion In Subsea Tunnel

The environment of subsea tunnels is more special than that of ordinary tunnels,the content of chloride ions Cl^-and sulfate ions SO₄^2-in seawater is rich,and the promotion of ion diffusion by high water pressure.The steel in the supporting structure system is easily depassivated and corroded by ion erosion,resulting in the weakening of its mechanical parameters and the degradation of the bond strength,which ultimately has an impact on the tunnel’s supporting structure system bearing capacity and safety.Therefore,the safety and durability of subsea tunnels during the operation period have always been a hot topic of research.The subsea tunnel designed based on NATM theory often has poor surrounding rock conditions in the section with section steel frame support.Therefore,the primary support will bear a large load.Due to the rusting of the section steel frame,it not only generates the rust expansion load,but also destroys the bonding effect between the section steel frame and the shotcrete,so that the slip between the section steel frame and the shotcrete occurs,and the section steel frame and the shotcrete cannot act jointly,thus the primary support’s bearing capacity is greatly reduced,and a large portion of the load is transferred to the secondary lining.However,there are few reports on the bond-slip degradation constitutive relationship between corroded section steel frame and concrete,the model of rust expansion force,and the mechanical evolution law of support system after section steel frame corrosion.Therefore,this paper takes a subsea tunnel constructed by drilling and blasting method as the engineering background,relying on the National Natural Science Foundation of China project-Research on bond-slip degradation constitutive relationship models between corroded profiled steelwork and sprayed concrete of submarine tunne1（51878567）,and adopts comprehensive research methods such as literature investigation,field test,indoor test,theoretical analysis and numerical simulation.The mechanical model of interface bond degradation and evolution mechanism of support system caused by section steel frame corrosion in subsea tunnel are studied.Firstly,based on the field investigation,the corrosion causes and the relationship between corrosion rate and time of section steel frame are obtained.Then,the electric accelerated corrosion test and push out test of corroded section steel arch frame concrete in subsea tunnel are carried out,and the bond slip degradation constitutive model and rust expansion force model are obtained.On this basis,the decay law of primary support and the mechanical evolution law of support system after steel frame corrosion are were explored.The research has achieved the following results:（1）Through on-site research,the basic data of cracking and water leakage of the primary support of the subsea tunnel are obtained,based on which the causes of the section steel frame corrosion in the subsea tunnel are analyzed.Finally,the relationship between corrosion rate of section steel frame and time is obtained based on field test.（2）The steel reinforced concrete members with different corrosion rates are obtained by electric accelerated corrosion test,and then the push out test is carried out.The load-bond（P-S）curve of corroded section steel frame concrete is obtained,and the bond failure mode and mechanical characteristics of the interface between corroded section steel frame and concrete are explored.Based on the bond stress-slip（τ-S）curve of corroded steel frame concrete,a bond-slip constitutive model of corroded steel frame concrete interface considering residual bond strength is established by introducing random damage theory.（3）Through the accelerated corrosion test of steel reinforced concrete members with different thickness of protective layer,the development process of rust expansion force between section steel frame and concrete is obtained:free corrosion stage,obvious increase stage of rust expansion force and expansion crack stage.The calculation model of rust expansion force is constructed according to the relationship curve between rust expansion force and corrosion rate.At the same time,the mechanical characteristic response of surrounding concrete in the development process of different rust expansion force is proved by ANSYS.（4）According to the stress characteristics of primary support（eccentric compression）,steel reinforced concrete eccentric members with different corrosion rates are made,and then eccentrically loaded.The failure mode of the primary support with corroded section steel frame,the characteristics and laws of the bearing capacity with the corrosion rate and eccentric distance changes are investigated,and the critical corrosion rate of the primary support meeting the plane section assumption is obtained.The three-dimensional solid load-structure computational mechanics model of the primary support varying with the corrosion rate of section steel frame is established,and the longitudinal force transfer（stress,bending moment and axial force）characteristics of the primary support are obtained,revealing the mechanical evolution mechanism of the primary support under the action of section steel frame corrosion,and the correction formula of bending moment and axial force are obtained by comparing with the two-dimensional simplified load-structure model.（5）The interaction mechanical calculation model of surrounding rock-primary support-secondary lining under the condition of section steel frame corrosion in subsea tunnel is established.The variation law of section steel frame corrosion on surrounding rock stress,displacement field,secondary lining bending moment and axial force is revealed,the load transfer mechanism between surrounding rock,primary support and secondary lining after section steel frame corrosion is revealed.Finally,the prediction formula of load transfer coefficient between primary support and secondary lining with the change of corrosion rate is given,and the long-term safety of secondary lining is evaluated.