Study On The Degradation Mechanism Of Lining Segment Of Subsea Shield Tunnel In Commission

As an important urban public transport infrastructure,subway has developed rapidly in recent years and ushered in a new round of construction climax.In order to meet the needs of urban development,some coastal cities have begun to try to build subsea subway tunnels.The shield tunnel in the sea area is subject to multiple environmental factors at the same time,such as chemical erosion of sea water,carbonization,long-term action of high-pressure water head,and earth pressure.In this special condition with multiple environments,the durability of tunnels will be more prominent.It is an urgent engineering problem to reveal the damage deterioration mechanism of subsea tunnel under the action of multiple environmental fact macro-microscopic perspective,which has important practical significance and strong practical needs.In this paper,taking the subsea shield tunnel between Maluan Central station and Jimeidao station of Xiamen rail transit line 6 as the engineering background,considering the combined action of chloride ion erosion,carbonization,water head pressure and tunnel surrounding rock pressure,focusing the two deterioration behaviors of concrete material strength reduction and corrosion-induced segment cracking,using the indoor test and numerical simulation methods,attempts to reveal the damage and deterioration mechanism of the lining structure of the subsea shield tunnel in the service period from the macro and micro perspective.The main conclusions are as follows:(1)The numerical analysis model of subsea tunnel considering earth pressure,total head of sea water and surface chloride concentration is established to explore the unsaturated seepage of sea water and chloride ion induced corrosion process in segment concrete under the coupling effect of multiple factors.It is found that the seawater infiltrating tunnel and chloride ion induced corrosion to tunnel are in a circular ring at the same time.The seawater infiltration depth and chloride ion induced corrosion depth in different positions of the tunnel are generally shown as the arch bottom > arch foot >arch shoulder > arch waist > arch crown,showing the characteristics that the deeper the buried depth of the tunnel is,the deeper the seawater infiltration depth is at the same time,in which the rebar at the arch bottom first reaches critical chloride concentration and rust.(2)Conventional triaxial compression tests were carried out on the water-saturated samples of dry concrete and the water-saturated samples of chloride solution to analyze their strength,deformation and energy evolution characteristics from the mechanical and energy perspectives.The results show that the chloride solution has a significant effect on the strength and deformation characteristics of segment concrete.The erosion of seawater greatly reduces the compressive strength and elastic modulus of concrete,and increases the ratio of dissipation energy to total energy.The concrete on the out side of the lining ring i.e.that has been eroded by seawater is more likely to be damaged under the same stress level and has a weaker bearing capacity than the other concrete.(3)Considering the chloride ion erosion on the outer rebars of the segment and the carbonation on the inner rebars,a non-uniform corrosion-induced cover cracking model of concrete is established using the method of XFEM,and the characteristics of crack growth and evolution of the reinforce concrete member having single rebar and a concrete segment member with multiple rebars are obtained.It was found that the propagation of corrosion-induced cracking crack was first fast and then slow.The closer it is to the surface of the rebar,the wider the crack width will be,and the farther it extends,the smaller the crack width will be.The crack,caused by the rusting of the outer rebars,in the concrete cover which is on the out side of the segment and eroded by seawater initiates earlier than that in the concrete cover on the inner side of the segment.The deterioration of the concrete material of the lining structure has a certain accelerating effect on the corrosion-reduced cracking of the concrete.(4)A tunnel degradation analysis model including the surrounding strata and sea water pressure is established,and the damage degradation simulation of subsea shield tunnel structure under the action of concrete strength loss,corrosion-induced cracking and concrete surface damage is carried out.It is found that the cracking characteristics of the tunnel are as follows: in the inner concrete linng ring,the cracks at arch crown and arch bottom penetrate the protective layer vertically,and the cracking effect in the rebar holes is very serious;the cracks at the arch shoulder,the arch waist and the arch foot are connected with each other after propagation,and the cover shows lamellar cracking.In the outer concrete linng ring,the carcks at arch crown and arch bottom connected with each other,and the cover shows lamellar cracking;At the arch waist,the crack is vertical to the protective layer,and at the arch shoulder and arch foot are cracks penetrate the cover obliquely.The main cracks around all the rebars in the inner and outer lining ring are perpendicular to the direction of the maximum principal stress and parallel to the direction of the minimum principal stress.