Multi-field Coupling And Freezing Damage Analysis For The Tunnel In The Cold Region

With the implementation of Chinese "One Belt,One Road" strategy,more and more mountain tunnels will be deployed and built in high-cold freeze-thaw areas.Therefore,it is important to study the failure mode of tunnels in cold region,and to summarize the weak areas of cold region tunnels.Taking the Xing’anling Uplink Tunnel,which has been in service for nearly a hundred years,as an example,during its operation,it has experienced freezing damage many times,resulting in serious personal safety and economic and property losses.Even though the investment in the maintenance of the Xing’anling Tunnel has been increased year after year,the frost damage problem is still in the dilemma of "long-term cure",which has attracted the attention of relevant departments.So far,the research of the lining failure mode and soil-lining interaction for cold region tunnel has rarely been carried out.For this reason,it is of great significance to deeply study the bearing capacity and the development of cracks of tunnel lining in cold region.In view of the above,this article adopts a combination of field test,model test,machine learning and theoretical analysis to study the bearing capacity and the development of cracks of tunnel lining.The main research work of this paper and some conclusions are as follows:(1)A series of model tests were carried out to investigate the soil-lining interaction of cold region tunnel.The representative test results of freezing depth,lining soil pressure,and lining internal force distribution were presented and discussed.Test results show that the tension instability is easy to occur at the side wall of the entrance section,and the risk of freezing damage to the lining of the tunnel body is much lower than that of the tunnel entrance.In actual engineering,it is necessary to comprehensively consider the differences of frost damage around the tunnel.(2)Focusing on the water-heat-mechanical coupling characteristics of the soil,a three-field coupling model of frozen soil was established.Subsequently,in view of the difference in freezing damage mechanism between the soil and concrete porous media,the phase field method of fracture mechanics was introduced,so as to realize the integration of the lining fracture energy field and the porous media multi-field coupling theory.Then,the coupling model of water-heat-mechanical-fracture energy field for the soil-lining system of tunnel is developed.(3)Based on the coupling model of water-heat-mechanical-fracture energy field for the soil-lining system of tunnel mentioned above,the selection of tunnel lining structure in the cold region is carried out.And the lining bearing capacity diagram is used to judge the failure mode of the tunnel lining.The results show that the circular lining maintains a high safety factor before and after the occurrence of frost heave,followed by the horseshoe tunnel.For new tunnels,a circular lining structure is recommended.For the horseshoe-shaped tunnels that have been in service for a long time,it is recommended to strengthen the lining at the arch toe and side walls,and strengthen the insulation and waterproof measures.(4)Applying the random forest algorithm,a prediction model for the distribution of longitudinal cracks in the Binzhou Railway tunnel has been established.Then,the importance of various influencing factors degree has been compared.The results found that: the air temperature in the tunnel greatly affects whether the tunnel has longitudinal cracks,and the service time will determine the development process of the longitudinal cracks.(5)Then,the specific development mode of lining cracks under the individual and combined effects of freezing and thawing,buried depth,and failure of waterproof layer has been researched.The results show that: for the lower environment temperature,the pore ice pressure reaches the critical value of lining failure,resulting in the formation of dense and small cracks in the lining water-soaked area;for the higher environment temperature,the pore pressure in the water-soaked area fails to completely crack the lining,but the damage to the lining caused the lining to form through cracks quickly.(6)by treating the tunnel lining as a thick-walled shell and combining with the displacement stress coordination conditions of freeze-thaw circle and unfrozen area,an analytical prediction model of soil-lining interaction was established for deep and shallow tunnels in cold regions.The two-stage excavation analysis method was adopted to consider the delayed installation effect of deep buried tunnel lining.Finally,using the orthogonal test method,the significant degree of the influence of each influencing factor on the axial force and lining bending moment of tunnel lining in the cold region was analyzed.