Dynamic Expansion Characteristics Of Artificial Frozen Soil Curtain Of Metro Tunnel Construction

In recent years,the artificial ground freezing(AGF)method has been widely used in the construction of subway tunnels domesticly and overseas,especially in soil layers such as soft soils and sandy gravel soils.However,there are still many problems to be solved regarding the dynamic expansion of the artificial frozen soil curtain and its multiple-field temporal and spatial variations.The investigation on the dynamic expansion characteristics of the artificial frozen soil curtain during its freezing and thawing process is not only related to the accurate evaluation of the soil frozen effect,but also can provide useful references for predicting and controlling the adverse effects such as the soil frost-heaving and thawing-settlement during the AGF process.In view of this,this article is mainly based on the research program of Hydraulic-thermal-mechanical Coupling Mechanism and Environmental Impact Control Technology for Horizontal Ground Freezing of Subway Tunnel Construction(No.51678500)sponsored by the National Natural Science Foundation of China,aimed at the research topic of the dynamic expansion of frozen soil curtain during the artificial soil-ground freezing in the metro tunnel construction,by using the comprehensive research methods combined with the theoretical analysis,indoor tests,numerical simulation as well as on-site monitoring,the temporal and spatial changes of the temperature and displacement fields during the AGF process and the concurrent three-dimensional dynamic expansion of frozen soil curtain are explored.Moreover,a novel intelligent monitoring technique suitable for the real-time evaluation on the dynamic mechanical characteristics of artificial frozen soils is also proposed.The main research results are summarized as follows:(1)Based on the employment of piezoelectric-ceramic smart sensors,indoor tests have been carried out,the feasibility and rationality of the stress-wave propagation active sensing technique and the electro-mechanical impedance method in real-time monitoring of soft soil freezing-thawing have been well demonstrated.The correlation between the impedance signal changes and the development of strength characteristics of frozen soil has been analyzed,and on basis of that,a reliable evaluation index for the real-time evaluation on the dynamic strength characteristics of soft soil during the freezing-thawing process is put forward.(2)Based on the theory of piezoelectric elasticity,a electro-mechanical coupling analysis model of piezoelectric patch-frozen soil is established,and the impedance analytical equations considering the actual electrical and mechanical boundaries and continuity conditions of the coupling system are derived.In addition,the related experimental research have been conducted,the reliability of the proposed model is further validated,the electro-mechanical response characteristics of the embedded piezoelectric sensor to the freeze-thaw induced changes of the soil mechanical characteristics are revealed,and the accurate prediction of dynamic changes of the soil strength characteristics is realized.The research works will lay a solid theoretical foundation for further promoting the application of the piezoelectric-based intelligent sensing method in the field monitoring of AGF.(3)With the consideration of the ice-water phase transition latent heat and the elastoplastic deformation of frozen soil,a thermal-mechanical coupling analysis model for the entire AGF process in the soil-layer metro tunnel construction is established.Additionally,combined with the AGF project of a turn-back line tunnel of the Guangzhou Metro construction,the three dimensional dynamic expansion process of the frozen soil curtain is simulated,the analysis on changes of the transient temperature and displacement fields in each phase of the active freezing-maintenance freezing-natural thawing periods is emphasized.Moreover,by comparing the field measured data with the numerical results,the reliability and rationality of the proposed model are verified.Based on the numerical calculation results,important references are obtained for the further design of applicable piezoelectric-based intelligent sensors in the field monitoring of artificial freezing soil curtains.(4)Based on the theory of the frozen-soil temperature field,by using the large-scale finite element analysis software,a two-dimensional numerical model is constructed for the calculation of the temperature field of freezing pipes in the typical sandy gravel soil layer,the numerical simulation of dynamic temperature field of the frozen soil around the freezing pipes with different spacings and layouts is performed,and the influences of multiple key design parameters such as the diameter of freezing pipe,the temperature of circulation brine,and the intermittent freezing mode on the soil frozen effect are analyzed.The dynamic characteristics of the temperature field of the frozen curtain under different key design parameters are studied.