| Freezing method,as one of the common weak soil reinforcement methods,has been widely used in mining engineering,traffic engineering and building foundation in my country in recent years.When the freezing method is applied to the construction of subway connecting passages,especially after encountering weak strata with water-rich pebbles,geological conditions and specifications are very different,and many new problems and difficulties have been encountered.The thesis takes a communication channel of Nanning subway as the engineering background,and uses theoretical analysis,numerical simulation,on-site measurement and other different research methods to systematically study the thickness of the frozen wall and its temperature and deformation characteristics of the subway connection channel in the water-rich gravel layer.And put forward design suggestions suitable for the thickness of the frozen wall of the subway connecting channel in the water-rich gravel layer.The main research results are as follows:(1)Analyze and evaluate the special properties of the water-rich pebble soft stratum and three traditional calculation methods for the thickness of the frozen wall.Comprehensive analysis is based on the connection between the special geological conditions of this project and the site engineering profile of the subway tunnel.The proposed structural mechanics method derives the theory,simplifies the calculation model,obtains the calculation method of the thickness of the frozen wall of the water-rich pebble layer of the contact channel based on the force method,and analyzes its application characteristics in the water-rich pebble layer.The thickness of the frozen wall can still be calculated by structural mechanics,and the main points of optimization are proposed.Relying on the actual project,according to the general situation of the project and the lithology of the formation,combined with the physical and mechanical tests of the artificial frozen soil at-10℃ conducted by the same group of personnel,the various parameters of the frozen soil are determined,and based on the strength control method and the force method,certain Safety factor,the thickness of the frozen soil wall is calculated to be 2.0m.(2)Using the finite element software ABAQUS,a numerical simulation method was adopted to analyze and study the temperature field of the frozen wall during the active freezing period and the excavation process of the subway connection channel during the frozen period.According to the results of numerical analysis:during the active freezing period,the point located inside the freezing tube reaches the active freezing temperature(-10℃)earlier,and the temperature difference between the inner and outer points increases with time.During the maintenance freezing period,when entering the excavation stage,the stress showed a downward trend.The maximum deformation of the frozen wall satisfies the allowable deformation condition from the simulation.The external load on the frozen wall meets the uniaxial compressive strength of the frozen soil,which further proves that the frozen wall Safety and reliability of design thickness.(3)Through the comparison between the field measurement and the finite element simulation,the change trend of the temperature,displacement and stress of the communication channel is analyzed,and the trend of the field measurement value and the numerical calculation result is found to be consistent.It shows that the calculation method,model and parameters of this model are basically feasible.Considering that the multi-strati graphi c phase transition can better reflect the change process of the actual engineering temperature field,at the same time,it is proved that the thickness of the frozen wall 2.0m designed according to this paper meets the deformation and strength of the frozen wall.The requirements of safety permits and research results can provide useful theoretical references for construction design of similar underground freezing projects.(4)The internal mechanism of freezing in water-rich pebble formation is briefly described.The composition of frozen soil in water-rich pebble layer is more complicated than that of ordinary frozen soil.With the change of external freezing temperature,internal temperature,ice crystal structure and unfrozen water content It also changes at any time,and the phase change exotherm changes accordingly.Frozen water-rich pebble layer soil is a complex multi-phase system,so it can only consider the influence of one or two factors under the same conditions,and analyze the impact of changes in this factor on the characteristics of frozen soil.In summary,this paper validates the calculation method of the frozen wall thickness of the subway connection channel by studying the thickness of the frozen wall of the subway connection channel in the water-rich gravel layer and its temperature and deformation characteristics,which provides a useful reference for subsequent similar studies. |
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