Study On The Thickness Design Of The Freezing Wall Of The Subway Liaison Channel In The Water Rich Gravel Stratum

Artificial freezing method uses artificial refrigeration technology to form solid ice in the soil of the soil,which greatly improves the strength of soil,making the frozen soil body form the frozen soil curtain(i.e.frozen wall)with high strength,good integrity,good water isolation and temporary support.The frozen wall formed by different freezing methods can be applied to the excavation sections with different structural forms,and can be applied to various water rich soft formations.For the whole freezing method,freezing design is the most important,and the quality of the freezing design is directly related to the success or failure of the whole project.Therefore,the freezing method of the freezing method for the tunnel excavation of No.2 liaison Channel Tunnel of Nanning Metro Line 1 is studied.The thickness design of the freezing wall of the contact channel of the water rich gravel layer is studied by formula calculation,model test and theoretical analysis,and the construction of the horizontal freezing method of high rich water bearing gravel stratum is put forward.Design method of frozen wall thickness for subway connecting passage.The main work is:(1)By comparing four traditional methods of calculating the thickness of frozen wall,combined with the characteristics of the subway liaison channel in the rich water gravel stratum,the soil pressure of the frozen wall is calculated by the water and soil pressure calculation.Based on the different gradation and grinding roundness of the water rich egg gravel,the permeability index is taken into consideration in the calculation of soil and water pressure.The earth pressure is calculated.Then,the contact passage section is simplified to an arch symmetrical structure,and the formula for calculating the thickness of the frozen wall suitable for the connecting passage of the subway is derived by using the concept of force method.Based on the engineering background of the No.2 liaison channel of the National University of Nanning rail transit National University to Qingchuan station,the frozen wall thickness of the water rich egg gravel stratum freezing method was calculated by using the design formula of the freezing wall thickness of the connecting passage of the water rich gravel stratum.The freezing wall thickness of the water rich egg gravel stratum freezing method was 2.0m.(2)In order to verify the reasonableness of the calculation method of the thickness of the freezing wall of the subway liaison channel in the water rich gravel stratum,the displacement and stress changes of the frozen wall are analyzed by the model test.According to the three theorems of similar theory,the similarity ratio of the physical quantities(geometric similarity ratio Cl=20)of the model test on the basis of similar theory is obtained by combining the similar characteristics of the freezing temperature field,the water separation field and the stress field.According to the similarity ratio of physical quantities,a multi-function model test platform for the construction of the water rich gravel stratum is designed.(3)Through the analysis of the monitoring results of the temperature field of the frozen wall in the model test,it is found that the temperature drop in the near end of the freezing tube is faster than the far end,the temperature in the inside of the frozen wall decreases faster than that in the outside,and the farther the freezing tube is from the freezing tube,the greater the difference of the measuring point(4)Through the analysis of the displacement field monitoring results of the model test frozen wall,the stage of displacement change can be divided into frost heave stage and excavation stage.The frost heave stage shows that the frost heave displacement of the frozen wall increases with the increasing time,and reaches the peak value at last.The displacement difference caused by frost heave in different positions is obvious,and the frost heave displacement is the largest in the vault,the arch angle is the second,and the arch bottom is the smallest.The excavation stage shows that the excavation displacement of each displacement monitoring point on the same section increases with the increase of time.When the excavation is carried out to the monitoring section,the excavation displacement will produce a sudden change,that is,the rich water gravel stratum has a great instantaneous dislocation,and finally reaches the peak displacement and then gradually tends to stability.Excavation causes the maximum displacement of arch bottom,the arch angle is the next,and the vault is the smallest.(5)Through monitoring and analyzing the stress field of the freezing wall of the model test,the variation law of the frost heaving force of the water rich gravel tunnel surrounding rock is obtained.The frost heaving force can be divided into four stages:the slight growth stage of frost heave force,the sharp increase stage of frost heaving force,the gentle growth stage of frost heaving force,and the stage of decreasing frost heaving force.The external load is mainly manifested as:during the active freezing period,the initial stress decreases slightly with time,and then increases gradually after reaching the minimum value.As the stress of the permafrost column increases sharply,the growth rate increases slowly until the critical point is reached,and the average stress is about 4 times that of the initial stress(6)During the test,the displacement of the displacement of the frozen wall is located at the top of the vault,reaching 0.767mm(actual 15.34mm),less than the permissible deformation value of the frozen wall(20mm).The maximum stress of the frozen wall is located at the bottom of the arch,and the maximum stress reaches 65.3kPa(actual about 1.3MPa),far less than the water rich egg gravel of-10 centigrade.Axial compressive strength(2.6MPa).Therefore,according to the test results,according to the thickness of frozen wall designed in this paper,the deformation and stress of frozen wall can meet the requirements of safety and stability.(7)Through the simulation of the actual project,the changes in temperature,displacement and stress of the frozen wall are monitored.Finally,the deformation and stress of the frozen wall meet the requirements of the safety permit,which further proves that the frozen wall thickness calculated by the method of freezing wall calculation in this paper is feasible and available.Similar engineering freezing design reference.