| The design and construction technology of artificial ground freezing method has been continuously improved in application,and the application scope has also been continuously expanded.With the wide use of freezing method,the construction environment of freezing method is becoming more and more complex.When freezing construction is carried out in the stratum with seepage,the upstream and downstream development of freezing temperature field will be different.At the same time,when there is more salt in the stratum,the distribution of temperature field and the development of freezing curtain will be affected.However,there are two unfavorable factors,seepage and salinity,in the coastal saline permeable formation,so it is difficult to predict the freezing effect.It is urgent to study the temporal and spatial evolution law of freezing temperature field in coastal saline formation.Based on this,the influence of seepage velocity and salt content on the space-time evolution law of freezing temperature field and the development characteristics of frozen curtain are studied in this paper.Standard quartz sand is selected as the component material of permeable formation.Firstly,the density,porosity and other physical parameters of the sand layer are determined.Based on the cooling characteristic curve of frozen soil,the freezing temperatures of different sand samples are obtained.It is found that when the freezing temperature is in the range of-0.1～-5.7℃,the lower the water content is,the higher the salt content is,the lower the freezing temperature is.The thermal conductivity of sand samples was measured based on the plane transient method.The results show that the lower the temperature is,the smaller the salt content is,the greater the thermal conductivity is.Temperature is the main factor affecting the thermal conductivity.The specific heat capacity of saline sand is calculated based on the principle of average equivalence.The higher the salt content,the smaller the specific heat capacity of saline sand.For saturated saline permeable formation,the coupling control equations of water,salt and heat are given.The equation is simplified into one dimension,and the theory is solved in the z direction with the help of numerical software.Based on the model,the changes of temperature,water and salt during freezing when the water content is 20%and the salt content is 2%are analyzed.The results show that the temperature near the cold end develops rapidly.The unfrozen water content in the frozen area decreases.The water content changes greatly on the frozen front.In the unfrozen area,due to the supplement of salt water,the water content remains in a saturated state of 20%.The salt content on the freezing front changes greatly.At the salt water contact end,the salt content is maintained at 2%.A single and double pipe freezing model test system for saline permeable formation is established.Orthogonal tests of three seepage velocities(0,3,6 m/d)and three salt contents(0%,2%,4%)were carried out.The test results of single tube freezing show that the temperature development in the downstream caused by seepage is faster than that in the upstream,and the temperature difference between the upstream and downstream on the C-axis of the interface is generally less than that of the D-axis of the main surface.Seepage velocity and salt content have obvious effects on the development of freezing curtain.Based on the single tube test,the double tube freezing test is carried out,and the spacing between the two tubes is 40 mm.Compared with single pipe freezing,the effects of seepage and salt on the temperature development of double pipes are similar.Under different test conditions,the overlapping time of frozen curtain varies greatly.When the seepage velocity is 6 m/d and the salt content is 4%,the overlapping time is 27.5 h,which is 162%longer than the circle time when the seepage velocity is 0 m/d and the salt content is 0%.The statistical analysis of the frozen curtain thickness R_u,R_d and R_s at different positions shows that both seepage and salt will inhibit the development of the curtain,with the maximum development rate of R_d and the minimum development rate of R_s.Seepage has the greatest inhibitory effect on the development rate of R_s.A quasi coupling model for coastal permeable formation freezing is established,and the applicability of the numerical model is verified by the model test results.When the seepage velocity is 4 m/d,the influence of different salt content is studied.Salt has a great impact on the upstream temperature,and the higher the salt content,the greater the temperature difference between the upstream and downstream.With the progress of freezing,the temperature difference first increases,then decreases,and then slowly increases.Salt also has an obvious effect on the freezing curtain.Under the joint action of seepage and salt,the time and position of intersection change obviously.The circle time fluctuates greatly,in the range of 12～51 h.When the seepage velocity is 8 m/d and the salt content is 6%,the frozen curtain cannot be crossed.The greater the seepage velocity,the higher the salt content,and the smaller the thickness R_u,R_d and R_s of the frozen curtain at the three places.The temperature field in the numerical model when frozen for 120 h is analyzed as the steady-state temperature field.Based on the superposition principle of temperature potential and the method of subsection analysis,the analytical solution of steady-state temperature field of double tube freezing is deduced and solved.The comparison between the numerical results and the analytical solution shows that the temperature distribution law is consistent on the x-axis and y-axis,the maximum temperature difference is 2.5℃,but the average error is small,and the analytical solution results of the steady-state freezing temperature field of double tubes are in good agreement.Based on the engineering geological conditions of the connecting channel of Nantong Rail Line 2 phase I project,a quasi coupled numerical model of the freezing of the connecting channel is established.The single circle freezing pipe is divided into four parts,including left wall,right wall,vault and inverted arch.The results show that except for the n-axis,the temperature of the outer measuring point is higher than that of the inner one,and the lap time and lap position of different parts are different.Among them,the intersection time of the left wall,the right wall,the vault and the inverted arch is 22,15,10 and 8 d respectively.It can be seen that the inverted arch is the first,the left wall is the latest,and the intersection time of the whole freezing scheme is 22 d.In the development behind the frozen curtain,the left wall is the thinnest and the right wall is the thickest,and the development of arch crown and inverted arch is basically the same.Figure[100]Table[53]Reference[157]... |
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