Study On Horizontal Freezing Temperature And Frost Heaving Deformation Of Ground Surface Of Shield’s Starting And Arriving

The safety start technique and arrival of shield machine are crucial to the shield methodwhen an underground railway tunnel is built. The artificial horizontal cup-shaped freezingtechnique has the benefits such as stopping water, keeping the subsistence of soil andenvironmental protection, but it is also restricted to be spread and exploited further because ofthe variability of temperature field with time during the formation of artificial freezing wall andthe frost heaving deformation influence on the surrounding. Based on the problems above, Inthis paper, the application of horizontal cup-shaped freezing wall in shield constructiontechniques for pass out of working pit engineering was presented by instances and the authormade a systematic study on the temperature field and the earth’s surface frost heavingdeformation during the formation of horizontal cup-shaped freezing wall by laboratoryexperiment, field survey, theoretical analysis and numerical simulation. The main conclusionsfor this paper were listed as follows:1）The results of laboratory experiment indicate that the initiative freezing temperature ofthe three kinds of typical soil layers in Nanjing region silt mass clay, silt clay and sandapproximately linearly increase when moisture content increase and temperature decreasewhen the Salt concentration increase. That’s more, water quality has great influence on theinitiative freezing temperature, but dry density and freezing and thawing cycle have littleinfluence on it. According to the test results coefficient of thermal conductivity increases withmoisture content and dry density increasing, and the frozen soils’ is greater than the normaltemperature soils’. Moisture migration extent is relevant to several factors, such as soil texture,freezing time, moisture content, dry density and so on and the moisture content influentsgreater.2) After establishing the computational model of the horizontal cup-shaped freezing walltemperature field, the model’s practicability is verified by the temperature field test result inNanjing metro line2Yixian Bridge subway station.This paper studies the effects of subzero saltwater temperature, thermal physics parameter andfreezing time on the horizontal cup-shaped freezing wall temperature field. Among the differentfactors’ value range, the short freezing pipe bottom’s freezing wall thickness can be30-54cm,the cup’s thickness can be80-140cm while freezing. The results of the closure of freezing wallshow that interior circle freezing pipes close the earliest, the part between the bottom of the cupand the cup’s wall close at the latest. 3) According to the finite element analysis, different factors which have influence on theground level displacement are researched. The results show that: horizontal or along axial oftunnel the nearer to the axial and sub-continued wall the ground level frozen deformation isgreater. What’s more, temperature of saline water, radius of tunnel, buried depth of tunnel andfrozen ratio have little influence on the position of the greatest displacement, but with thethickness of plates increase, the position is near to the sub-continued wall. Among thedifferent factors’ value range, the further the distance to frozen wall, the displacement of groundlevel decrease attenuatly. Based on the influence of temperature of saline water, parameter oftunnel, buried depth of tunnel about frozen displacement is greater and taking the requirementsof kinds of architecture into consideration,20m away from the tunnel portal and export isdetermined to be monitored.4) With establishing the stability model of horizontal cup-shaped freezing wall,the resultssuggest that the stress and displacement caused by changes of the thickness of cup bottom andcylinder are much less than allowable value, and the diameter of tunnel is smaller of thestability of formations. In addition, the reasonable thickness of cup bottom can be2.0~2.5m,and the length of cylinder is almost the length of shield machine. The last but not least, thethickness of cylinder can be1.0~1.2m, the deeper the tunnel, greater the thickness, and thespecific thickness and buried depth will be determined by the physical and mechanicalproperties of formations.The researches solve several problems such as parameter selection, development oftemperature field, prediction of ground level deformation ect. And the researches haveimportant significance on application and spread of horizontal cup-shaped freezing in shield’sturnover.