Research On The Forming And Thawing Of Frozen Soil Walls By Artificial Ground Freezing Method

Because the artificial ground freezing method(AGF) is unlimited basically bysupporting range and depth and can prevent effectively the gushed water anddeformation of surrounding soil body in construction, it is one of the main means tocomplete underground engineering. The artificial ground freezing is a complexprocess influenced by the operation status of refrigeration system, geologicalconditions, heat dissipation through boundary, construction steps and other factors.The behaviour of frozen soil is strongly temperature dependent and the temperaturefield of the frozen soil changes with time. So the research on the dynamic change offrozen soil temperature field is of great importance.A station-to-station tunnel of the Shanghai Rail Transit Line No.4 hadencountered soil and water inburst and tunnel lining collapse due to construction ofthe cross-passage by AGF. The limited depth freezing method with four rows ofvertical freeze-tubes was applied as an aid to recover the collapsed tunnel to form afrozen soil wall, which was used to resist the soil and water pressure and to hold theundamaged tunnel lining between the undamaged part and the collapsed part. Thein-situ testing of the development and changing features of the temperature field inthe frozen soil wall was applied to supervise the construction. After the freezingconstruction finished, field experiment studies on changing features of thetemperature field for natural thawing and artificial thawing were performed. Based onthese freezing and thawing projects, researches on forming and thawing of frozen soilwalls by field experiment, theoretical analysis and numeric analysis were carried outin this dissertation. The major works and results of this dissertation are as follows:(1) A summary of calculation methods on the soil thermal conductivity, specificheat, thermal conductivity coefficient and latent heat are made by analysing theinfluencing factors of soil thermophysical properties. Based on these formulas, athermal parameter table of frozen soils in Shanghai is given. In case of absence ofexperimental test, the thermal parameters of the frozen soils for temperature fieldcalculation could be obtained according to the given table to seek numerical interpolation.(2) Analysing of the in-situ test data of limited depth freezing construction withfour-row freeze-tubes, the development characteristics of the frozen soil wall insideand outside of the rows of freeze-tubes are revealed overall. Mutiple-row freezing isbeneficial to strengthen the freezing efficiency, to form a larger frozen soil wall and toacquire lower average temperature. Due to the limited depth freezing, the frozen soilwall has an entasis characteristic. In the future similar engineering, design andconstruction method could be improved according to these characteristics.(3) Through the field experiments of natural thawing and artificial thawing, aconclusion is drawn that the natural thawing velocity is very slowly and the artificialthawing can accelerate the thawing velocity. For large volume frozen soil developedby multi-row freezing, it is a better choice to use artificial thawing method, so thatgrouting be conducted in time to control soil thaw settlement.(4) According to the heat balance principle that the rate of heat flow through thefreeze pipes must be sufficient to sustain growth of frozen soil wall, theoreticalfreezing models are established for single row of freeze pipes, multiple rows of freezepipes and arch layout of freeze pipes. Using the established theoretical models, thetotal energy extracted from the ground and the time to freeze per increase in total wallthickness are caculated. The calculation results agree well with the in-situ test results.(5) Using the established theoretical models of single row of freeze pipes, theinfluencing factors on the total energy extracted from the ground and the freeze timeare analyzed in detail. The influencing factors include the values of freeze influencecoefficients, the brine temperature of freeze pipe, water content in soil and thedistance between freeze pipes.(6) Based on the heat balance principle, the theoretical models of artificialthawing for single thaw pipe, single row of thaw pipes and multiple rows of thawpipes are established. Using the established theoretical models, the total energyextracted from the frozen soil and the time to thaw per increase in total wall thicknessare caculated, the artificial thawing velocity of single row and multiple rows of thawpipes are caculated as well.(7) Influences of the ground temperature on the frozen soil wall by using analytic method and finite difference method are analyzed. It could be draw that thenatural thawing problem of frozen soil wall with limited thickness simplified tosemi-infinite geometry problem is improper. The change law of temperature field ofthe natural thawing in frozen soil is: a)The temperatures in the different places offrozen soil wall rise up relatively quickly and close to phase transformationtemperature. b) The phase interface moves with uniform speed to the middle of frozensoil wall, and the moving speed accelerating at the time of the phase interface beingfinished to the middle of frozen soil wall. c) The temperature in thawed soil risesslowly until the entire frozen wall is thawed. For the natural thawing of frozen soilwall formed by single-row freezing, it is shown that approximate parabolarelationships exist between the thawing time, as well as thawing velocity and thethicknees of frozen soil wall. For the frozen soil wall formed by multiple-rowfreezing, the relationships are approximate linear.(8) Influences of heat dissipation through boundary on the temperature of thefrozen soil wall under the conditions of freezing or freeze stopping are analyzed bythe finite difference method. While freezing, the phase interface in frozen soil wallmoves quickly at the early stage. The higher the environment temperature is, thefaster the speed of the phase interface moving. Later, it grows slower and slower andfinally, the phase interface is no longer moving when the heat balance betweenfreezing and heat dissipation through boundary reaches. As freezeing stops, the frozensoil wall is affected significantly by the heat dissipation through boundary and thephase interface moves at a uniform speed until the entire frozen soil thawed. Thermalinsulation material on the face of frozen soil wall could reduce influences of heatdissipation through boundary on the temperature of the frozen soil wall.(9) Natural thawing of frozen soil for cross passage construction is a combinedaction of ground temperature and heat dissipation through boundary. The analysisresults show that heat dissipation through boundary is the main function to naturalthawing of the frozen soil wall in cross passage.(10) Influence of concrete hydration heat on the temperature of the frozen soilwall is analyzed by the finite difference method. The temperature of concrete rises upgreatly while the phase interface in frozen soil wall moves quickly and stops on the third day. Later, the temperature goes down under the action of the low temperature ofthe frozen soil and heat dissipation through boundary and the soil refreezes. Theanalysis result shows that the quality of concrete will not be affected by the lowtemperature of frozen soil.