Expeirmental Research On Stresses And Thermal Conductivity Of PVC Freezing Pipe Frozen In Shield Starting And Arirving Freezing Constructi On

Certain soil improvement measures must be taken before shield starting and arrivingto ensure the soil outside the entrances be stable and have certain strength. It is commonlyadopted the low carbon seamless steel tube as freezing pipe used in freezing process byshield tunneling, but cutter head cannot remove the freezing steel pipe driven into soil,which causes the difficulty for shield driving and brings the construction risk. The study onpipe drawing-free technique in freezing consolidation by shield tunneling will advance thesafety of engineering and have agood social and economic efficiency.Conducted bending test, withstand voltage test, thermal stress test of PVC pipe at lowtemperatures, model test of comparing thermal conductivity of freezing pipes made ofdifferent materials with temperature field, and after combining the numerical simulationand industrial field tests, this research subject obtained results as follows:①The bendingstrength of bonded PVC pipe at normal temperature is6.12MPa,the deflection is22.33mm;after the treatment of low temperaturesalt water, the bending strength is12%more than thatat normal temperature, becomes14.51mm, and the deflection is35%less than that atnormal temperature, becomes6.85MPa,the tensile shear strength is more than2.48MPa.Meet the strength and deformationdemand of shield starting and arriving.②As thewithstand voltage test shows, when the pressure in pipe reaches0.8MPa, there shows nooccurrence of water seepage and leakage in0.5hours. After the withstand voltage test, thepressurizing test is conducted again with increasing bending resistance to4MPa, and thereshows no seepage and leakage phenomenon.Meet the demand of brine freezing watertightness.③The average development speed of PVC pipe freezing soil curtain is about0.7～0.8times of development speed of steel pipe freezing, and by using the analyticcalculating transcendental equation of freezing temperature field of single pipe, thepipe-freezing front of PVC freezing pipe is proportionalto the square root of freezing time.At the initial stage of congealment, the temperature differences between wall of PVCborehole and brine are big, and the maximum temperature difference is up to11.5℃. Butthe difference will gradually decrease during the middle and late stages of congealment.The lowest temperature of PVC freezing pipe wall can be-25℃, keeping stable of5℃ differences with brine temperature, and the average temperature keep a low level as well,which will meet the demand of freezing starting of shield.In the end, a field experiment of shield tunneling starting without pipe drawing andreinforcing by brine freezing should be carried out. From the debris statistics collectedfrom the experiment, we can find that the size of broken freezing pipe can be limitedwithin about100mm which will be excavated easily by the spiral propulsion, andinspecting its leakage characteristic and thermal conductivity in the frozen groundandconstruction technology in practical.In summary, stresses and thermal conductivityofPVCfreezingpipe can meet theexempt-pull-out frozen pipetechnology demand of freezing starting and arriving of shield.