Study On Frost Heave Characteristics Of Liquid Nitrogen Brine Combined With Vertical Freezing

In the process of shield tunnel,artificial freezing method is often used to strengthen the soil at the end to prevent leakage accidents.As the most common form of freezing,brine freezing often produces excessive frost heave deformation in the soil during the freezing process,which is of great harm to surface buildings and underground pipelines.If another kind of refrigerant liquid nitrogen is used,the ultra low temperature is used to freeze the end soil quickly,and the thermal insulation maintenance with brine is used to reduce frost heave in the later stage,which is a worthy research direction.In this thesis,due to the tight construction period in the opening silo project of the Pearl River Delta water resources allocation project,the combination of liquid nitrogen and brine with vertical freezing is used as the background,and the numerical simulation and model test are combined to study the frost heave characteristics of the soil at the shield end under the combination of liquid nitrogen and brine with vertical freezing.The distribution characteristics and development rules of soil temperature field and displacement field during the freezing period are obtained.Firstly,the distribution and development of temperature field and displacement field of soil under typical parameters were obtained by numerical simulation,and the frost heave displacement of soil under the same thickness of frozen wall was compared with that of saline frozen soil.Then,the effects of five single factors,namely liquid nitrogen freezing time,liquid nitrogen freezing tube row spacing,brine freezing tube row spacing,initial ground temperature and brine temperature,on freezing wall thickness,freezing wall average temperature and maximum vertical and horizontal displacement of the surface were studied.The influence size and significance of each factor were obtained through orthogonal test.Finally,based on the similarity theory,model test was used to obtain the temperature and displacement data of soil mass during the freezing process,and the distribution and change law of temperature and displacement fields were obtained.The difference of frost heave displacement between the two was analyzed through the control test of saline freezing,and the simulation effect was evaluated according to the comparative analysis of the data of numerical simulation and model test.The main conclusions are as follows:(1)The development of freeze wall thickness in main and interface paths can be divided into rapid development stage,stable development stage and slow weakening stage.The distance of liquid nitrogen freezing tube has the greatest influence on the thickness of freezing wall,and the thickness of freezing wall increases approximately linearly with the increase of the distance of liquid nitrogen freezing tube.(2)The development law of the average temperature of the freezing wall shows that the temperature decreases linearly and sharply in the first 5days of the ultra-low temperature freezing of liquid nitrogen,and then increases significantly after maintenance with brine.The temperature tends to stabilize after 15 days of freezing.The brine temperature has the greatest influence on the freezing wall average temperature,and the freezing wall average temperature decreases linearly with the brine temperature decreasing.(3)The vertical displacement of the surface surged in the period of liquid nitrogen freezing,and the maximum uplift was 89.5mm on the sixth day of freezing,which was weakened weakly by brine maintenance in the later period.On the same vertical section,the vertical displacement shows that the surface decreases with the increase of buried depth.Brine temperature has the greatest influence on the maximum vertical displacement of the surface,and the maximum vertical displacement of the surface increases linearly with the decrease of brine temperature.(4)On the 5th day of freezing,the maximum X positive displacement in X direction is 8.82 mm and the maximum X negative displacement in X direction is-8.69 mm.On the 6th day of freezing,the maximum horizontal displacement in Y direction reaches 14.3mm.Brine temperature has the greatest influence on the maximum horizontal displacement of the surface,and the maximum horizontal displacement of the surface increases linearly with the decrease of brine temperature.(5)In numerical simulation and model test,when the thickness of freezing wall is 2.5m～2.8m,combined freezing can reduce the displacement of 4.55mm～61.7mm and 0.28mm～1.79 mm,respectively,compared with saline freezing,and the reduction range can reach 7.7%～43.2% and 10.4%～24.3%.Through this study,the displacement inside the formation and on the surface can be predicted when liquid nitrogen brine is combined with vertical freezing under similar engineering background,which provides a basis for frost heave prevention and control and a reference for the design and construction ideas of artificial freezing method in the freezing reinforcement project of shield end soil.There are 59 figures and 12 tables as well as 93 references in the thesis.