Study On Characteristics Of Artificial Frozen Soil Under Seepage-flow Conditions

With the development and utilization of underground space,loose aquifer strata are often encountered and brings great potential safety problems to engineering construction.Artificial ground freezing technology is widely used to reinforce unstable strata,which acts as a barrier and waterproof.However,due to natural conditions or human impact such as large-scale extraction of groundwater,the groundwater flow rate is too fast,which will have an adverse impact on freezing construction.The seepage flow will transfer the cooling capacity of freezing pipe to downstream,thereby extending the intersection time.It could increase the cost of construction significantly.In addition,the seepage is also very likely to cause uneven frozen wall shape,the rise in average temperature,and the decrease in freezing quality.All these changes are related to the velocity of groundwater,the temperature of refrigerant and the spacing of freezing pipes.With the expansion of the application fields of freezing method and the increasing complexity of construction environment,higher requirements for freezing method are put forward.It is necessary to determine the design parameters of artificial freezing method in complicated geological conditions.In this paper,the freezing characteristics of artificial frozen soil under seepage-flow conditions are investigated by laboratory experiments and numerical simulation.The specific research are as follows:(1)A simplified two-hole freezing model is proposed based on the actual application.According to the similarity criterion,a model test system for artificial frozen soil under seepage-flow conditions is designed,including seepage system,freezing system and temperature monitoring and recording system.Besides,The model box adopts double-deck design,which improves the heat preservation effect.(2)A single factor laboratory test was carried out by changing the water head and alcohol temperature.The influence of the seepage velocity and the refrigerant temperature on the freezing temperature field is studied on the basis of changes in temperature of point,the development process of freezing temperature field and the temperature distribution of characteristic surface.The seepage field has an adverse impact on the temperature field,which makes the temperature of the upstream and downstream obviously different,and accelerates the cooling process in the downstream of the characteristic plane and leads to nonuniform temperature distribution.Reducing the temperature of the refrigerant will shorten the cooling stage and lower the temperature.(3)Based on the data of temperature field,the development law of frozen wall is investigated.The influence of different seepage velocity and refrigerant temperature on the core evaluation indexes of freezing engineering,such as intersection time,thickness of frozen wall,development velocity,nonuniformity and average temperature,is analyzed.It is found that the seepage makes the time of intersection increase obviously,and makes the thickness and average temperature of frozen wall develop unevenly.Reducing the refrigerant temperature can decrease the intersection time,accelerate the advance speed of the freezing front and improve the freezing quality.(4)The numerical simulation is used to expand the fields of model test.The influence of the distance between freezing pipes and the direction of seepage on the intersection time,the thickness of freezing wall and the average temperature are studied.The sensitivity of parameters is analyzed by orthogonal design experiment.