Comprehensive Study On One-dimensional Thawing Process Of Frozen Soil Under Variable Load Or High Temperature Thawing

Permafrost is a common porous material in daily life.Permafrost usually consists of soil particles,ice,water,and air.Permafrost can be divided into natural permafrost and artificial permafrost according to the different forming modes.The soil settlement caused by thawing permafrost will lead to engineering diseases.Large-scale projects in permafrost areas,including road projects,large-scale oil and gas pipeline projects and large-scale power transmission and transformation projects,are all affected by thawing and settling of permafrost.In addition,in the process of urban underground engineering construction,forced thawing is often used to make the artificial permafrost formed by the freezing method melt quickly.If the soil thawing deformation caused by the rapid thawing of the artificial permafrost is not well controlled,it will cause serious damage to the above-ground buildings and urban roads.It can be seen that it is necessary to study the melting process of permafrost.At present,there are few researches on the thawing process of permafrost under the condition of variable load and high temperature.Therefore,this paper carried out a comprehensive study on the one-dimensional thawing process of permafrost under the condition of variable load or high temperature thawing.(1)This paper introduces the theories of soil mechanics and porous media required to construct a one-dimensional thawing thermal consolidation model of frozen soil under high temperature thawing conditions,including effective stress principle,onedimensional soil consolidation theory and one-dimensional thawing consolidation model of frozen soil.On this basis,a one-dimensional thawing thermal consolidation model of frozen soil under the condition of high temperature thawing is established.The model includes the heat transfer part and the thermal consolidation part.The former is the phase change heat conduction process of the unfrozen water in the frozen soil,and the latter is the thermal consolidation process of the soil in the thawed area under the action of high temperature,load and dead weight.In this model,the thermal expansion of soil particles and water caused by temperature is ignored and reduced to MN model.(2)Aiming at the one-dimensional thawing process of frozen soil under the condition of general variable load,a one-dimensional thawing process model of frozen soil under the condition of variable load was established,in which the existence of unfrozen water in the frozen soil was considered.According to the theoretical model,the analytical solution and numerical solution of the model are derived.The analytical solution of temperature field shows that,even considering the influence of unfrozen water in frozen soil,the variation of thawing front position with time still has the same form as Newman’s solution,namely,is the square root of time.The analytical solution of pore pressure in the process of consolidation of thaw zone under power function external load is derived through similarity transformation.Based on this,the analytical solution of thaw and consolidation of thaw zone under any type of external load is derived by using superposition principle.In the numerical solution,for the temperature field part,by modifying the volume heat capacity of the control volume where the melting front is located,the discontinuity of the moving speed of the melting front is effectively avoided.For the consolidation part of the melt zone,the time-varying melt zone is transformed into a fixed region by coordinate transformation,and the problem of starting the solution process of the moving boundary problem is solved by using the analytical solution as the initial value.The reasonableness of the analytical solution and the numerical solution are verified by the calculation examples.It is also obtained that there are differences in the excess pore water pressure,the average degree of consolidation of soil and the settlement of soil during the one-dimensional thawing process of frozen soil under the exponential external load and the instantaneous constant external load.(3)For the study of high temperature thawing of permafrost under the condition of one-dimensional melting process,to build the high temperature permafrost under the condition of one-dimensional melt test apparatus,through the indoor test system,explores the moisture content,temperature,localized to one dimensional temperature field in the process of melting permafrost,melt front movement and soil thawing settlement amount and moisture transfer,the influence of the characteristics of melting time and access to energy.1)In the one-dimensional thawing process of frozen soil under the condition of high temperature thawing,the variation of soil temperature field can be divided into three stages: the stage of rapid temperature rise,the stage of slow temperature rise and the stage of stable temperature.2)In the one-dimensional thawing process of frozen soil under high temperature thawing condition,the position of thawing front is proportional to the square root of time,which is consistent with the Newman solution form and similar to the analytical solution form of temperature field in the one-dimensional thawing process model of frozen soil under variable load condition.The movement velocity of the melting front decreases with the increase of water content,but increases with the increase of heating temperature.The movement velocity of the melting front in Gansu silty clay is higher than that in Xuzhou clay.3)The soil thaw settlement increases with the increase of water content and heating temperature,and the thaw settlement of Gansu silty clay is greater than that of Xuzhou clay.The Origin software was used to fit the curves of thawing amount of frozen soil with time under different conditions,and the empirical formula of thawing amount in one-dimensional thawing process under different conditions at high temperature was obtained.4)In the one-dimensional thawing process of frozen soil under the condition of high temperature thawing,the water content in the lower part of soil increases while the water content in the middle and upper part decreases.5)The characteristic thawing time of Xuzhou clay is larger than that of Gansu silty clay.The characteristic thawing time of Xuzhou clay is larger than that of Gansu silty clay.6)The energy obtained by soil is smaller with the increase of water content,but increases with the increase of heating temperature.The energy obtained by Gansu silty clay is greater than that obtained by Xuzhou clay.The data obtained from the laboratory test can provide a comparative reference for the verification of one-dimensional thawing thermal consolidation model of frozen soil under the condition of later high-temperature thawing.This paper has 87 figures,14 tables,and 88 references.