Study On The Freezing Depth Of Clayey Soil In Seasonal Permafrost Region

Freezing and thawing of seasonal permafrost is an important physical process in soil.Reasonable determination of freezing depth is the key to anti-freezing design in seasonal permafrost area.The maximum freeze-thaw interface is an important parameter for the design of subgrade height to prevent frost heave in seasonal frozen soil area.The deepening of freeze-thaw active layer of frozen soil will lead to land subsidence,oil and gas pipelines and related facilities.Meanwhile,soil frost heave itself will not only cause foundation damage,but also cause frost damage to the foundation of bridge and culvert.When permafrost thaws,the subgrade and foundation bed will settle to different degrees under the action of gravity and overburden load.In order to study the freezing depth of clay in seasonal permafrost regions,the temperature characteristics of some seasonal permafrost regions in China were calculated,and then the temperature boundary conditions of laboratory tests were determined.The one-way cycle freeze-thaw test of silty clay was carried out.The maximum freezing depth of silty clay with different moisture content under different temperature conditions was obtained by laboratory freeze-thaw test,soil freezing depth prediction formula study,and hydrothermal coupling numerical analysis of frozen soil.(1)The temperature of some seasonal permafrost regions in China was obtained.On the basis of temperature data,three temperature conditions with temperature amplitude of 25 and annual average temperature of 12.5℃,0℃ and-12.5℃ were designed.One-way freeze-thaw cycle tests were carried out on four samples with soil mass moisture content of 10%,20%,25% and 30% respectively.The maximum freezing depth of silty clay with four kinds of water content at different temperatures is obtained,and the rule between freezing depth and water content is expounded.(2)by comparing the analysis of the measured values and Stephan formula,Lu Ji Yang’s formula,specification for design of building foundation formula,the permafrost area building foundation design code and the geological investigation of permafrost engineering formula of the predicted values,show that Lu Ji Yang’s formula to predict frozen depth relative to other formula is more accurate.Combined with the volume moisture content of unfrozen water,the calculation value of freezing depth in "Code for Design of Building Foundation in Permafrost Area" is revised,and the error is within 6%.It is suggested to use this revised formula to calculate the design freezing depth in the clay area of seasonal permafrost area.(3)Based on the basic equation of soil moisture and heat transfer,the equilibrium equation of water phase transition and temperature,and considering the effect of phase transition on water characteristic parameters and thermal characteristic parameters,as well as the effect of latent heat of phase transition on heat transfer process,a hydrothermal coupling mathematical model of frozen soil was established.Second development of COMSOL Multiphysics finite element calculation software,using the numerical model of three different temperature boundary conditions,four kinds of water content indoor soil freezing test numerical simulation,and compared with the test results.The results show that the program can accurately simulate the evolution characteristics of temperature field and water field during soil freezing process.