Study On Pore Pressure Evolution During Freezing And Thawing Process Of Deep Clay Under High Stress

Frost heave and thawing sedimentation of soil is not conducive to the stability of structure and surrounding environment.In the freezing-thawing process of deep soil,deep soil undergoes freezing-thawing under high stress,and the change of pore water pressure in this process is closely related to freezing-thawing and thawing sedimentation.Taking deep clay as the research object,this paper carried out an experiment on the evolution law of freeze-thaw pore pressure under closed system and open system with the help of the self-developed pore water pressure testing system,and analyzed the relationship between various physical quantities.The main results obtained are as follows:(1)The freezing-thawing pore water pressure test device of high-stress deep soil and the extractable pore water pressure gauge are designed by ourselves.The device can carry out the pore water pressure test in the freezing-thawing process of both closed and open systems.The maximum axial pressure is 8MPa and the working temperature is-25??20?.(2)Through carrying out the pore water pressure test of the clay in the deep part of the closed system,it was found that the pore pressure in different positions decreased at the initial cooling stage,the pore pressure in the middle and upper part of the sample increased rapidly before the temperature reached the phase transition,and the pore pressure in all areas decreased rapidly with the temperature decreasing in the freezing stage.At the beginning of the temperature rise,the pore pressure in different positions of the sample increased.When the temperature rose to 0?,the pore pressure in all regions decreased slightly,and then gradually increased as the temperature rose.(3)In the loaded-freeze-thaw mode of the closed system,the upper hole pressure decreases with the increase of frost heaving deformation during the freezing process,and the two are in a linear form.The change of pore pressure in the upper region of the sample during the temperaturerising process with the decrease of displacement can be divided into four stages: sharp increase,slow decrease,slow increase and stable change.(4)Under the unidirectional freezing-thawing condition of the open system,the pore pressure in the upper region and the middle and lower regions of the soil decreased at the initial cooling stage.When the temperature dropped to the phase transition point,the pore pressure in all regions increased,and the pore pressure in all regions decreased as the temperature continued to decrease.The variation of pore pressure in the melting process is basically the same as that in the closed system.(5)In the open-system freeze-thaw mode with load,the deformation of freeze-heaving increases with the increase of freezing depth.The axial displacement of the sample changes with the passage of time in the whole test process can be divided into four stages,namely,the decrease of preloading,the increase of freezing expansion,the decrease of melting settlement and the decrease of drainage consolidation.The pore pressure at the upper and lower ends of the sample decreased first and then increased with the soil settlement during the melting stage.During the consolidation stage,the pore pressure in different areas of the sample had a linear relationship with the settlement deformation.(6)Under the condition of the closed system,when the axial pressure is different at the lowest temperature point,the influence law of the change of initial water content on pore pressure is different.When the initial water content is the same,the larger the axial pressure is,the larger the characteristic value of pore pressure is.Under the condition of open system,at the lowest temperature point,the initial water content is the same,and the characteristic value of pore pressure in the upper area of the sample decreases first and then increases with the increase of axial pressure.With the same axial pressure,the higher the initial water content is,the greater the characteristic value of pore pressure in the upper area of the sample is.(7)There is a difference in the variation of pore water pressure between open and closed systems.In the freeze-thaw process,the pore pressure in the upper region of the closed system sample is greater than that in the open system,while the pore pressure in the lower region of the closed system sample is less than that in the open system.(8)Under the conditions of this test,the axial frost-heaving deformation of the samples in the closed system during the freezing stage and the axial thawing deformation of the samples in the closed system after the temperature rise are greater than that in the open system.This paper has 70 figures,39 tables and 83 references.