Pore Pressure And Mechanical Properties Of High Temperature Frozen Soil Under Static Triaxial Condition

Under the long-term sustained load,frozen soil foundations are prone to significant creep deformation and even structural damage.As a result,constructions in cold regions on permafrost foundations are susceptible to various diseases such as uneven settlement,cracking,and other ailments.This not only poses a threat to the long-term stable operation of engineering structures but also causes significant economic losses and resource wastage in later stages of engineering maintenance.High-temperature permafrost is one of the main causes of ground deformation.High-temperature permafrost refers to permafrost with temperatures ranging from-1.5℃ to 0℃.As it is in the phase transition zone,there is a large amount of unfrozen water in the hightemperature permafrost.Under long-term static loads,significant pore water pressure is generated,and the change in pore water pressure shows a significant increase and dissipation phenomenon,which can cause changes in the stress field of the permafrost.Under static loading,the soil particles inside the soil body will experience mutual squeezing and displacement,as well as the development of pore ice pressure,melting and refreezing,microcracks,and other changes,which can cause damage to the permafrost and affect the macroscopic deformation characteristics of the permafrost.Additionally,some mechanical energy inside the permafrost will be converted into thermal energy under static loads,causing an increase in soil temperature,an increase in unfrozen water content,and a deterioration of soil mechanical properties.Therefore,studying the changes in pore water pressure,soil deformation,and soil temperature under static loads in high-temperature permafrost is of great significance for researching the stability of permafrost foundations.This article conducted tests to determine the physical properties of hightemperature frozen soil,as well as uniaxial creep tests and static triaxial rapid failure tests under different initial dry densities,test temperatures,sample densities,and axial stress conditions.The study investigated the factors influencing pore water pressure changes,deformation characteristics,frozen soil strength,and internal effective stress under static loading and creep conditions.Optical electron microscopy scanning and CT microscop ic structure tests were used to analyze the effects of different factors on pore changes within the frozen soil under static loading.The following main conclusions were drawn.(1)According to the freezing point experiment,it was found that the freezing temperature of high-temperature frozen soil decreases with the increase of the average dry density of the soil sample.The volumetric unfrozen water content decreases with the increase of the initial dry density.The difference in volumetric unfrozen water content among soil bodies with different initial dry densities is due to the combined effect of capillary action and soil particle surface adsorption caused by pore structure.(2)triaxial creep tests under static loading were carried out to investigate the changes in pore water pressure and deformation characteristics of frozen soil,as well as the factors affecting the strength and effective stress within the soil under different initial dry densities,test temperatures,sample dry densities,and axial stress conditions.It was found that the pore water pressure changed with the development of accumulated displacement during the static loading triaxial creep tests.Initially,the accumulation rate of the displacement curve was relatively fast,and the pore water pressure increased rapidly.When the accumulated displacement of the sample reached a certain value,structural defects such as cracks occurred in the soil sample,resulting in a decrease and dissipation of the pore pressure.Conversely,when the accumulated displacement was small,and the soil sample had no microcracks or structural defects,there was no significant dissipation of the pore pressure.Moreover,under the same axial stress conditions,the higher the frozen soil temperature,and the smaller the sample’s initial dry density,the more rapid the response of the pore pressure within the soil and the faster the accumulation rate of the displacement.(3)Through the changes of pore pressure,stress-strain curve and effective stress curve,it is found that reducing the test temperature can enhance the compressive deformation resistance of frozen soil.Under static load,the lower the temperature of frozen soil,the greater the stress increment,the stress-strain curve presents a stable growth trend,and the higher the sample strength.The higher the dry density of the sample,the lower the content of unfrozen water,the denser the soil skeleton,the smaller the void between particles,and the larger the required deformation and failure stress.In addition,a higher temperature will make the axial stress on the sample be smaller,resulting in greater pore water pressure in the soil,and thus resulting in a smaller effective stress in frozen soil.(4)SEM scans of air-dried clay samples subjected to static triaxial creep under different conditions were obtained,with experimental temperature,initial dry density,and axial stress as variables.It was found that the porosity gradually decreases with decreasing temperature,increasing dry density,and increasing axial stress.The mechanical properties of Qinghai-Tibetan clay are closely related to its microstructure,which is the fundamental factor determining its macroscopic mechanical characteristics.These findings illustrate the influence of soil structure on its macroscopic mechanical properties and provide a method reference for establishing a relationship between microstructural parameters and macroscopic physical and mechanical properties.