Study On Influence Of Groundwater On Thermal-mechanical Characteristics In Permafrost Regions

Under the background of rising global temperature,with the increase of precipitation,the melting of glacial ice and underground ice greatly increases the possibility of groundwater.And the convective heat of groundwater makes the frozen soil in the nearby range warm up or even melt,which will seriously threaten the service performance of the pile foundation project in the permafrost regions.Therefore,the problem of pile foundation caused by groundwater thermal effects during the operation of pile foundations is a new challenge faced by frozen soil engineering.In this paper,by preparing samples of frozen soil-structure interface,the process of mechanical characteristics of frozen soil-structure interface induced by heating is explored,and the mathematical expression of friction characteristics of frozen soil-structure interface coupled with multiple factors is constructed.Based on the survey data of pile foundation engineering,and combined with the mathematical representation of the friction characteristics of the frozen soil-structure interface,the calculation model of the temperature field and stress field of the pile-soil system is constructed to analyze the thermal disturbance variation of the groundwater thermal effect on the temperature field and the degration of the bearing capacity of the pile foundation.It can provide experimental and theoretical basis for cognition on strength degradation mechanism and friction characteristics of frozen soil-structure interface,and is a useful supplement to the design pile foundation and disaster recognition in permafrost regions.According to the evolution variation of the friction characteristics of the frozen soil-structure interface during the thawing process,the shear test of the frozen soil-structure interface during the thawing process is carried out and it is obtained that the shear strength of the interface decreases with increasing temperature during the thawing process.At a moisture content of 30%and 40%,the shear strength is approximately linear with temperature.When the water content rises to 50%and 60%,the shear strength drops sharply at the stage of-1℃～-0.5℃.The shear strength of the interface exhibits varying degrees of loss as the temperature of the frozen soil-structure interface increases.The variation of internal friction angle with temperature of different water content is similar to the trend of shear strength with temperature.Based on the variation of friction angle of the frozen soil-structure interface,a mathematical expression of the friction coefficient under the coupling of temperature and water content is constructed.According to the thermal disturbance of groundwater and groundwater regional differences on the temperature field of the pile-soil system in the future years,the thermal analysis of the temperature field of the pile-soil system under without groundwater and groundwater temperatures of 0.6℃,0.8℃ and 1.0℃ was carried out and showed that the presence of groundwater caused the surrounding frozen soil to heat up or even melt.The melting range of groundwater at 0.6℃,0.8℃,and 1.0℃ after 30 years is 9.18m,11.17m,and 13.22m,respectively.Therefore,the presence of groundwater is more likely to cause the instability of the pile.According to the degradation variation of pile foundation bearing behavior under the groundwater thermal effect in future years,the numerical calculation analysis of bearing capacity of pile foundation in different years was carried out and it was found that the axial force decreases along the length of the pile under the action of ultimate load,and the axial force curve converges faster the closer to the pile end with the groundwater temperature increases.As the effect of groundwater on the thermal disturbance of frozen soil continues to intensify,the attenuation trend of the side friction resistance of the lower part of pile foundation becomes more obvious.According to the variation of Q-S curve of pile,it can be divided into elastic stage,elastoplastic stage and plastic stage.The Q-S curve of the pile is obviously shifted to the left and the settlement continues to rapidly increase when the groundwater acts.At the ultimate load of the pile foundation,the displacement of the soil around the pile of the lower part of the pile foundation has increased significantly and the distance of the pile-soil system displacement spreading farther in the radial direction increases with the groundwater temperature increases.