Experimental Study On Strength Characteristics Of The Frozen Soil-Pile Interface And Numerical Simulation Analysis On Bearing Capacity Of Pile Foundation In Permafrost Region

Under the background of global warming and engineering disturbance,with the rapid increase of human activities and engineering construction in permafrost area,the degradation of permafrost makes the bearing capacity of pile foundation in permafrost area face a severe test.The nature of permafrost is very complex,and the research results of pile foundation in permafrost are not as perfect as those of pile foundation in general soil,and there is a lack of systematic research on the generation and development law of freezing force at pile-soil interface.Taking the pile foundation engineering in the permafrost region of Qinghai-Tibet Plateau as the research object,this paper adopts the means of combining laboratory test and numerical calculation,and focuses on the influencing factors and formation mechanism of the freezing strength of the pile-soil interface.On this basis,with the typical bridge engineering in permafrost region as the engineering background,ABAQUS software was used for numerical simulation analysis to explore the problems of pile-soil freezing strength,pile foundation bearing capacity and stability in permafrost region.Based on the physical and mechanical properties of permafrost on the QinghaiTibet Plateau,a pile compression test system was designed for the freezing strength of soil-structure interface under low temperature conditions.The characteristics and variation rules of the freezing strength of the interface under different factors were discussed,and the distribution law of normal compressive stress on the interface along the pile body was further understood.On this basis,the numerical simulation of indoor pile compression test was carried out,and the results were compared with the test data,in order to reveal the formation mechanism of freezing strength at pile-soil interface.The results show that there is a temperature gradient that decreases from the bottom to the top and from the inside to the outside in the freezing process of soil.The smaller the depth of the pile is,the greater the frost heaving force is,and the frost heaving force generally increases during the freezing time.In the process of pile compression,the shear stress change of pile-soil interface can be divided into two categories.One is three stages,including the linear growth stage,the abrupt decline stage and the residual stage.The other is strain hardening,in which the shear stress continues to increase with the increase of pile displacement until it reaches a certain value.The shear stress change rate of pile-soil interface is closely related to temperature and water content.The freezing strength,allowable displacement of failure and residual freezing strength shift of pile-soil interface gradually decrease with the increase of buried depth.The decrease of temperature or the increase of moisture content will increase the freezing strength of contact surface significantly.At the end of freezing test,the normal pressure of pilesoil interface increases along the buried depth and presents a convex distribution law.At the end of test loading,the normal pressure of pile-soil interface presents a distribution rule of "large at both ends and small in the middle".Furthermore,in order to better fit the engineering practice in the qinghai-tibet plateau permafrost regions qingshui river bridge as the background,using ABAQUS software,carried out in permafrost regions of pile-soil interface strength,pile foundation bearing capacity and stability of the frozen numerical simulation research,analysis of frozen the settlement of single pile foundation under the action of a hierarchical load law,formulated under the condition of low temperature change regularity of pile foundation and permafrost interface freeze strength and influence mechanism.The results show that the freezing force at the pile side basically increases along with the increase of the buried depth,and the main freezing force is located in the permafrost.With the rise of the temperature of seasonal thawing layer,the freezing force of pile foundation decreases,and the freezing force transfer effect of upper loading on the side contact surface at the lower part of pile is better.In the process of loading,the horizontal stress of soil presents an increasing trend,and there is an obvious stratification phenomenon,which indicates that different depth and different types of soil have a great influence on the distribution of horizontal stress in soil,and the greater horizontal stress concentrates on the tip of pile at the end of loading.The safety factor of pile foundation was calculated by the design code and fitted with the temperature of seasonal freeze-thaw layer.It was found that the higher the temperature of seasonal freeze-thaw layer,the more sensitive the safety factor was to the temperature change.