Study On The Frost Jacking Effect And Bearing Characteristics Of Pile For Overhead Contact Systems Mast Of Qinghai-Tibet Railway

With the development of social economy,the infrastructure construction in high latitude and high altitude seasonal frozen soil regions and permafrost regions is on the rise,which inevitably encounters more and more frozen soil engineering problems.Pile foundations are widely used in infrastructure construction in frozen soil regions due to the advantages of low thermal disturbance,strong adaptability and stable bearing capacity.The problem of frost jacking effect and bearing characteristics of pile foundation under the action of freezing and thawing cycle of frozen soil foundation is an engineering theoretical and technical problem that needs to be solved urgently,which is related to the freeze-drawing deformation of soil,pile foundation type,mechanical characteristics of pile-soil contact surface,but also has complex and close connection with engineering geological conditions,climate environment,soil moisture content,external load and space-time effect and other factors.Therefore,a systematic study on the frost jacking effect and bearing characteristics of pile foundation in frozen soil regions can provide technical reference for the design,construction and operation and maintenance of pile foundation structure in frozen soil regions,and also provide reference for the analysis of the interaction between foundation and frozen soil in similar engineering practice,which has important engineering science and engineering application value.This paper takes the frost jacking behavior of pile foundation for overhead contact systems mast of Qinghai-Tibet Railway electrification project as the research topic.Through indoor model tests,theoretical analysis and numerical simulation,the research is carried out on the hydrothermal gas migration and frost deformation response of unsaturated soil during freezing process,shear characteristics of frozen soil-concrete interface,theoretical calculation method of frost jacking load transfer of pile foundation,resisting frost jacking effect of shaped pile foundation,and stability of pile foundation under long-term freezing and thawing cycle action.The main research contents and results are as follows:(1)Study of liquid water,water vapor and heat transport and frost heave deformation of unsaturated soil under one-dimensional/two-dimensional freezing action.In order to reveal the liquid water and water vapor transport process and deformation mechanism in unsaturated soils under one-dimensional and two-dimensional freezing action,a coupled computational model of liquid water,water vapor,heat transport and frost heave deformation of unsaturated soil under freezing action is constructed,and the whole process of liquid water and water vapor transport,water accumulation and condensation into ice in unsaturated frozen soil is reproduced by means of numerical simulation.It is shown that the liquid water and water vapor in the unfrozen zone during the freezing process of unsaturated soil migrate toward the freezing edge driven by the temperature gradient,where they accumulate leading to the reduction of unsaturated pore space and the decrease of equivalent vapor content.The soil water field is obviously redistributed during the two-dimensional freezing action,and the highest migration of liquid water and vapor to the diagonal direction of the soil is caused by the superposition effect of the negative temperature gradient in both directions,which generates frost heave deformation along the two directions perpendicular to the negative temperature action surface,with the largest frost heave deformation in the diagonal direction of the soil.(2)Model test study on frost jacking characteristics of pile for overhead contact systems mast(OCSM)of electrification reconstruction project of Qinghai-Tibet railway.In order to study the effect of resisting frost jacking of piles with different sections(equal section circular pile,straight cone cylindrical pile and curved cone cylindrical pile),the indoor model tests with three freeze-thaw cycles were carried out using the subgrade filling materials of Qinghai-Tibet railway as the test soil sample.The ground temperature distributions,jacking displacement and force characteristics of OCSM pile foundations under the action of freezing-thawing are obtained.The study shows that the subgrade freezing(melting)is two-dimensional that is jointly affected by cold(heat)energy in two directions of subgrade top and slope surface.The ground temperature on both sides of the subgrade and slope of OCSM pile foundation is asymmetrically distributed,and the maximum melting depth near the foundation is about 30.0 cm.The vertical frost jacking displacement of straight cone cylindrical pile and curved cone cylindrical pile during a freezing period is only 46% and58% of that equal section circular pile.The frost jacking force of straight cone cylindrical pile and curved cone cylindrical pile is only 80% and 71% of that equal section circular pile,which means that the curved cone cylindrical pile has the characteristics of small vertical frost jacking displacement and low frost jacking force,and has better resisting frost jacking effect.The soil water content increases from 10% to 15%,and the average tangential frost heave stress in the range of frost depth increases significantly,leading to an increase in the frost jacking force of the pile foundation.(3)Study on shear characteristics of frozen soil-concrete interface and calculation method of pile foundation freeze-drawing bearing theory.A series of negative temperature direct shear tests of frozen sand soil and concrete with different ice film thickness at the contact interface were carried out to analyze the influence of ice film thickness on the shear mechanical deformation characteristics and shear strength parameters of the interface.Combined with multiple regression method,a trilinear shear constitutive model with multiple factors was established.Based on the shear-displacement method,the trilinear shear constitutive model is introduced to establish a theoretical calculation model for the frost jacking behaviour of single pile during the freezing process of frozen soil foundations,and solved by analytical and difference method.The dynamic development law of the frost jacking force during the freezing process was obtained by programming.The rationality of the model was verified by field tests and model tests,and the influencing factors of the frost jacking response of the pile foundation were analyzed.The results show that the ice film thickness has a significant effect on the shear behavior of the interface,and the relationship between peak shear strength and residual shear strength and ice film thickness can be approximated as a hyperbolic function.There is an "effective influence thickness" of ice film thickness on the interfacial shear strength.As the ice film thickness increases,the peak cohesion increases significantly and the residual cohesion also increases to some extent.The peak of axial force in the pile length range is near the freezing depth,and with the freezing process,the axial force gradually increases,the peak moves down,and some of the pile-soil interface produces slip phenomenon.The length of the slippage section gradually increases,and the length of the slippage section is about 0.65 times the frost depth when frozen to the maximum frost depth.The frost jacking force does not increase linearly with the increase of frost heaving ratio of soil,which is mainly affected by the peak freezing strength and residual strength.The shorter the pile length,the higher the freezing stress is required to resist frost jacking.The influence of horizontal frost heave force in the existing subgrade on the horizontal displacement and bending moment of the OCSM pile cannot be ignored.(4)Numerical simulation study of frost jacking stability of pile foundations under the action of long-term freezing and thawing cycles.A numerical calculation model of pile-frozen soil interaction under the action of freezing and thawing cycles was constructed to predict the cumulative frost jacking response and service performance of OCSM piles in existing subgrade of Qinghai-Tibet Railway.The study shows that the frost heave deformation of the foundation soil during the freezing period mainly occurs in the active layer.The frost deformation of the soil around the pile and the frost jacking displacement of the pile are basically stable until February,and the accumulated frost jacking displacement of the pile foundation during a freezing period is 1.6 mm.The horizontal frost heave force on the left and right side of the pile(subgrade side and side slope side)during the freezing of the active layer is asymmetrically distributed,the horizontal frost heave force at the surface of the subgrade side is the largest after soil freezing and stabilization,and the horizontal frost heave force on the side of the slope increases first and then decreases along the length of the pile.The pile axial force of OCSM pile foundation near the location of frost depth is the largest,the maximum value is 250.6 k N,and the maximum tensile stress of the cross section reaches1055.3 k Pa,so the design of OCSM pile foundation should focus on the tensile strength of the pile material near the maximum frost depth.The accumulated frost jacking displacement of the OCSM pile foundation under the long-term freezing and thawing cycle action increases year by year,and the accumulated frost jacking displacement of the curved cone cylindrical pile reaches 34.4 mm in the 30 th year,and the frost jacking displacement of straight cone cylindrical pile and curved cone cylindrical pile is only 35.3% and 31.0% of that equal section circular pile.