Numerical Simulation Research On Deformation Characteristics Of Pile Foundation Under Non-uniform Frost Heaving Condition

The construction of electrified railway is of great significance to China ’s economic,social and cultural development.Catenary is an important part of railway electrification system.Pile foundation is a common foundation form of catenary.Therefore,the deformation characteristics of catenary pile foundation have a significant impact on the safe and stable operation of railway engineering.Some areas along the electrified railway are mainly alpine mountainous areas,and seasonal frozen soil is widely distributed.The frost heave and thaw settlement of subgrade in seasonal frozen soil area will not only affect the smoothness of track structure and train operation,but also lead to problems such as train speed limit in winter.At the same time,it will greatly affect the safety and stability of the catenary pillar foundation.If the treatment is not appropriate,it will lead to engineering diseases such as freezing and overturning of the catenary pillar foundation.In order to study the deformation characteristics of pile foundation under non-uniform frost heave conditions,the frost heave deformation of catenary foundation of electrified railway in seasonal frozen soil area is taken as the research background.With the help of finite element software,the three-field coupling of water-heat-force is realized.Considering different pile types,pile lengths and different slope rates of subgrade of catenary pillar foundation,numerical simulation is carried out to analyze the variation law of temperature field,water field and deformation field,so as to provide design parameters for catenary pillar foundation in seasonal frozen soil area.The mechanical properties of frozen soil-concrete interface were studied by laboratory tests,which provided basic conditions for numerical simulation.The ’ geotechnical test assistant-particle analysis test calculation software ’ and ’ frozen soil-structure contact surface shear creep test monitoring system ’ were independently developed to match the test.At the same time,based on the large-scale stress-controlled shear instrument made by the research group,the freezing strength test of frozen silt-concrete structure was carried out.The temperature is set to-0.3℃,-1℃,-2℃,the ice content is 20 %,30 %,40 %,50 %,70 %,the roughness is 0 mm,0.538 mm,0.775 mm and 1.225 mm,the normal stress is 50 k Pa,100 k Pa,150 k Pa,a total of 11 groups of tests.The results of shear test show that the stress-strain curve of direct shear test mainly includes linear deformation stage,accelerated deformation stage and sliding failure stage.Under the smaller stress state,the stress-strain curve is linear.When the shear load further increases,the deformation of the contact surface begins to enter the acceleration stage,and the deformation rate continues to increase.As the shear load continues to increase,the strain increases sharply,and sliding failure occurs on the contact surface.The source of shear strength of the contact surface mainly includes the cohesion and friction of the contact surface,which will be affected by factors such as temperature,moisture content,roughness and normal pressure.The shear strength of frozen soil-concrete interface increases first and then decreases with the increase of water content.The temperature is negatively correlated with the shear strength of the contact surface;the normal pressure and roughness are positively correlated with the shear strength of the contact surface.Based on the water migration law of unsaturated thawed soil,the ice-water phase transition term is added to the differential equation to obtain the water migration equation of frozen soil,which is transformed into the form of coefficient-type partial differential equations provided by the finite element software.The diffusion coefficient,source term,retardation coefficient and absorption coefficient of the partial differential equation of the water field constructed in the PDE interface are obtained.Similarly,the temperature control equation considering the latent heat of phase change is transformed into the form of coefficient-type partial differential equations provided by the finite element method,and the partial differential equation coefficients for constructing the temperature field are obtained.The solid-liquid ratio is introduced to couple the temperature field and water field,and the coupling results are verified based on the indoor model test.Based on the coupling results of temperature field and moisture field,the temperature and moisture numerical simulation data of the first year,the 15 th year and the 30 th year were extracted.The results show that considering the global climate with a temperature rise of2.6 ℃ in 50 years,the temperature inside the subgrade during the freezing and thawing periods shows an increasing trend;in the 30 years of simulation analysis,the lowest temperature in the freezing period increased by 1.6 ℃,and the highest temperature in the thawing period increased by 1.4℃.The maximum freezing depth gradually decreases,and the maximum freezing depth decreases by 0.64 m.The influence of ambient temperature on the temperature of foundation soil gradually weakens with the increase of soil depth.Finally,after reaching a certain depth,the influence of ambient temperature on foundation soil disappears completely.By analyzing the changes of ice content and unfrozen water content in freezing period and water content in thawing period,it can be found that the overall ice content and unfrozen water content of subgrade show seasonal variation.During the thawing period,with the increase of the distance from the pile,the unfrozen water content of the soil at the same depth shows a decreasing trend;during the freezing period,with the increase of the distance from the pile,the ice content of the soil at the same depth shows a decreasing trend.Based on the calculation results of temperature field and water field,the displacement of pile-soil system is calculated,and the influence of slope rate,pile type and pile length on pile displacement is studied.By extracting the numerical simulation results,the relationship between pile displacement and subgrade slope rate,pile type and pile length is analyzed.The vertical displacement of the pile changes in a sinusoidal form.As time goes on,the maximum vertical displacement and horizontal displacement of the pile in each cycle gradually decrease.Compared with the straight pile,the cumulative frost heave of the conical pile is small,and the cumulative frost heave in the 30 th year is only 63.4 % of that of the straight pile with the same slope rate,which has good uplift resistance.Due to the existence of the side slope of the tapered pile,the stress state of the pile foundation is changed,resulting in the decrease of the horizontal stress of the pile side.At the same time,due to the influence of cold shrinkage,with the continuous development of freezing depth,the normal frost heaving force will eventually become tensile stress,and the tensile force will continue to increase.When the tensile force exceeds the tensile limit of the structure between the soil and the pile,cracks will occur between the soil and the conical pile,and the tangential frost heaving force will dissipate.