Bearing Characteristics Of Permafrost Pile Foundation Considering The Creep Effect Of Pile-soil Contact Surface

The settlement and deformation of foundation and pile foundation of buildings（structures）in cold regions is one of the key problems in the study of frozen soil engineering,which seriously affects the normal performance and service safety of buildings（structures）.This problem restricts the construction and development of projects in cold regions.The problem of permafrost degradation caused by global warming has aggravated this phenomenon.Creep deformation of pile foundation is one of the main deformations of bridge pile foundation in service stage in cold regions,and small load can also cause creep deformation of pile body.However,due to the special sensitivity of temperature and time correlation of pile foundation in permafrost,the creep law between pile and soil is more complex and unstable.Therefore,it is urgent to study the settlement deformation law and settlement prevention and control measures of pile foundation in permafrost under different constant load conditions and permafrost degradation caused by climate warming.The shear creep test of frozen soil-structure interface is one of the effective and convenient ways to study this problem.In this paper,the stress-controlled large-scale direct shear apparatus is used to carry out the direct shear test and creep test of the frozen Qinghai-Tibet silt-concrete contact surface,and the expression that conforms to the actual variation law of the direct shear test curve is constructed.On the basis of clarifying the influence of various factors on the results of direct shear test,the shear creep test of frozen Qinghai-Tibet silty sand-concrete interface was carried out synchronously to clarify the phenomenon of creep test,and to explore the variation law,creep rate and morphological characteristics of creep test curves under different conditions,so as to reveal the internal mechanism of creep.Considering the special critical value of creep curve shape and change rate,the criterion of contact surface failure is defined,and the long-term strength curve and corresponding parameters are obtained.Based on the rheological theory component model,an improved contact surface creep constitutive model considering time and stress level is constructed,and the variation law of parameters in the model is judged.Combined with the finite element theory,the secondary development of the improved contact surface creep constitutive model in the numerical calculation software is realized.Finally,the long-term bearing characteristics of pile foundation in permafrost regions are explored by means of numerical simulation.The main research contents and innovations are as follows:（1）Design of Shear Test And Analysis of Direct Shear Test Results of Frozen Qinghai-Tibet Silty Sand-Concrete InterfaceThe shear test was carried out by using the existing stress-controlled large-scale direct shear apparatus,and the functions and characteristics of each component and system in the test equipment were clarified.The shear creep test monitoring system of frozen soil-structure interface was developed.Based on the shear creep test results of the frozen Qinghai-Tibet silt-concrete contact surface,the direct shear stress-displacement curves of the contact surface under different test conditions are drawn.The identification curve is mainly composed of four parts:tangential consolidation stage,elastic stable development stage,plastic deformation dominant stage and final slip failure stage.The SWeibull1 growth curve expression is introduced to describe the stress-displacement curve,and the least square method is used to invert the parameters.It has clear physical meaning and good agreement,and can describe the direct shear stress-displacement curve of the contact surface under different conditions.（2）Analysis of Shear Creep Test Results of Frozen Qinghai-Tibet Silty Sand-Concrete InterfaceBased on the shear creep test results of the frozen Qinghai-Tibet silt-concrete contact surface,the shear creep curves of the contact surface under different test conditions are drawn.It can be seen that the creep curve of the contact surface is similar to the creep curve of the rock and soil mass,which can be divided into three categories:attenuation creep stage,constant creep stage and accelerated creep stage.The effects of different shear stress levels,normal stress,temperature and contact surface roughness on the shear creep curve shape and creep rate of the contact surface were investigated,and the influence mechanism of different conditions on the creep characteristics of the contact surface was revealed.According to the shear creep morphology and failure rate in the acceleration stage,the criterion for determining the long-term strength of the contact surface is defined.The creep deformation value corresponding to the maximum value of the constant creep damage rate in the acceleration stage is defined as the failure value of the contact surface under the same condition,and then the long-term strength curve of the contact surface is drawn according to the criterion.The first-order decaying exponential function can better describe the curve and the parameters have clear physical significance.Finally,the influence of different conditions on the long-term strength parameters is explored.（3）Establishment of Improved Shear Creep Constitutive Model of Contact SurfaceBased on the theory of rheological model elements,a shear creep constitutive model of contact surface considering time and stress level is constructed.Based on the traditional Nishihara model,in the viscoelastic Kelvin body,the linear viscous element is changed into a nonlinear element,and the viscosity coefficientη₁ is expressed as a function of time and stress.In the viscoplastic Bingham body,the damage variable D is introduced to describe the creep failure process of the contact surface,and a new improved shear creep constitutive model is obtained.The shear creep test data of frozen Qinghai-Tibet silty sand-concrete interface are compared and analyzed to verify the rationality and applicability of the improved model.The results show that the improved Nishihara model can better describe the whole creep process of frozen soil-concrete interface,and has better applicability than the traditional model.（4）Secondary Development of Shear Visco-Elastic-Plastic Constitutive Model of Contact SurfaceBased on the finite element theory of the contact surface and the rheological model element theory,the initial strain method and the incremental iteration method are used.The Fortran programming language is used to write the finite element calculation format of the viscoelastic-plastic constitutive model of the contact surface in Visual Studio.The built-in FRIC subroutine of ABAQUS is embedded to realize the secondary development of the viscoelastic-plastic constitutive model of the contact surface.A numerical calculation model consistent with the experimental conditions is established,and the simulation results,experimental data and improved constitutive model expressions are compared and analyzed.The results show that the experimental data,the analytical solution of the constitutive model and the numerical solution of the simulation results are highly consistent.The verification results combined with the research method of numerical simulation have certain reference significance for re-expanding the simplified shear creep test law of the contact surface to the long-term bearing characteristics of the pile-soil system itself.（5）Numerical Simulation Study on Long-Term Bearing Characteristics of Pile Foundation in Permafrost RegionBased on the engineering background of the Yangtze River Source Bridge（Tuotuo River Bridge of Qinghai-Tibet Railway）in the permafrost region of the Qinghai-Tibet Plateau and the large river melting area,combined with the ABAQUS finite element numerical simulation software and the shear visco-elastic-plastic constitutive model of the contact surface obtained by secondary development,the long-term bearing characteristics of pile foundations in permafrost regions under different pile top loads,temperature rise caused by permafrost degradation,and different pile lengths and pile diameters were studied.The research results have good theoretical basis and reference value for the design,construction,maintenance and reinforcement of pile foundation in permafrost regions.