| Recent years,infrastructure construction in permafrost regions has been carried out on a large scale.The completion and use of the Qinghai-Tibet Highway,Qinghai-Tibet Railway,Qinghai-Tibet DC Transmission Line Engineering and pipeline project mark the progress and development of the technical level of construction in China’s cold regions.As one of the important basic types of foundations in permafrost regions,pile foundation is widely used in the infrastructure construction of permafrost regions.However,due to the special physical and thermodynamic properties of frozen soil,considering the different temperatures,different types of frozen soil and the creep characteristics of frozen soil,the study of long-term bearing and deformation of permafrost piles becomes the key technical issues of pile foundation design and service.Considering the creep characteristics of the relationship between pile and soil,the mechanical relationship between pile and frozen soil is simplified to the problem of interface between frozen soil and structure,which is the key scientific problem to solve such problems.Based on the development of large-scale creep tester,the direct shear test of frozen soil-concrete interface was used to obtain the shear strength and shear strength parameters of the interface and their variation with various test factors.Based on the shear creep test of interface,considering the influence factors of the direct shear test,the shear stress level and the loading time on the creep characteristics of the interface,the shear creep law of the contact surface was studied.According to the creep deformation characteristics of the interface,the characteristic values of the failure state of the interface were defined,the long-term strength criterion of the interface was established,and the long-term strength and long-term strength parameters of the interface were studied.Furthermore,the phenomenological creep equation of the frozen soil-concrete interface was established based on the creep stress-strain isochronal curve of the interface and the phenomenological theory.Considering the strengthening and weakening effects of the interface during the creep process,the shear creep constitutive model of the interface was established based on the Viilov’s creep model theory.Then,the interface creep constitutive model was implanted into ABAQUS to realize the numerical analysis of interface theory.The main research contents and innovations of this paper are as follows:(1)The direct shear tests of frozen soil-concrete interface under different temperatures,sample moisture content and normal stress conditions were studied to investigate the mechanism of shear stress on the interface and the stress-strain constitutive equation of the direct shear test of the contact surface was proposed.Furthermore,the shear strength criterion that took the effects of temperature and moisture content on the cohesion and internal friction angle into account was established.The results show that the shear stress-displacement curves of the direct shear test of the interface has elastic deformation stage,plastic deformation stage and sliding failure stage successively,which reflects the shear failure mechanism of the frozen soil-concrete interface macroscopically.The shear strength of the interface is affected by the test temperature,the moisture content of the sample,and the normal stress.It can be attributed to the influence of various factors on the shear strength of the interface:the lower the test temperature,the greater the shear strength of the interface;with the increase of normal stress,the shear strength of the interface increases;with the increase of water content,the shear strength of the interface increases first and then decreases,and finally the shear resistance of pure ice-concrete contact surface strength.What’s more,there is a critical moisture content with the maximum shear strength of the interface,and the critical moisture content is 80%to 93%of the saturated moisture content of the test silt.Therefore,the shear strength of the interface can be improved by increasing the moisture content of the frozen soil appropriately and the combination of hyperbolic function and linear function can simulate the direct shear stress-displacement relationship of frozen silt-concrete interface.(2)Taking the shear stress level,initial moisture content of soil sample,the effect of test temperature and normal stress on creep properties into consideration,the similarities and differences of shear creep characteristics between frozen silt-concrete interface,pure ice-concrete interface and frozen silt were analyzed in view of the creep shear test data.From the stage type of the creep curve,the test results show that the shear creep of the interface is similar to that of the ice-concrete interface when the moisture content of the soil is greater than w_p+35±5%,and the total performance of the interface is non-attenuation creep.While the soil moisture content is less than w_p+35±5%,the creep of frozen soil-concrete interface and the shear creep of frozen soil both include attenuation creep and non-attenuation creep.In terms of the amount of creep deformation and the types of failure,the creep deformation of the frozen soil-concrete interface and the pure ice-concrete interface is relatively small,which is brittle,but the type of creep failure of the frozen soil is plastic.Shear stress affects creep deformation rate and creep time exponentially.The creep long-term strength of frozen silt-concrete interface decreases linearly with the increase of moisture content,and the creep long-term strength of interface increases linearly with the decrease of temperature.Under the condition of-2°C,the creep cohesion of the frozen silt-concrete interface with a water content of 22%is only half of that in the direct shear test.Therefore,with the gradual development of creep,the reduction of the contact strength of the interface cannot be ignored.(3)Based on the deformation state of the creep test of the interface,the interface attenuation creep stress can be expressed as the product of the power function of the strain and the time power function.The strain rate in the steady phase is the power function of the shear stress,and the strain rate in the progressive flow phase is an exponential function for time.Combined with the theory of aging,the relationship between strain,stress function and time function under different creep states is established.Then,time hardening parameters are introduced and a unified phenomenological creep model of the contact surface is proposed.(4)Combining the stress-strain relationship of the interface creep,the creep deformation mechanism is discussed and the hardening factor and damage factor are introduced to describe the strengthening and weakening effects of creep deformation.The creep constitutive model for the whole process of the contact surface is established by identifying the creep model of the interface.Based on the contact finite element theory,the penalty function method of contact finite element is used to simulate the frozen soil-concrete contact problem,and the UINTER subroutine of the contact creep constitutive model is established in the form of normal soft contact,which is used to calculate the creep of frozen soil-concrete interface.The research conclusions in this paper makes the understanding of long-term bearing capacity and long-term deformation of pile foundation in permafrost regions more rational,and can provide theoretical basis for the design and operation of permafrost pile foundations. |
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