| Pile foundation has been widely used in the permafrost region due to its good bearing capacity and structural advantages,and has become one of the most commonly used foundation forms in the permafrost region.Due to the particularity of permafrost,the clear bearing characteristics between permafrost and concrete are related to the long-term bearing performance and deformation stability of pile foundation in permafrost,as well as the structural safety of key projects such as upper high-rise buildings,high-speed railways and expressways.This paper takes the shear creep characteristics of the interface between frozen sand and concrete as the research object.Under-2 ℃ temperature,in different shear stress simulation of upper load,different method to the stress of pile simulated soil lateral earth pressure and freezing force,the shear stress and normal stress,frozen soil moisture content,surface roughness and other four factors effect on the interface shear creep property,analysis of creep law of creep mechanism,explore the interface contact surface creep model is established.In this paper,the shear test of constant creep under constant temperature and pressure is carried out by separately loading method.Four kinds of shear stress levels(54.15 k Pa,110.4k Pa,166.65 k Pa,222.9 k Pa)and three kinds of normal stress levels(100 k Pa,150 k Pa,200 k Pa),three kinds of concrete roughness conditions(roughness 0 mm,0.059 mm and 0.098 mm),seven moisture content conditions(including 6%,12%,16%,etc.3 kinds of frozen soil and low ice content 23%,36%,60%,80% and so on four kind of permafrost high ice content)under the shear creep characteristics of frozen soil and concrete interface has carried on the comparison and analysis,a total of 14 groups often creep shear test.In addition,in order to determine the shear stress condition of the contact surface,a group of fast shear tests of the contact surface were carried out under the same condition as the normal creep test.In order to compare and analyze the difference between separate loading and fractional loading,a group of shear creep tests of interface under fractional loading were carried out.The increase of shear stress level will directly affect the deformation stability of the contact surface.When the shear stress level increases from 54.15 k Pa to 222.9 k Pa,the shear creep curve of the contact surface changes from attenuated creep to non-attenuated creep,and the total time from loading to failure of the specimen at 222.9 k Pa is one third of that at 166.65 k Pa.In other words,the larger the shear stress is,the more unstable the deformation of the contact surface is,and the larger the corresponding instantaneous creep variable is.The shorter the duration of stable creep is,the higher the rate of stable deformation is,and the more easily the sample is destroyed.However,normal stress can improve the interface contact force and make the contact between frozen sand and concrete surface closer.Therefore,the influence law of normal stress level on shear creep characteristics of contact surface is opposite to that of shear stress level.In other words,the larger the normal stress is,the more stable the deformation of the contact surface is.The increase of the normal stress can significantly enhance the creep strength of the contact surface.With the increase of frozen soil moisture content,the creep deformation of the contact surface decreases first and then increases in the same period of time.When the moisture content is 16%,the deformation of the contact surface is the least under the same test conditions.At the same time,due to the increase of water content,the unfrozen water content in the frozen sand increases.When the water content of frozen sand reaches 80%,the deformation development of the contact surface shows obvious viscous characteristics.It can be found from the experimental study of the contact surface between frozen sand and concrete with different roughness that the increase of roughness can obviously improve the bearing capacity of the contact surface,and the larger the roughness is in a certain range,the more stable the deformation of the contact surface is.When the creep characteristics of contact surface are studied by using the methods of separate loading and fractional loading,it is found that there is little difference between them at lower shear stress levels(54.15 k Pa and 110.4 k Pa),and similar research laws can be obtained.However,when the shear stress level increased to 166.65 k Pa,due to the accumulation and development of the contact surface deformation under the first two levels of load,the graded loading test was destroyed quickly after the third level of load was added.The graded loading could not truly reflect the shear creep characteristics of the contact surface under high stress level.Based on the experimental study and the analysis of shear creep law of the contact surface,the shear creep mechanism of the contact surface between frozen sand and concrete is discussed through the applicability analysis of the model,and the creep model which conforms to the test law is established.It is found that the Boggs model and Nishihara model can reveal the attenuation creep phenomenon of contact surface under small stress,and the creep process of unstable creep stage and stable creep stage under high stress,but can not describe the accelerated creep test law well.Based on the phenomenological theory,the relationship between creep rate and time under different shear stress conditions is analyzed,and a phenomenological creep model of contact surface with good applicability is obtained.Through verification,the calculated value of the model is in good agreement with the experimental value. |
