Study On The Degenerate Mechanism Of Pile-soil Interface Under Cyclic Loading

The study of pile-soil interface under cyclic loading is an important research direction of pile-soil interaction.Roughness is one of the important factors of its mechanical response.In this paper,the self-made new visual constant stiffness interface shear instrument combined with Digital Image Correlation Measurement?DIC?is used to simulate the monotonous straight shear and cyclic shear test of sand-steel interface.PFC^2D particle flow numerical simulation is used as the platform,study the macroscopic and microscomic mechanism of the weakening effect of pile-soil interface.The main work and achievements of this paper are as follows.?1?The development of large-scale visual constant stiffness interfacial shear tester,the use of SLR camera through the plexiglass window on the sand-steel interface image acquisition,combined with digital image-related measurement technology,the whole process to capture the macroscopic and microscopic behavior of sand-steel interface's particle flow.?2?The results of monotonically straight shear test of sand-steel interface show that there is no obvious shear stress peak on the shear stress-shear displacement curve of the specimen and smooth steel when the initial normal stress is the same.With the increase of relative roughness,The strain softening phenomenon is gradually obvious,the interface shear strength and shear strength required to achieve the shear displacement also increase.When the relative roughness of the interface is the same,the greater the initial normal stress,the greater the interface shear strength and shear strength required to achieve the shear displacement.?3?The monotonically straight shear test results of sand-steel interface also show that when the shear interface is smooth steel plate,the specimen shows a slight shear and keeps it relatively stable.When the shear interface is a rough steel plate,the specimen shows shrinkage after shear,the end of the test,the rate of expansion has not yet reached a stable.When the initial normal stress is the same,the rate of expansion increases with the increase of the relative roughness of the interface.When the relative roughness of the interface is the same,the larger the initial normal pressure is,the smaller the dilatation rate is.?4?The results of cyclic shear test show that the interface shear stress and the normal stress are logarithmic type weakened with the increase of the number of cyclic shear when the relative roughness of the interface is the same,and the weaker amount is 26%⁴6%and 19%⁶2%,and the weakening coefficient of the shear stress ratio and the normal stress ratio is 0.07⁰.13 and 0.02⁰.18,respectively,and the weakening coefficient increases with the increase of the interface roughness.The normal displacement of the cyclic shear test is in the state of shrinkage,and the shear volume changes mainly from the first 5 cycles,accounting for 36%⁴7%of the total shear volume,and the total shear shrinkage increases with the increase of roughness.?5?After the end of the cyclic shear,the friction angle between the sand and the steel plate is in the range of 29°³5°,and the interface friction angle increases with the increase of the relative roughness.In the range of steel plate from 0¹cm,the median particle size range of the sand sample is 0.68⁰.88mm,which is about 4%²5%lower than that of the original specimen.The relative breaking rate is 1.86%¹0.25%,and the larger the relative roughness of the interface,the smaller the median particle size of the sand sample,and the larger the relative breaking rate.?6?The results of visual analysis show that the shear band thickness of monotone direct shear test is about 3⁴ times of the median particle size,and with the increase of interface relative roughness gradually increased.The shear band thickness of cyclic shear is about 4⁵ times of the median particle size,and its development mainly occurs in the first 5 cycles,and the thickness of the shear band tends to be stable after the 10th cycle.?7?Using the cluster in PFC^2D to randomly create different shapes of granular clusters,replacing the traditional disc particles,establishing of broken PFC^2D constant stiffness cycle shear model.The numerical simulation results show that the shear stress is weakened by the logarithmic function with the increase of the number of cycles,and the weakening coefficient is about 30%⁴6%.The weakening coefficient of shear stress ratio is 0.10⁰.16,and with the increase of interface roughness,the weakening coefficient increases gradually.In the numerical simulation,the break of clusters means the particles are sheared.The thickness of the shear band is about 1² times of the median particle size,and with the increase of the relative roughness,the thickness of the shear band increases.