Research On Influence Of End Contact And End Restriction In Soil Tri-axial Tests

Tri-axial test is one of the most common soil tests, which brings out the soil constitute model parameters through the measurement of axial strain, radial strain and volume strain. However, tri-axial test is inevitably influenced by end contact and end restriction, hence it is of great significance to determine the influence of the end problems for the improvement of tri-axial test equipment and understanding the soil constitute law.In this paper, the stainless steel cylinder with higher strength is used in the compression test on the tri-axial test equipment. The test loading system deformation is standardized and loading system mechanics property is analyzed quantitatively, based on which the whole measurement method is adjusted, and the results is consistent with that of the digital image measurement. The result shows that the loading system deformation lasts throughout the loading process, and it is obvious in the earlier stage, and it also illustrates the reason why the whole measurement method and digital image local deformation measurement method are different, and further states the rationality of digital image measurement.The tri-axial test is numerically simulated to analyze specimen interior stress field variation and the effect on specimen deformation during the loading process. The Duncan-Chang model is developed along with the application condition based on the platform of ABAQUS. It is applicable. In the earlier stage for the simulation test, and can accurately display the influence of the minimal principle stress on specimen local strength. But it cannot show the plastic deformation and stress softening, and it degenerates to approximate elastic model in the late stage, so the result is not reasonable. In order to illustrate the soil mechanics property during different loading stage, the Mohr-Coulomb elastoplastic model is employed in late stage. The numerical results show that, due to end constriction, "hard core" appears at the two ends of the specimen, which have greater strength and stiffness than the middle part. Under the compression of the hard core, the middle part will first yield, and then develops into plastic band zone. The axial deformation increases greatly, and the specimen looks like a drum in the appearance. The state of the plastic zone is related to the end constriction and soil material parameter. The deformation obtained at the late stage is in agreement with that of reality, and can rationally expound the drum pattern deformation of tri-axial test soil specimen.