| The shearing surface of direct shear test on soil is limited to the plane between upper and lower boxes,which is not along the weakest surface of sample.The forming mechanism of shear band in direct shear test is still not clear.In this paper,the formation mechanism and expansion process of shear band of different soil materials in direct shear test are studied by small visual direct shear test of sand / clay and large visual direct shear test of gravel soil.The effects of sample size,material type and normal stress on the expansion of shear band are summarized.At the same time,the numerical simulation of the indoor direct shear test is carried out by using the finite difference program FLAC3 D and the discrete element program PFC3 D,and the shear band expansion law obtained from the physical test is supplemented.The main research contents are as follows:First of all,the original strain-controlled direct shear instrument in the laboratory is visually modified,and five kinds of transparent shear boxes and new compression frames with different aspect ratio are designed.Combined with the meso-image recording device,the shear tests of sand are carried out.The strength parameters and the shapes of shear band in each tests are compared and summarized.The influence of shear box size on test results and the reasonable sample size are obtained.According to the diagrams of displacement fields at different shear times under same normal stress,the developing processes of shear band are divided into five stages: initial compaction stage,local yield stage,shear band development stage,peak stress stage and softening stage.The development law of shear band when shear stress reaches peak point under different normal stresses is analyzed.The curve fitting and strength analyses of the arch secondary shear band are carried out to study the normal stresses,shear stresses and shear strengths on the main and secondary shear bands.Secondly,the visual shear tests of clay samples are carried out by the shear equipment of sand tests.The development law of shear band of clay specimen at different shear times in same normal direction and that when shear stress reaches peak point under different normal stresses are compared and summarized.It is found that shear failure of clay specimen is approximately along the horizontal shear band in shear processes and the sample sizes and normal stresses have little influence on the shear band.At the same time,the developing law of shear band is further verified by pasting white strips on the surface of clay samples.Then,the gravel soil samples are tested by large-scale visual shear instrument available in the laboratory.It is found that the process from the beginning to the formation of a stable arch shear band is very short,and the shear displacement is only 10% of the total shear displacement.After that,the shear band remains stable in shear tests,and the normal stresses have little effect on the shear band.Finally,the numerical simulations of shear tests are carried out by finite difference program FLAC3 D and discrete element program PFC3 D to further study the developing laws of shear bands base on various soils.At the same time,the applicabilities of two softwares for the forming and developing mechanism of shear band based on different soil materials are compared. |
PDF Full Text Request