Study On Shear Failure Process Of Soil Specimen In Triaxial Test Based On The Whole Surface Digital Measurement Technology

The formation of shear band and the progressive failure theory of soils are the most concerned subjects in geotechnical engineering.The shear zone of soil specimen is a typical strain localization phenomenon,the occurrence of strain localization zone(shear band)means that the failure occurs in geotechnical structures and the structures begin to lose stability,which also indicates the imminent occurrence of instability and destruction of geotechnical structures.Therefore,the study on the shear band of soil has attracted extensive attention from scholars both at home and abroad.At present,the research on shear band problems includes theoretical analysis,numerical simulation and experimental research.While the results based on theoretical analysis and numerical simulation are mostly deviated from experimental observations.For the whole process of deformation of the shear zone,some qualitative analysis methods are adopted,but the quantitative analysis criterion of failure process is still not given.In addition,the existing constitutive models of soils are constructed by taking the specimen in a triaxial test as an elementary body,but the strain localization phenomenon indicates that the deformation of a soil mass is not uniform,especially after the shear band is generated in the soil structure.Therefore,when analyzing soil specimanes with localized deformation characteristics,these constitutive models are no longer applicable.This thesis focuses on these two issues,and a whole-surface digital image measurement system is used to measure and analyze the whole deformation and local deformation of the specimen in real time,which provides an effective method to study shear band.Firstly,in this thesis,standard sand,silica micro-powder,Guiping clay and kaolin clay are carried out in the triaxial compression test,the stress-strain curves of the entire specimen and the arbitrary point are obtained by the whole-surface deformation digital image measurement system,and the deformation of the specimen is observed clearly by the displacement field and the strain field.It can be found that:at the beginning of the test,the deformation of the specimen is approximately uniform and there is no obvious local deformation.As the test progresses,the specimen exhibits a non-uniform deformation tendency.At the time of residual stress,the failure zone is continuously extended and finally form a shear band.There are two types of specimen deformation:one is the soil in the shear band,for which the deformation continues to increase,the stress remains the same,and the state of deformation is similar to the sliding friction model of sliding block;the other is the soil outside the shear band,in the later period of the test,the deformation and the force remain unchanged,which basically shows a similar balance of the state.At this time,the deformation of the specimen mainly concentrates inside the shear band,and deformation is exclusively due to the blocks of the specimen sliding along the shear band.When the shear bands of triaxial specimens begin to form,when the shear bands are completely formed and what is the law of their development and change,these problems have confused experts and scholars of soil mechanics.Based on the stress and strain at any point on the specimen obtained by the whole-surface digital image measurement technique,two methods for determining the beginning and the formation of the shear band are proposed.In the first method,the stress level,S,is calculated to determine whether the soil is in failure at a point.The stress level of each point on the specimen is calculated based on the strain,Young's modulus and Poisson'^ ratio.Define the stress levels S=qp/qf,it's indicated that shear failure occurred on the specimen when 5=1,the area enveloped by the curve defined by 5=1 on the stress-level contour map is the shear band of the specimen.The second method is called as the combined method of stress-strain for determining the shear band.The latest moment corresponding to turning point of axial strain among 192 points on the surface of soil specimens in each test is defined as the moment when the shear band is completely formed;according to the stress,the earliest moment corresponding to peak stress of one of all the marked points is defined as the moment for the onset of the shear band.The phenomenon of strain localization shows that the deformation of the soil is not uniform,and the deformation of the specimen at different stages of the test are not the same.By analyzing the local deformation characteristics of soil specimens and the deformation mechanism at different stages,the deformation process and stress-strain curve of soil specimens are divided into pre-failure stage,in-failure stage and post-failure stage.In the pre-failure stage,the deformation of the soil specimens is generally uniform,and basically satisfies the same stress-strain curve.In this state,the entire specimen can be treated as a continuum element.In the in-failure stage,the soil specimens begin to break and develop gradually from a certain point(REV)until the shear band cleaves the specimen.The observed deformation of the soil is the result of the combination of deformations in the shear band and out the shear band.In the post-failure state,the deformation is fully due to the blocks of the specimen sliding along the shear band,the sliding of the upper and bottom oblique bodies along the shear band,so it can not calculate the "strain" of the soil specimens based on this situation.From the initial uniform deformation of the specimen to that the shear zone completely cleave the specimen until failure,the development of the whole deformation reflects the damage process of the specimen structure.For analyzing the structure damage variation law of soil specimens during the shear process,the damage variable W that describes the degree of damage development is proposed,and the evolution equation of shear damage is established.The variance of the axial strain at each marked point on the rabber membrane is used to define the damage variables of the specimen before the shear band completely cleaves the specimen,and the damage state of the specimen is obtained by analyzing the law of damage variables.In the process of triaxial test,the confining pressure and the type of the soil and so on will have an influence on the damage evolution process.In the shear failure analysis,when the shear band of the specimen is completely formed,the soil is considered to be completely damaged,and at this moment W=1.The value of W satisfies a gradual increase from 0 to 1 during the beginning of the test until the shear band is fully formed.