Method Of Whole Surface Deformation Measurement For Soil Specimen In Triaxial Tests And Its Application

As a subject with great concern to practice, development of soil mechanics is firmly relied on the improvement of soil testing technology. As one of the most important test species, triaxial test has been playing an essential role in the development of soil mechanics. The successful application for computer aided measurement technology in soil triaxial test overcame some defects and insufficiency of traditional deformation measurement technology, thus, provided a more accurate and effective deformation measurement method.In the first part of the dissertation, an automatic digital image processing method is presented for whole surface measurement of specimen deformation in triaxial tests. This method incorporates digital imaging techniques and computer-aided analysis to assess deformation throughout a triaxial test. A detailed description of hardware including development of multi-function triaxial apparatus for soil, design and manufacture of triaxial pressure cell as well as design of camera bracket and lighting resources is provided. Based on the analysis for imaging characteristics of cylinder sample in triaxial tests, data analysis methodology, including distortion correction, data separation, pixel equivalent normalization, data splicing, and strain field calculation is presented.Further more, possible sources for both system errors and random errors have been discussed. Calibrations have been made to get ride of system errors such as lens distortion. As for random errors, necessary improvements for experimental environment and data processing have been made. After all these measures taken, other error sources remained are not quantified separately, instead, accuracy of the entire system is examined. The accuracy of method is quantified through comparison of strain results obtained from both image processing technique and a static strain tester.The second part of this dissertation concentrates on the applications of this digital image processing method in measuring deformation of triaxial specimens. It includes:(1) Discussion on the application of local deformation data. Based on qualitative analysis of local deformation characteristics, strain non-uniformity coefficient is defined and calculated, thus, quantitative comparisons are then given out for local deformations along specimen height. The effectiveness for using of local deformation data (central35mm height for specimen of size80mm*39.1mm) in eliminating end restraint as well as the reasonability, veracity and superiority for application of local deformation data in calculation of strength and deformation parameters have been verified through the comparison of test results obtained from both traditional and end-lubricated triaxial tests, and also parameters of Duncan and Chang’s model.(2) Study for deformation of specimen and formation of shear bands in triaxial tests of cohesionless soil. The development of shear deformation and failure patterns have been studied based on comparative analysis of specimen images, surface axial strain field maps, deviator stress-axial strain curves and local deformation data for triaxial tests on silt sand and fly ash. It is found that surface axial strain fields and local deformation data are more sensitive in analyzing local non-uniform deformations as compared with specimen images and overall deformation data, and, more clearly and intuitively judgment for the formation and development of shear bands can be obtained using strain field maps. It is found that for dense sand samples, shear zones occur before peak intensity, and, after gradual development, fully form after peak intensity. As for loose sand, shear zones occur after peak intensity and tend to develop rapidly after formation. When initial defects existing, shear failures tend to occur in positions of initial defects and eventually go through the initial defects. And, when entirely failure happens, the tendency of specimenns’deformation and failure mode can also be obtained from the analysis of surface axial strain field maps and local deformation data.