By use of the digital image technology, the deformation of the nodes of the marked grid on the rubber membrane of soil specimen is observed in triaxial test. And the strain distribution on the whole surface of the specimen during the test can be obtained. Then stress distribution can be estimated according to the strain, the actually measured elasticity modulus and poisson's ratio of the specimen. Further, the distribution of the stress level on the whole surface of the specimen can be calculated according to the stress distribution. And the assessment whether the shear failure takes place at a point on the specimen's surface can be made. In this way, by means of triaxial compression tests, the occurrence and the development of shear failure of the soil specimen are investigated, including where the shear failure takes place and how the shear band extends. The beginning and extending of the shear band, and the shape of shear band such as its width, length and angle are observed for the silica micropowder through the tests considering the effect of the different density and confining pressure.
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