Abstract: A three-dimensional digital image measurement technique by using two CCD cameras placed at the front and back of the triaxial testing machine， respectively， was proposed. A plastic membrane with white and black grids was used to replace the traditional plastic membrane. The images of the specimen obtained during the triaxial testing were able to capture the deformation of the whole specimen surface. By extracting and tracking the corner points of white and black grids， and detecting the edge of the specimen， the front and back images were stitched together and three-dimensional deformation measurement of the soil specimen was realized based on the assumption that the horizontal cross section of the soil specimen will keep horizontal circular plane before and after deformation. By combining with the numerical analysis with ANSYS finite element software， 3D surface strain contour can be obtained. This technique was applied to triaxial testing of diatomite and it achieved very good measurement effects.

Key words: triaxial test, digital image, three dimensional measurement, corner point extraction, edge detection, two CCD camera