Abstract: Application of digital image processing technique to measuring specimen deformation in triaxial test can solve a series of problems about deformation measurement in traditional triaxial test. This paper introduces the development of triaxial image measuring system, which can be divided into three stages: (1) Digital image measurement system based on edge recognition, which determines the radial deformation of soil specimen by tracking the changes of the edge location and axial deformation of soil specimen by identifying the white mark line. (2) Digital image measurement system based on corner recognition, which measures and tracking each corner point (node) position, and obtains the displacement of soil surface node at any moment; we can obtain the displacement (deformation) field and strain field of the soil surface by using the finite element technique. (3) Whole surface digital image measurement system, based on the front surface corner recognition, together with two reflection mirrors situated inside the triaxial cell, a 360 degree coverage of specimen’s deformation could be achieved using only one video camera. Digital image measurement system for measuring specimen deformation in triaxial test has the unparalleled advantages comparing with traditional measurement methods, especially in the research of the constitutive model.

Key words: digital image processing technique, edge recognition, sub-pixel corner identification, whole surface strain field, triaxial test for soil