Deformation field measurement for granular soil model using image analysis

Recently digital camera with high resolution provides a simple, economical and practical solution to the deformation measurement for granular soil model, without resource to the embedded target markers or mesh grid drawn on the model. The image matching method was employed to track the displacement of the sand particles on models and the concept of isoparametric element used widely in FEM was introduced to validate the image and calculate the strain of soil. Subpixels precision of displacement measurement was achieved by means of bilinear interpolation, and not only the translating search but also the rotating search strategy were proposed to recognize the deformation accurately induced by rotation of local zone in soil model. Precision validation test shows that the standard deviation of measuring translation displacement was up to 0.19 pixels and numerical test also shows that standard deviation of rotating search was up to 0.08 pixels for measuring displacement of 15 degree rotation. The performance of this technique is illustrated by an example application in a centrifuge test of bearing capacity of sand foundation. The result consists of the deformation filed distribution of displacement, maximum shear strain and volumetric strain.This method can be used to analyze the local deformation, shear band and progressive failure quantitatively for granular soil.