An image-based deformation measurement system for the geotechnical centrifuge

Abstract

New techniques of image capture and analysis have been recently applied to geotechnical centrifuge modelling. These techniques improve the utility of the geotechnical centrifuge by increasing the detail and precision of deformation measurements. This paper describes the use of a simple image-based deformation measurement system on a drum centrifuge. Precise deformation fields comprising many thousands of measurement points can be evaluated from images of plane strain tests using small soil models typically comprising 2 litres of soil. Digital still photography is used to capture high resolution images. A weighted shutter allows a continuous sequence of images to be captured without the need for an onboard PC or communication with the camera via sliprings. Particle Image Velocimetry (PIV) and close range photogrammetry are used to process the resulting images. Target markers are not needed since PIV operates on the spatial variation in image brightness to measure displacement. Close range photogrammetry allows image distortion to be corrected. Distortion leads to a 5% variation in image scale (or pixel size) for this test geometry. An example test of a skirted strip foundation under eccentric vertical load is analysed. The instantaneous velocity field at failure is manipulated to allow the realism of simple upper bound failure mechanisms to be examined. It is interesting to note that although the measured failure load matches almost exactly the optimal upper bound solution, the observed deformation mechanism differs significantly.