Computer vision-based technique to measure displacement in selected soil tests

The potential of normal case photography using charge-coupled-device (CCD) cameras to extract deformation (strain) in soil specimens of two soil tests, i.e., the unconfined compression test and the direct shear test, was investigated. A PC-based digital vision system was used to obtain accurately measured linear displacement. Using remolded soil specimens, comparisons between displacement measurements using ASTM conventional methods and the normal case photography method showed that use of the latter method is promising and could be used as a substitute for strain gages. Experimental investigation showed that differences between displacement measurements using conventional ASTM procedures and computer vision technique were consistently within 0.04 +/- 0.15 to 0.3 +/- 0.23 mm for unconfined compression tests and direct shear tests, respectively. This was compatible with the image scale where one pixel on the image domain was equivalent to about 0.4 mm on object space coordinates. Statistical correlations between strains by the two methods supported this result. Image scale and resolution were found to be the two major factors affecting the accuracy of the measurements. The results of this work are expected to open the door for geotechnical engineers and agencies responsible for soil testing standards to incorporate image-based analysis in soil testing. This will indeed bridge the gap between manual and fully automated soil testing measurements.