Computer Vision-Based Technique to Measure Displacement in Selected Soil Tests

The paper investigates the accuracy of normal case photography as a means of measuring linear deformations of soil specimens. The results of using this approach were compared with results from conventional procedures for two soil tests, namely unconfined compression and direct shear tests. Charge-coupled-device (CCD) video cameras were used to measure deformation or strain in soil specimens. A personal computer based digital vision system was used to obtain accurately measured linear displacement data. Using remolded soil specimens, comparisons between displacement measurements using American Society for Testing and Materials (ASTM) conventional methods and normal case photography methods 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 the computer vision technique were consistently within 0.04 plus or minus 0.15 to 0.3 plus or minus 0.23 mm for unconfined compression tests and direst 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 can further the search for more fully automated soil testing measurements.