3.8 Measurement of Soil Displacements Using Image Processing

By means of a micro camera, the soil movement images during the tests can be obtained. After tests, the soil displacements can be measured from the video pictures by means of image processing software. Both the total displacement vectors and incremental displacement vectors can be obtained by measuring the positions at different time stages related to the reference point, as shown in Figure 3.13. Although video based image acquisition and processing system is an effective approach to capture the deformation field of soil models, the distortion caused by the wide angle lens of the micro camera can be significant. To eliminate this distortion error, correction was necessary on the distortion pattern. However, the resolution of the image obtained from the video is still not high enough. For this reason, the accuracy of the displacement of soil points measured with this method is 0.5 mm to 1.0 mm. With better lighting and camera, the quality of the image can be improved.

For the measurement of the displacement field of the soil, cross lines are fixed on the perspex as reference points. To reduce the surface friction between the wall and the soil mass for the plane strain model, the points had to be marked on the outside wall surface. The perspex wall are 80 mm thick to take the high pressure during the high-g tests. In such a situation, parallelex error would arise when the light was refracted through the thick perspex wall. To avoid such an error, the view axial for measuring the displacement of soil need to be at right angle with the surface of the perspex wall. Based on the principle of right angle of view axial, a simple approach called point tracing method to avoid parallax error used before and after test was developed and used in this study as shown in Figure 3.14.