Abstract: Laser speckle image of transparent soil, which is a novel method for studying the soil mechanics problems, can be used to measure the displacement field inside the soil mass through digital image cross correlation (DIC) method. Tests are conducted on fused quartz particles with four different grain sizes, i.e. fine quartz (0.1-0.3 mm), medium quartz (0.3-0.5 mm), coarse quartz (0.5-2.0 mm), and fine gravel quartz (2-5 mm). This paper reveals the influences of grain sizes of fused quartz on the quality of laser speckle image, the selection of appropriate query window size and the displacement measurement accuracy of transparent soil. The results show that the gray histograms and the speckle patterns are different for transparent soils of different grain sizes. With the same incident laser light, the quality of image for smaller sizes is better than that of the larger sizes. The histograms of obtained displacement error indicate that the displacement error is dependent on the query window size and grain size. By adjusting the query window size with the average gray level gradient, the transparent soil models of different grain sizes can provide the equivalent measurement accuracy. Digital and physical translation tests show that the transparent soil models of smaller sizes tend to give higher measurement accuracy.

Key words: transparent soil, fused quartz, grain sizes, digital image, laser speckle, measurement accuracy