Uniformity evaluation of cohesionless specimens using digital image analysis

Nonuniformity within specimens of granular materials is considered to be one of the major reasons for the dependence of the observed static/dynamic response during laboratory testing on the method of preparation. In this paper, a resin-impregnation/digital image analysis method to examine the uniformity of cohesionless specimens at microscopic, macroscopic, and global scales is described. The approach relies on the equivalence between volume fraction and the expected value of area fraction where the area fraction of voids on two-dimensional sections is obtained using a computer-based digital image analysis system. A sampling strategy which allows porosity variations in the vertical direction as well as in the radial direction of the specimen to be examined is proposed. The method is illustrated with the results of measurements performed on a typical specimen and shows that a much larger amount of nonuniformity exists when the soil is examined at a microscopic level. A statistical procedure to evaluate the accuracy of a volume-fraction measurement using areal analysis is also described. The proposed method offers much promise for examining the uniformity of specimens accurately and efficiently.