Three-point imaging test for AASHTO soil classification

Abstract

A rapid, digital image-based test has been developed for the classification of highway subgrade materials according to the AASHTO system. The test involves segregating particles in a sedimentation column, then determining the elevations at which 2.0-, 0.425-, and 0.075-mm grains (No. 10, No. 40, and No. 200 sieves, respectively) are found. If it is assumed that porosity variations throughout the column are insignificant, as established in previous studies, then the distances between these elevations translate to the percentages that pass through the various sieves. The method uses a previously developed calibration between an image wavelet decomposition index and the pixels per particle diameter. At a camera magnification of 32.5 pixels/mm, the entire grain size range from 2.0 to 0.075 mm is resolved. Thus, only a single fixed magnification lens is needed for the system. The accuracy of the new test is demonstrated by comparing the digital image-based results and conventional sieving for three soil specimens: well-graded sand, uniformly fine sand, and sandy silt. The advantages of the three-point imaging test for AASHTO soil classification over conventional sieving and hydrometer tests include reduced testing time, low noise, possibly lower equipment costs, and the elimination of airborne particles in the lab.