Particle Shape Estimates of Uniform Sands: Visual and Automated Methods Comparison

Abstract

The importance of particle shape, when considering the engineering behavior of sand, has been well-documented. Angular sands tend to have greater maximum and minimum void ratios, a larger friction angle, and a greater compressibility potential than their rounded counterparts under similar conditions. These differences in particle shape affect the behavior of the sands during in situ testing and in estimating soil behavior in a number of other capacities. This paper compares the results of roundness (R) and sphericity (S) estimates of ten individual particles and then later collectively on six sand specimens by several observers. An existing visual comparison method, manual methods, and automated methods are used to classify particle shape. Coefficient of variation (CV) values between 9 and 45% were obtained between the observers for visual estimates of single particles and of collective observations of sand specimens. These results are within CV values obtained when estimating other geotechnical parameters for soils and when considering in situ test variability. The visual estimates were comparable to the automated imaging system (AIMS) method for angularity measurements for the natural fine to medium sands but not for the very angular and crushed sands. Visual sphericity estimates for all sands also appeared to differ from the AIMS 2D form measurements. Research regarding automated quantification methods should continue because there would be a benefit in quickly and accurately measuring the particle shape of sand as in quality control applications for aggregates; however, there currently appears to be some limitations with the automated methods especially for fine to medium sands, sands with quartz or other translucent minerals, or very angular sands. In the meantime, qualitative particle shape estimates by using a simple visual procedure are able to provide usable first-order estimates of roundness (angularity) and sphericity sufficient for many practical geotechnical engineering applications.