Surface Smoothness Evaluation of Etched and Unaltered Sand Specimens with Mechanical Behavior Assessment

Abstract

Particle smoothness refers to the surface texture of a sand particle on a small scale. This parameter may be used in conjunction with roundness (angularity) and sphericity in describing particle shape for engineering purposes. Smoothness is challenging to measure and quantify. This note describes preliminary evaluation approaches used to compare two identically graded sand specimens; one with an unaltered, smooth surface texture, and the other chemically etched to create a rougher surface texture. Visual observation indicated that the unaltered specimens were essentially the same as the etched specimens in terms of macroscale particle shape features (i.e., roundness and sphericity). On the other hand, the particles were significantly different in terms of the surface texture and roughness, as shown by visual techniques using scanning electron microscope and optical microscopy images. A tentative, easy-to-implement, visual particle smoothness scale is offered, designed to complement the roundness and sphericity estimates and measures commonly used to describe particle shape. Next, several simple laboratory index tests are completed on the unaltered and the etched specimens to observe differences in mechanical behavior. Limit density, flow rate, and angle of repose tests are undertaken. The results of the testing indicate measurable differences in the mechanical behavior of the unaltered versus the etched specimens. This further supports the importance of particle surface texture on the mechanical behavior of poorly graded sands. Additional research is underway to more completely quantify the contribution of particle smoothness of sands with respect to the other macroscale particle shape features.

Evaluating Sand Particle Surface Smoothness Using a New Computer-Based Approach to Improve the Characterization of Macroscale Parameters

“…The authors [2] previously explored techniques concerning both etched and unetched glass bead specimens. The surface smoothness of these particles was estimated using a method that began with a visual comparison, guided by Figure 1.…”

“…Tessari et al [2] put forth a particle shape parameter including R, S, and s: modified ρ, or ρ mod . This parameter is the average of R, S, and s, as shown in Equation ( 2):…”

“…This was carried out as a means of further investigating the possible impacts of s on the overall behavior of the sands, and to add to the body of knowledge on the subject. Note that Tessari et al [2] contributed to this exercise previously as well, using 30-40 glass beads of varying surface roughness. The results of that study confirmed and provided examples of how s (without a change in R or S) affects the physical behavior of particles.…”

“…The considered sands included glass beads, Ottawa sand, Manistee dune sand from Manistee, Michigan, Traverse City sand collected from Traverse City, Michigan, Gay stamp sand from the former Mohawk and Wolverine stamp mill in the village of Gay, Michigan, and a crushed limestone sand specimen. We should note the addition of several new sands in the context of these surface smoothness evaluations beyond what was carried out previously by the authors [2]. The grading of the soils by sieve was performed to limit the number of variables, as there may be an additional relationship between the size of the particle and its smoothness in the overall macroscale response.…”

“…Smoothness, s, was estimated for these specimens using two broad approaches: (1) a proposed quantitative and/or qualitative visual classification scale (either for evaluating a number of individual particles, or for rapidly evaluating a small specimen of representative particles collectively), and (2) a newly developed numerical routine, using open-source software, designed to quantify surface texture and intended to be useful in validating/checking the results of the visual evaluation approaches. Note that a simple approach for optical/visual evaluation of s (using a hand lens and/or naked eye) was previously described by the authors [2]. That study involved two of the same specimens (glass beads): one with its "default" smooth surface finish, and the other with its surface etched to create a noticeably rough finish.…”