Surface smoothness evaluation of etched and unaltered sand specimens with mechanical behavior assessment

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.