Particle Roundness and Sphericity from Images of Assemblies by Chart Estimates and Computer Methods

Soil particle roundness (R) and sphericity (S) are two important intrinsic properties that govern a soil’s mechanical behavior. Although R and S have well-established mathematical definitions dating back to the 1930s, the values are much more typically estimated using charts developed in the 1940s and 1950s. The charts, are based on the earlier mathematical definitions. Using these charts, a class of undergraduate civil engineering students at the University of Michigan were asked to estimate (Rc and Sc) from images of twenty geologically and geographically diverse sands spanning a range of actual R and S values. The images were of three-dimensional (3D) assemblies of the sands as they would be found in images taken remotely or in situ. The students’ estimates were statistically analyzed and compared with rigorously determined R and S using a recently developed computational geometry algorithm. Overall, the students’ estimates were scattered, particularly for natural sands exhibiting intermediate values of R and low values of S. On average, the students underestimated Rc and Sc. Reasons for the diverse responses and underestimates of the actual R and S are proposed.