A simple apparatus for measuring the angles of repose of masses of granular material in various media, based on the rotating-drum principle, is described. Two angles of repose are measured by this method. The critical angle of repose (ac), is defined as the angle through which a mass of granular material can be rotated before it fails by avalanching, and the angle of rest (aR), is defined as the inclination of the slope after avalanching has ceased.
Measurements of the angles of repose of several materials in air and under water with the “rotatable-drum apparatus” indicate that for materials with similar surface characteristics the angles of repose increase with departure of the grains from a spherical form. For masses of spherical particles the angles of repose increase with increasing intergranular friction caused by changes in the surface characteristics of the spheres. For mixtures of spheres and cubes the angles of repose are proportional to the volumes of the end members present. For natural quartz sands the critical angle of repose (ac), is always greater in air than under water, but the angle of rest (aR), is the same in both media. Measurement of the inclinations of slip faces of subaerial dunes and subaqueous deltas indicates that these faces stand at, or less than, the angle of rest (aR), of the sand from which they are built.
It is concluded that shape and surface characteristics of the particles have the greatest effect on the angles of repose.