The pressure exerted by granular material: an application of the principles of dilatancy

Abstract

The author had spent about three years endeavouring unsuccessfully to obtain experimental confirmation of the recognised theories of earth pressure and was attempting to measure the “ coefficient of friction ” of sand (on which all the theories are based) when a research student engaged on this work, Mr. C. P. R. de Villiers, called his attention to a remarkable phenomenon. This was recognised as an example of Dilatancy as described by Osborne Reynolds, and quickly led to the recognition of dilatancy as the fundamental property of granular material on which its behaviour and the forces it exerts ultimately depend. Two well known methods were being used to measure the coefficient of friction; the apparatus is shown in figs. 1 and 2. The outer vessels contained sand and the couples to rotate the disc (fig. 1) and the cylinder (fig. 2) were measured. With neither apparatus could repeat results be obtained. The impossibility of getting repeat results had been the fundamental trouble in all our work. But while making these tests Mr. de Villiers observed that the surface of the sand heaved up at the instant when rotation of the disc or cylinder began. This was particularly surprising in the disc apparatus (fig. 1) in which the moving disc was about 6 inches below the surface of the sand. To check whether the sand was really expanding, as it appeared to be, a simple experiment was made with the cylindrical apparatus (fig. 2). The cylinder was completely filled with sand while it was gently tapped ; no ramming was applied and the hole in the top through which it was filled was left open. When an attempt was made to rotate the inner cylinder it was found to be locked, and when the cylinder was finally forced round the couple required was 270 times the ordinary friction couple. When the sand was emptied out a lot of fine dust was found with it showing that rotation had not occurred until the sand was crushed. These experiments showed that the closeness of packing of the grains of sand was an essential factor in determining its behaviour and furnished the key to the irregular results of all our previous experiments. Small changes in packing may produce large effects. To illustrate the magnitude of the effect of closeness of packing a simple experiment on foundation pressures may be quoted. A small flat plate was laid on the flat surface of sand in a large box, and the force was measured to cause the plate to sink into the sand. When the sand was loosely poured into the box and trickled off level the force was only about one-tenth of that required when the sand was shaken and rammed to get it into the close-packed con­dition. (The densities of the sand in the two conditions were about 91 and 110 lb. per cubic foot; this small difference offers no explanation of the large difference in bearing pressure.)