In connection with the experiments of Leygue, the writer wishes to refer to the dry sand and wheat arching experiments described in the final discussion of his paper, “The Bracing of Trenches and Tunnels, with Practical Formulas for Earth Pressures,”[Footnote 23] ] and especially to the experiment made with dry sand in a 2-in. pipe, described in his paper on “Pressure, Resistance, and Stability of Earth.”[Footnote 24] ] For clearer reference, the latter experiment is again described:
A 2-in. pipe, 18 in. long, was filled with dry sand for a depth of 12 in., and a thin piece of tissue paper was pasted across the bottom. Then, with a wooden piston bearing on the sand, the latter would support the blow of a sledge hammer or the weight of a man without breaking the tissue paper.
All these experiments seem to prove very clearly that pressure is distributed laterally, at least through perfectly dry material, and, with certain values known, it should not be difficult to compute the value of this thrust. For example, if, in the pipe experiment, the exact coefficient of friction of the granular particles against the inside of the pipe is determined to be 0.4, then it would take a pressure equal to the complement of this, or 0.6 of the weight, to hold the granular particles against the vertical side. Now, if the exact depth of sand is determined at which no further pressure (by reason of adding to this depth) is transmitted to the bottom, then, by finding the difference or loss of pressure at the bottom, it should be a simple matter to compute the amount of thrust developing sufficient friction over the given area to sustain the additional weight. This is what the writer, for want of a better term, has called “cohesive friction” induced by lateral pressure. While such experiments on a small scale give definite results with very dry sand or wheat, they will not be conclusive unless made on a very large scale in cases where dampness adds materially to the normal cohesion of small masses.
As to the experiments on revolving boards or walls referred to by the author, it would appear that, as noted, they eliminate consideration of the effect of pressure as increasing the values of the coefficient of friction and cohesion.
The writer believes that it can be shown by [Fig. 22] that the lateral pressure against sheeted faces or retaining walls is greater toward the top. [Fig. 22] is adapted from Fig. 5 in the writer’s paper, “Pressure, Resistance, and Stability of Earth.”
Fig. 22.
It is supposed that a retaining wall has first been built along the face,
, and that it has been back-filled with ordinary dry sand to the curved line