Note Return to text — EXPERIMENTS ON RETAINING WALLS AND PRESSURES ON TUNNELS.

As preliminary to the discussion, however, it is well to give the essentials of Leygue’s experimental proof that cohesion and friction exist at the same time. A box without a bottom, about 4 in. square in cross-section and 4 in. high, was made into a little carriage by the addition of four wheels. The latter ran on the sides of a trough filled with sand which the bottom of the box nearly touched. The box was partly filled with sand, and the trough and box were then inclined at the angle at which motion of the box just began, the sand in the box resting on the sand in the trough, developing friction or cohesion or both, just before motion began. Only friction was exerted after motion began. The solution involves the theory of the inclined plane, but, to explain the principles of the method, it will suffice to suppose the trough and the sand in it to be horizontal, and that the bottomless box filled with sand is just on the point of moving, due to a horizontal force applied to it. The weight of the box and a part of the weight of the sand in it held up by the friction of the sides, is directly supported by the wheels resting on the sides of the trough; so that only a fraction of the weight, , of the sand in the box is supported directly by the sand in the trough. Call this amount . Then, for equilibrium, calling the horizontal force, less the resistance of the carriage wheels, we have,[Footnote 5]