Such results should be of great service to future experimenters as proving two things: (1) that dry sand, with as small a coefficient of cohesion as possible, should be used (perhaps grain would be a more suitable material), and (2) that no experimental wall should be less than from 6 to 10 ft. high.
Even if the wall is, say, 6 ft. high, if damp clayey earth is used as the filling, with a coefficient of adhesion,
, then all the diagrams of forces, as in [Fig. 11], will be the same as before, or similar figures, and the discrepancies noted in [Table 3], will be as pronounced as ever. All the experiments on retaining boards, except some of Curie’s, have been with very small models, and the results have brought the common theory under suspicion, if not into disrepute.
The writer hopes that the foregoing investigation and results may be instrumental in establishing more confidence in the theory, and in showing when cohesive forces may be practically neglected and when they must be included.
As an illustration, the results for a vertical wall 10 ft. high are presented in [Table 4], taking
and
. In the first wall, the surface of the earth was horizontal; in the second wall its slope was 1 rise to 2 base.