, to the culvert, and thus the vertical pressure on the top of the culvert will be greater than in the first supposed case; but, if the reverse obtains, or if the culvert settles more than the earth outside the lines,
and
, or if the arch or covering stone descends in the middle relatively to the abutments, then part of the weight of the earth vertically over the culvert is transferred to the sides. For a comparatively rigid arch, the settlement is perhaps not enough to warrant us in making the maximum unit pressure less than
. Exactly what settlement would warrant the use of the theory set forth in the Appendix it is impossible to say. If the unit pressure is taken as
, we can rest assured that in most cases the real pressure is materially less.
[Footnote 1:] The writer refers to his “Retaining Walls,” Van Nostrand’s Science Series, No. 3, for the demonstrations pertaining to the above constructions, and to the derivations of formulas.] [Return to text]