from 3 to 5 ft. apart vertically; this timbering apparently was too
slight for pressures which, theoretically, might be expected from the
natural slope of the material. Just what pressures develop on the sides
of the structures in these deep trenches after pulling the top sheeting
(the bottom sheeting being left in place) is, of course, a matter of
conjecture. There can be no doubt that there is an arching of the
material, as suggested by the author. How much this may be assisted by
the practical non-disturbance of the virgin material is, of course,
indeterminate. That substructures and retaining walls designed according
to the Rankine or similar theories have an additional factor of safety
from too generous an assumption in regard to earth pressure is
practically admitted everywhere. It is almost an engineering axiom that
retaining walls generally fail because of insufficient foundation only.

For the foregoing reasons, and particularly from observations on the
effect of earth pressures on wooden timbers used as bracing, the writer
believes that, ordinarily, the theoretical earth pressures computed by
Rankine and Coulomb are not realized by one-half, and sometimes not even
by one-third or one-quarter in trenches well under-drained, rapidly
excavated, and thoroughly braced.


J.C. MEEM, M. AM. SOC. C. E. (by letter).--The writer has been
much interested in this discussion, and believes that it will be of
general value to the profession. It is unfortunate, however, that
several of the points raised have been due to a careless reading of, or
failure to understand, the paper.

Taking up the discussion in detail, the writer will first answer the
criticisms of Mr. Goodrich. He says: