the earth excavated from the trench being too
near the edge of the cut.

For some years the writer has been making extended observations on deep
trenches, and, thus far, has failed to find evidence, except in aqueous
material, of earth pressures which might be expected from the known
natural slope of the material after exposure to the elements; and this
latter feature may explain why sheeted trenches stand so much better
than expected. If air had free access to the material, cohesion would be
destroyed, and theoretical pressures would be more easily developed.
With closely-sheeted trenches, weathering is practically excluded, and
the bracing, which seemingly is far too light, holds up the trench with
scarcely a mark of pressure. As an instance, in 1893, the writer was
successfully digging sewer trenches from 10 to 14 ft. deep, through
gravel, in the central part of Connecticut, without bracing; because of
demands of the work in another part of the city, a length of several
hundred feet of trench was left open for three days, resulting in the
caving-in of the sides. The elements had destroyed the cohesion, and the
sides of the trenches no longer stood vertically.

Recently, in the vicinity of Boston, trenches, 32 ft. wide, and from 25
to 35 ft. deep, with heavy buildings on one side, have been braced with
8 by 10-in. stringers, and bracers at 10-ft. centers longitudinally, and
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