become wet
before the excavation is completed.

In caisson work, if the caisson can be kept absolutely plumb, it can be
sunk without having to overcome much friction, while, on the other hand,
if it is not kept plumb, the material is more or less disturbed and
begins to bind, causing considerable friction. The author claims that
the pressure does not increase with the depth, but all caisson men will
probably remember that the friction to be overcome per square foot of
surface increases with the depth.

In calculating retaining walls, many engineers add the weight of the
soil to the water, and calculate for from 90 to 100 lb. per cu. ft. The
speaker is satisfied that in the so-called New York quicksand it is
sufficient to use the weight of the water only. If the sand increased
the side pressure above the water pressure, engineers would expect to
use more compressed air to hold it back, while, as a matter of fact, the
air pressure used seldom varies much from that called for by the
hydrostatic head.

Although allowance for water pressure is sufficient for designing
retaining walls in New York quicksand, it is far from sufficient in
certain silty materials. For instance, in Maryland, a coffer-dam,
excavated to a depth of 30 ft. in silt and water, had the bottom shoved
in 2 ft., in spite of the fact that the waling pieces were 5 ft. apart
vertically at the top and 3 ft. at the bottom, and were braced with 12
by 12-in. timbers, every 7 ft. horizontally. The walings split, and the
cross-braces cut into the waling pieces from 1 to 2 in.; in other words,
the pressure seemed to be almost irresistible. This is quite a contrast
to certain excavations in Brooklyn, which, without any bracing whatever,
were safely carried down 15 ft.

Any engineer who tries to guess at the angle of repose, and, from the
resulting calculations, economizes on his bottom struts, will find that
sooner or later an accident on one job will cause enough loss of life
and money to pay for conservative timbers for the rest of his life. So
much for side pressures. As to the pressure in the roof of a tunnel,
probably every engineer will agree that almost any material except
unfrozen water will tend to arch more or less, but how much it is
impossible to say. It is doubtful whether any experienced engineer would
ever try to carry all the weight over the roof, except in the case of
back-fill, and even then he would have to make his own assumption (which
sounds