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 more polite than "guess").

The author has stated, however, that when the tunnel roof and sides are
in place, no further trouble need be feared. On the contrary, in 1885,
the Canadian Pacific Railroad built a tunnel through clayey material and
lined it with ordinary 12 by 12-in. timber framing, about 2 or 3 ft.
apart. After the tunnel was completed, it collapsed. It was re-excavated
and lined with 12 by 12-in. timbers side by side, and it collapsed
again; then the tunnel was abandoned, and, for some 20 years, the track,
carried around on a 23° curve, was used until a new tunnel was built
farther in. This trouble could have been caused either by the sliding or
swelling of the material, and the speaker is inclined to believe that it
was caused by swelling, for it is known, of course, that most material
has been deposited by Nature under great pressure, and, by excavating in
certain materials, the air and moisture would cause those materials to
swell and become an irresistible force.

To carry the load, Mr. Meem prefers to rely on the points of the piles
rather than the side friction. In such cases the pile would act as a
post, and would probably fail when ordinarily loaded, unless firmly
supported at the sides. The speaker has seen piles driven from 80 to 90
ft. in 10 min., which offered almost no resistance, and yet, a few days