The extension of the shell back from the diaphragm is for the purpose
of affording opportunity to put in place the finished tunnel lining
whatever it may be, masonry, cast-iron, cast-iron and masonry, or steel
plates and masonry. Where the material is saturated with water as is
the case in all subaqueous tunneling it is necessary to use compressed
air in connection with the shield. The intensity of air pressure is
determined by the depth of the tunnel below the surface of the water
above it. The tunnelers work in what are called caissons to which they
have access through an air lock. In many cases quick transition from
the compressed air in the caisson to the open air at the surface results
fatally to the workers. The caisson disease is popularly called "the
bends" a kind of paralysis which is more or less baffling to medical
science. Some men are able to bear a greater pressure than others. It
depends on the natural stamina of the worker and his state of health.
The further down the greater the pressure. The normal atmospheric
pressure at the surface is about fourteen pounds to the square inch.
Men in normal health should be able to stand a pressure of seventy-six
pounds to the square inch and this would call for a depth of 178 feet
under water surface, which far exceeds any depth worked under compressed
air. For a long time one hundred feet were regarded as a maximum depth
and at that depth men were not permitted to work more than an hour in
one shift. The ordinary subaqueous tunnel pressure is about forty-five
pounds and this corresponds to a head of 104 feet. In working in the
Hudson Tunnels the pressure was scarcely ever above thirty-three pounds,
yet many suffered from the "bends."

What is called a freezing method is now proposed to overcome the water
in soft earth tunneling. Its chief feature is the excavating first of
a small central tunnel to be used as a refrigerating chamber or ice
box in freezing the surrounding material solid so that it can be dug