the East River and on
the New Jersey side of the North River, than in more homogeneous
material such as is found in the greater part of the North River. During
the progress of construction under the East River, there were frequent
blow-outs through fissures opened in the river-bed, and the bottom of
the river over the tunnel had to be blanketed continuously with clay, to
check the flow of the escaping air.

In view of the serious difficulties which it was thought might be
encountered in the application of the shield method to the East River
work, other methods for the execution of this part of the project
received special consideration, one of the methods considered being the
freezing process. It was proposed to drive a small pilot tunnel and
freeze the ground for a sufficient distance around it by circulating
brine through a system of pipes established in the tunnel. The pilot
tunnel was then to be removed and the full-sized tunnel was to be
excavated in the frozen material and its lining placed in position. By
this means, it was intended to avoid the danger incident to the use of
compressed air in material of greatly varying character. This method
contained too many elements of uncertainty to justify its adoption; but
as the Management considered it desirable to have, if possible, an
alternative method, an extended experiment was made with the freezing
process. A pilot tunnel, 7 ft. 6 in. in diameter, was driven in the bed
of the East River for a distance of 160 ft., circulating pipes were
established in it, and brine at a very low temperature was passed
through the pipes until the ground was frozen for a distance of about
11.5 ft. around the tunnel. Observations to determine the rate of
cooling and other important points connected with the process were
carefully made. When it was found that the construction of the tunnels
was progressing satisfactorily by the shield method, and that so much
time was required to freeze the material that the freezing process could
not be used to advantage in this particular case, the experiment was
discontinued.

_Design of the Sub-River Tunnels._--The sub-river tunnels consist of a
circular cast-iron shell, of the segmental, bolted type, having an
outside diameter of 23 ft., lined with concrete having a normal
thickness of 2 ft. from the outside of the shell. Through each plate of
the shell there is a small hole, closed with a screw plug, through which
grout may be forced into the