these circumstances, it was the desire of the Management that the Board
should receive and consider proposed methods of construction from all
available sources; and during the first year of its labors much of its
time was devoted to the examination and discussion of projects submitted
for its consideration by engineers and practical builders, some of these
projects having decided merit. Most of the methods proposed involved
temporary structures, or the use of floating plant, in the navigable
channels of the river. This was objectionable in view of the resulting
obstruction to the enormous river traffic. After full consideration of
the subject, it was decided to adopt the shield method with compressed
air for the construction of the tunnels under both rivers, this being
the only method recommended by the Chief Engineers, and having the great
advantage of conducting all operations below the bottom of the river,
thus avoiding obstruction of the channel.

Experience has shown, as was anticipated, that it is much more difficult
to construct tunnels in such material as occurs in 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