In all parts of the work problems were encountered requiring for their
solution large expenditures and much engineering skill; but many of
these difficulties had been frequently met in previous engineering
experience, and the methods of overcoming them were well understood.
Thus, in the Meadows Division, a long and heavy embankment, part of
which was on submerged meadow land, and many bridge foundations had to
be constructed; in the Bergen Hill tunnels, very tough trap rock was
encountered; in the tunnels under the city, the work was much
complicated and its cost increased greatly by the necessity of caring
for sewers, water and gas pipes, and the foundations of adjacent
buildings; and many troublesome problems were met in the construction of
the tunnels connecting the East River tunnels with the Sunnyside Yard.

The novel features of the project, however, were the great tunnels
extending the line under the North and East Rivers. Tunnels of the kind
contemplated, to be used for heavy and rapid railroad traffic, had never
been constructed through materials similar to those forming the beds of
the North and East Rivers. Questions arising in connection with the
design and method of construction of the tunnels will be considered
later. Here they are referred to only in their relation to the location
and grades of the line, in which connection the conditions controlling
their establishment were the most important elements.

_Location and Grades._--It was desirable to make the tunnels between
the bulkhead lines of the rivers as straight as possible, and it was
necessary to place them at sufficient depth below the dredging plane of
the War Department (which in the North and East Rivers is 40 and 26 ft.
below mean low water, respectively) to insure them against possible
injury from heavy anchors or sunken vessels. Furthermore, they had to