then filled with grout. To avoid the possibility of pushing the iron
backward after the air pressure was on, rings of segmental plates, 5/8
in. thick and 13-7/8 in. wide, were inserted in eighteen circumferential
joints in each tunnel between the rings as they were erected. The plates
contained slotted holes to match those in the segments. After the rings
left the shield, the plates were driven outward, and projected about 5
in. When the tunnel was grouted, the plates were embedded.

The bulkheads were completed, and the tunnels were put under air
pressure on the following dates:

Line _D_, on October 5th, 1905;
Line _C_, on November 6th, 1905;
Line _B_, on November 25th, 1905;
Line _A_, on December 1st, 1905.

This marked the end of the preparatory period.

In the deepest part of the river, near the pier-head line on the
Manhattan side, there was only 8 ft. of natural cover over the tops of
the tunnels. This cover consisted of the fine sand previously described,
and it was certain that the air would escape freely from the tunnels
through it. To give a greater depth of cover and to check the loss of
air, the contractor prepared to cover the lines of the tunnels with
blankets of clay, which, however, had been provided for in the
specifications. Permits, as described later, were obtained at different
times from the Secretary of War, for dumping clay in varying thicknesses
over the line of work. The dumping for the blanket allowed under the
first permit was completed in February, 1906. The thickness of this
blanket varied considerably, but averaged 10 or 12 ft. on the Manhattan