other hand, where progress was rapid, caused practically no settlement
in the racks.


CLAY BLANKET.

As previously mentioned, clay was dumped over the tunnels in varying
depths at different times. A material was required which would pack
into a compact mass and would not readily erode under the influence of
the tidal currents of the river and the escape of the great volumes of
air which often kept the water in the vicinity of the shields in violent
motion. Suitable clay could not be found in the immediate vicinity of
the work. Materials from Shooter's Island and from Haverstraw were tried
for the purpose. The Government authorities did not approve of the
former, and the greater portion of that used came from the latter point.
Although a number of different permits governing the work were granted,
there were three important ones. The first permit allowed a blanket
which roughly followed the profile of the tunnels, with an average
thickness of 10 ft. on the Manhattan side and somewhat less on the Long
Island City side. The second general permit allowed the blanket to be
built up to a plane 27 ft. below low water. This proved effective in
checking the tendency to blow, but allowed considerable loss of air.
Finally, dumping was allowed over limited and marked areas up to a plane
of 20 ft. below low water. Wherever advantage was taken of this last
authority, the excessive loss of air was almost entirely stopped. After
all the shields had been well advanced out into the river, the blanket
behind them was dredged up, and the clay used over again in advance of
the shield.

Soundings were taken daily over the shields, and, if marked erosion was