_Materials and Inception of the Work._--The materials met in Tunnel A
are typical of all four tunnels. From the Long Island shafts westward,
in succession, there were 124 ft. of all-rock section, 125 ft. of part
rock and part earth section, 22 ft. of all-rock section, 56 ft. of part
rock and part earth section, 387 ft. of all-rock section, 70 ft. of part
earth and part rock section, and 1,333 ft. of all-earth section.
[Illustration: PLATE LXVII]
The materials passed through are indicated on Plate XIII. The rock was
similar to that of the Blackwell's Island Reef, and was likewise covered
by a layer of sand and boulders. The remainder of the soft ground was
divided into three classes. The first was a very fine red sand, which
occurred in a layer varying in thickness from 6 ft. to at least 15 ft.
It may have been much deeper above the tunnel. It is the quicksand
usually encountered in all deep foundations in New York City. The
following is the result of the sifting test of this sand:
Held on No. 30 sieve 0.6%
Passed No. 30, " " No. 40 " 0.4%
" No. 40, " " No. 50 " 0.7%
" No. 50, " " No. 60 " 2.4%
" No. 60, " " No. 80 " 14.9%
" No. 80, " " No. 100 " 54.0%
" No. 100, " " No. 200 " 8.0%
" No. 200 " 19.0%
------
100.0%
This means that grains of all but 4% of it were less than 0.0071 in. in
diameter. The 19% which passed the No. 200 sieve, the grains of which
were 0.0026 in. or less in diameter, when observed with a microscope
appeared to be perfectly clean grains of quartz; to the eye it looked
like ordinary building sand, sharp, and well graded from large to small
grains. This sand, with a surplus of water, was quick. With the water
blown out of it by air pressure, it is stable, stands up well, and is
very easy to work. It appears to be the same as the reddish quicksand
found in most deep excavations around New York City.
The second material was pronounced "bull's liver" by the miners as soon
as it was uncovered. "Bull's liver" seems to be a common term among
English-speaking miners the world over. It is doubtful, however, if it
is always applied to the same thing. In this case it consisted of layers
of blue clay and very fine red sand. The clay seemed to be perfectly
pure and entirely free from sand. It would break easily with a clean,
almost crystalline, fracture, and yet it was soft and would work up
easily. The layers of clay varied in thickness from 1/16 in. to 1 in.,
while the thickness of the sand layer varied from 1/4 in. to several
inches. The sand was the same as the quicksand already described.
The "bull's liver" was ideal material in which to work a shield. It
stood up as well and held the air about as well as clay, and was much
easier to handle.
The third material was a layer of fine gray sand which was encountered
in the top of all the tunnels for about 400 ft. just east of Blackwell's
Island Reef. It was very open, and had grains of rather uniform size.
During the starting out of the tunnels from the shafts, and for more
than a year afterward, the roof of the working chamber in the caissons
and the locks previously described under the Long Island shafts took the
place of the bulkhead across the tunnels for confining the air pressure.
The first work in air pressure was to remove the shield plug closing the
opening in the side of the shaft. This being done, the shield was shoved
through the opening, and excavation begun.
At the start the shields were fitted with movable platforms, but no
hoods of any kind were placed until after the rock excavation was
completed.