Up to August, 1907, the joints between the segments of the cast-iron
lining were caulked with iron filings and sal ammoniac, mixed in the
proportion of 400 to 1 by weight. With the air pressure balancing the
hydrostatic head near the tunnel axis, it was difficult to make the
rust-joint caulking tight below the axis against the opposing water
pressure; this form of caulking was also injured in many places by
water dripping from service pipes attached to the tunnel lining. A few
trials of lead wire caulked cold gave such satisfactory results that it
was adopted as a substitute. Pneumatic hammers were used successfully on
the lead caulking, but were only used to a small extent on the rust
borings, which were mostly hand caulked. Immediately before placing the
concrete lining, all leaks, whether in the rust borings or lead, were
repaired with lead, and the remainder of the groove was filled with 1 to
1 Portland cement mortar, leaving the joints absolutely water-tight at
that time. The subsequent development of small seepages through the
concrete would seem to indicate that the repair work should have been
carried on far enough in advance of the concreting to permit the
detection of secondary leaks which might develop slowly. The average
labor cost chargeable against the caulking was 12 cents per lin. ft., to
which should be added 21.8 cents for "top charges."
Unfortunately, it was necessary to place the greater part of the
concrete lining in the river tunnels during the summer months when the
temperature at the point of work frequently exceeded 85 deg.; and the
temperature of the concrete while setting was much higher. This abnormal
heat, due to chemical action in the cement, soon passed away, and, with
the approach of winter, the contraction of the concrete resulted in
transverse cracks. By the middle of the winter these had developed quite
uniformly at the ends of each 30-ft. section of concrete arch as placed,
and frequently finer cracks showed at about the center of each 30-ft.
section.
While the temperature of the concrete was falling, a like change was
taking place in the cast-iron lining, with resulting contraction. The
lining had been erected in compressed air, the temperature of which
averaged about 70 deg. in winter and higher in summer. Compressed air having
been taken off in the summer of 1908, the tunnels then acquired the
lower temperature of the surrounding earth, slowly falling until
mid-winter. The contraction of the concrete, firmly bedded around the
flanges of the iron, and showing cracks at fairly uniform intervals,
probably localized the small corresponding movements of the iron near
the concrete cracks, and resulted in a loosening of the caulking at
these points. With the advent of cold weather, damp spots appeared in
numerous places on the concrete, and small seepages showed through quite
regularly at the temperature cracks, in some cases developing
sufficiently to be called leaks. Only a few, however, were measurable in
amount.
Early in January small brass plugs were firmly set on opposite sides of
a large number of cracks, and caliper readings and air temperature
observations were taken regularly throughout the winter and spring. The
widths of the cracks and the amount of leakage at them increased with
each drop in temperature and decreased as the temperature rose again,
but until spring the width of the cracks did not return to the same
point with each return of temperature.
The leakage was similar in all four tunnels, but was largest in amount
in Tunnel _D_, where, at the beginning of February, the ordinary flow
was about 0.0097 cu. ft. per sec., equivalent to 0.00000347 cu. ft. per
sec. per lin. ft. of tunnel. Of this amount 0.0065 cu. ft. per sec.
could be accounted for at eight of the cracks showing measurable
leakage, leaving 0.0032 cu. ft. per sec. or 0.00000081 cu. ft. per sec.
per lin. ft. of tunnel to be accounted for as general seepage
distributed over the whole length.
It was not feasible to stop every leak in the tunnel, most of which were
indicated simply by damp spots on the concrete; a rather simple method
was devised, however, for stopping the leaks at the eight or ten places
in each tunnel where water dripped from the arch or flowed down the face
of the concrete. The worst leak in any tunnel flowed about 0.0023 cu.
ft. per sec. To stop these leaks, rows of 1-in. holes, at about 4-in.
centers, were drilled with jap drills through the concrete to the flange
of the iron. These rows were from 3 to 18 ft. long, extending 1 ft. or
more beyond the limits of the leak. The bottoms of the holes were
directly on the caulking groove and the pounding of the drill usually
drove the caulking back, so that the leak became dry or nearly so after
the holes were drilled. If left alone the leaks would gradually break
out again in a few hours or a few days and flow more water than before.
They were allowed to do this, however, in only a few cases as
experiments. After the holes were drilled, the bottom 4 in. next the
flange was filled with soft neat cement mortar. Immediately on top of
this was placed two plugs of neat cement about 2-1/2 in. long, which
were 5 or 6 hours old and rather hard. Each was tamped in with a round
caulking tool of the size of the hole driven with a sledge hammer. On
top of this were driven in the same way two more plugs of neat cement of
the same size, which were hard set. These broke up under the blows of
the hammer, and caulked the hole tight. When finished, the tamping tool
would ring as though it was in solid rock. Great pressure was exerted on
the plastic mortar in the bottom of the hole, which resulted in the
re-caulking of the joint of the iron. No further measurable leakage
developed in the repaired cracks, during a period of four months, and
the total leakage has been reduced to about 0.002 cu. ft. per sec. in
each tunnel, an average of 0.00000051 cu. ft. per sec. per lin. ft.