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.