were loaded, and also about
every third one of the broaching holes, and, unless the rock was very bad,
it usually broke sharply at the broaching line. Occasionally, the broaching
holes which were not loaded were filled with sand, which gave rather better
results than leaving them open.

In the steam-shovel work on the east side of Ninth Avenue, spring holes
were used. They were formed by drilling a 20-ft. hole and exploding at the
bottom of it, without tamping, two or three sticks of dynamite, and
repeating this process with heavier charges until there had been formed at
the bottom of the hole a large cavity which would hold from 100 to 200 lb.
of dynamite. Face holes and breast holes were also drilled, and it was
possible by this method to drill and break up a cut 20 ft. deep and 15 ft.
thick. The only place where spring holes were used on this work was on the
east side of Ninth Avenue where the heavy cutting was sometimes extended
beyond the east house line.

From the best records obtainable, the average progress in drilling was
about 33 lin. ft. per 8-hour shift. The average number of cubic yards of
excavation per drill shift was 13.9, and the average amount of drilling per
cubic yard of excavation was 2.4 ft.; this covered more than 27,000 drill
shifts.

The dynamite was practically all 60%, and the average excavation per pound
of dynamite was 2.2 cu. yd. The contractor employed an inspector of
batteries and fuses, who, using an instrument for that purpose, tested the
wiring of each blast prior to firing, in order to discover any short
circuits, and thus prevent the danger of leaving unexploded dynamite in the
holes.

The average quantity of excavation per derrick shift of 10 hours, covering
7,400 shifts, 87% of the excavation being rock, was 50 cu. yd., and the
average force per shift, including only foreman and laborers, was 13 men.
It was found that a derrick operating at the top of a 20-ft. cut would
handle about 40 cu. yd. per shift, whereas, if operating at the bottom of
the cut, it would handle about 60 cu. yd. per shift. The elevator derricks
at Tenth Avenue were very efficient, and each could take care of the
material from four derricks at the bottom, hoisting 250 cu. yd. per shift a
height of 60 ft.

_Concrete Retaining and Face Walls._--It was essential to have the greatest
space possible at the bottom of the excavation, and, inasmuch as the yard
was to be left open, it was necessary to provide some