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 facing for the rock on the sides in order to prevent disintegration, due to exposure, and give a finished appearance to the work. Above the rock surface a retaining wall of gravity section was designed, the top being slightly higher than the yards of the adjoining properties. The face wall was designed to be as thin as possible, in order to allow the maximum space for tracks.
The excavation, therefore, was laid out so that the back of the retaining wall would not encroach on the adjoining property, but would practically coincide with the property line at positions of maximum depth.
The batter on the face of the wall was 2 in. per ft., and a bridge seat 3-1/2 ft. wide was formed at an elevation of 22 ft., minimum clearance, above the top of the rail. This bridge seat was made level. The maximum height of the south wall is 49 ft., and of the north wall 65 ft.
The face walls were classed as "Upper Face Walls," extending from the base of the retaining wall to the bridge seat, and as "Lower Face Walls," extending from the bridge seat to the base of the wall. The general design is shown on Fig. 8.