_Drilling._--Where no timbering was required, several different methods were used in drilling and excavating the solid rock, though in all cases a center top heading was driven. The four diagrams, Figs. 1, 2, 3, and 4, give typical examples of these methods and show, in the order of their numbers, the general tendency of the development from a small heading kept some distance ahead of the bench, to a large heading with the bench kept close to it. The notes on each diagram give the general details of the quantity of drilling and powder used, methods of blasting, etc., and on the progress profile, Fig. 6, is indicated those portions of the tunnels in which each method was used.
All the drills used throughout the work by Mr. Bradley were Rand No. 34 sluggers, with 3⅝-in. cylinders, and the steel was that known as the “Black Diamond Brand,” 1⅜-in., octagon. It was used in 2, 4, 6, 8, 10, and 12-ft. lengths; toward the end of the work it was proposed to use 14-ft. lengths, but owing to some delay in delivery this length was never obtained. The starters, 18 to 24 in. long, were sharpened to 2¾ to 3-in. gauge, which was generally held up to depths of 6 ft.; then the gauge gradually decreased until it was 1¾ to 2¼ in. at the bottom of a 12-ft. hole. Frequently, as many as three or four starters were used in starting a hole, and generally two sharpenings were required for each 2 ft. drilled, after the first 6 ft. It is estimated that about ¼ in. of steel was used for each sharpening, and that there was an average of one sharpening for every foot drilled.
The total quantity of steel used up, lost, or scrapped on the whole work was almost exactly 1 ft. for each 10 cu. yd. excavated, equal to 1¼ in. of steel per yard, distributed approximately as follows:
Sharpening ¾ to ⅞ in. Other losses ½ to ⅜ ” --------------- Total 1¼ in. per cu. yd.
An “Ajax” drill sharpener was used, and proved very satisfactory. Rubber and cotton hose, covered with woven marlin, was used for the bench (3 in. inside diameter, in 50-ft. lengths), for drills (1 in. in diameter, in 25-ft. lengths), and for steam shovels (2½ in. in diameter, in 50-ft. lengths). Hose coverings of wound marlin, and of woven marlin with spiral steel wire covering were tried, but were not satisfactory, owing to the unwinding of the marlin and the bending of the steel covering.