When the excavation for the open-cut work of the Terminal Station had advanced to the line of Tenth Avenue, the contractor started a heading from this point and drove westward under Tenth Avenue until the headings driven eastward from the cut-and-cover portion, were met.
This was done to expedite the work under Tenth Avenue, where the ground was not very good, where there were several important gas and water mains in the street, and where, moreover, the tunnels were of exceptionally large span (24 ft. 6 in.), making a total width of some 60 ft. for the excavation. The excavation for the New York Tunnels was practically finished in January, 1908.
Drilling and Blasting.—The foregoing short description will serve to show in a general way the scheme adopted in making the excavation. A few details on drilling and blasting methods may not be out of place.
Percussive drills run by air pressure were used. They were Ingersoll-Sergeant, Nos. 3½, A-86, C-24, and F-24. The air came from the high-pressure compressor previously described. This compressor, without assistance, could supply air for nine drills, but, when fed by compressed air from the lower pressure, its capacity was increased three or four times.
The air was compressed to 100 lb. per sq. in. in the power-house, and was delivered at about 80 lb. per sq. in. at the drills. A 3-in. air line was used. The drill steel was 1⅛-to 1⅜-in. octagonal. The holes were about 3¼ in. in diameter at starting and 2⅝ in. at the full depth of 10 ft. The powder used on the New York side was 40% Forcite, the near presence of heavy buildings and lack of much rock cover necessitating light charges and many holes spaced close together.
To compensate the contractor for the inevitable excavation done outside the neat lines of the masonry lining, the excavation was paid for to the "Standard Section Line" which was 12 in. outside the neat lines on top and sides and 6 in. outside at the bottom of the cross-section. The actual amount of excavation done was about 11% greater than that paid for. The distance excavated beyond the neat line, because of the very heavy timbering necessary, was about 2.1 ft. instead of the 1 ft. allowed, and at the bottom about 0.85 ft. instead of the 0.50 ft. paid for.
For a period of 5 months detailed records were kept of the drilling and blasting. About 12,900 cu. yd. of excavation are included. A sketch and table showing the method of driving the heading, the number and location of the holes drilled, and the amount of powder used, is given in [Fig. 5]. From this and similar figures the information in [Table 5] is derived.
| Portion of excavation. | Feet of Hole drilled per cubic Yard of Execavation | Pounds of Power Used per Cubic Yard of Excavation | ||||
|---|---|---|---|---|---|---|
| 15-ft. 4-in. span-- twin tunnel | 19-ft. 6-in. span-- twin tunnel | 24-ft. 6-in. span-- twin tunnel | 15-ft. 4-in. | 19-ft. 6-in. | 24-ft. 6-in. | |
| Wall-plate heading[C] | 13.00 | 10.97 | 10.97 | 3.77 | 2.85 | 2.85 |
| Total heading[C] | 7.87 | 8.17 | 7.81 | 2.31 | 2.02 | 1.78 |
| Bench and raker bench[C] | 5.97 | 6.15 | 7.56 | 0.94 | 0.93 | 1.13 |
| Trench[C] | 9.82 | 15.96 | 18.10 | 1.84 | 2.49 | 2.73 |
| Average for section[C] | 6.69 | 7.43 | 8.95 | 1.28 | 1.30 | 1.45 |
| Actual amount[D] | 6.82 | 7.27 | 8.95 | 1.22 | 1.24 | 1.27 |