A great deal of the extra quantity was due to rock falling from the core-wall side whenever one working face was behind the other. Blasting at the face behind generally loosened more or less rock on the core-wall side of the tunnel which was ahead, in one or two instances breaking entirely through, as shown in Fig. 2, Plate XXVI, the hole in the core-wall in this case being utilized by building a storage chamber in it.
Table 3 gives some of the statistics of drilling in the Simplon Tunnel, as compared with the drilling on this work, the figures for the Simplon being taken from papers read before the Institution of Civil Engineers of Great Britain.
TABLE 3. -------------------------------------------+--------------+---------- | | | Bergen Hill. | Simplon. -------------------------------------------+--------------+---------- Drills set up in heading, percentage of | | total elapsed time | 50% | 60% Actually drilling the rock, percentage of | | total elapsed time | 25% | 50% Average advance per round (attack) | 8.5 ft. | 3.8 ft. Average time for each attack | 36 hours. | 5 hours. Average advance per day of 24 hours | 5 ft. | 18 ft. † Depth of holes | 10 ft. | 4.6 ft. Diameter of holes | 2¾ in. | 2¾ in. Linear feet drilled per hour, per drill | 2.7 | 7.0 Linear feet drilled per cubic yard | 5.0 | 6.0 Pounds of dynamite per cubic yard | 3.4 to 5.7 | 8½ Average depth drilled with one sharpening | 12 in. | 6½ in. Total number of men per day of 24 hours* | 450 | 3,300 -------------------------------------------+--------------+----------
[* On Bergen Hill Tunnels, for two full working faces at the Hackensack end, about 3,000 ft. in from portal (March, 1908). At Simplon, two full faces and two headings, at a distance of about 5,000 ft. in from the portal (January, 1900). These both include lining as well as excavation. The lining of the Bergen Hill Tunnels progressed about twice as fast as the excavation; it is inferred that on the Simplon it progressed at about the same rate as the excavation.]
[† At the Italian end, in Antigoric gneiss, which is stated to be very hard rock.]
The figures in Table 3 are for “heading only” in both cases, except for the last item (number of men), the heading in the Simplon Tunnels being about 60 sq. ft., as compared with the heading of Method No 4 (which has been used for comparison), of 210 sq. ft.