[9] Network of steel rods embedded in concrete.
Ventilation.
—In tunneling, one of the most important problems to be solved is that of ventilation, and it is for this reason that the Simplon tunnel consists of two parallel headings with cross cuts at intervals of 220 yds. At Brigue, a shaft 164 ft. deep was sunk through the overlying rock until the “gallery of direction” was encountered. Up this chimney the foul air is drawn by wood fires, the fresh air—a volume of 19,000,000 cu. ft. per day, or 13,200 cu. ft. per minute—entering by heading No. 2, penetrating up to the last cross gallery, and returning by tunnel No. 1. The entrances of No. 1 and the “gallery of direction,” besides those of all the intermediate cross galleries, are closed by doors. By this arrangement, however, fresh air does not reach the working faces; therefore a pipe, 8 ins. in diameter, is led from the fresh air in No. 2 to within 15 yds. of the face of each heading, and up this pipe a draft of air is induced by means of a jet of water, the volume to each face being 800 cu. ft. per minute. One single jet of water from the high-pressure mains, with a diameter of 1⁄16 in., is capable of supplying over 1000 cu. ft. of air per minute at the end of 160 yds. of pipe, and during the attack the men at the drills are in a constant breeze with the thermometer standing at 70° F. At Iselle, air is blown into the entrance of heading No. 2 at the rate of 14,100 cu. ft. per minute by two fans driven from the turbine shaft. This air travels from the fans along a pipe 18 ins. in diameter, till a point 15 yds. up the tunnel is reached, where beyond a door the pipe narrows to form a nozzle 10 ins. in diameter. This door is kept open to allow the outside air to be induced up the tunnel, as the headings are at present only 2500 yds. long, giving a resistance of not quite sufficient power to cause the air to return. The fresh air then travels up No. 2, crossing over the top of the “gallery of direction,” from which it is shut off by doors, to the last cross gallery, returning by No. 1, and finally leaving either by the “gallery of direction” or by No. 1. A system of cooling the air and driving it on by means of a large number of water-jets will be installed in No. 2 where that heading crosses over the “gallery of direction,” but at present there is no need for it.
The average temperature at the face is 73° F. during the drilling operation, 76° F. after firing the charges, and a maximum of 80° F., lately attaining to 86° F. on the south side, with 80° F. and 85° F. before and after firing. The temperature of the rock is taken at every 110 yds. in holes 5 ft. deep, and shows a gradual increase according to the depth of over-laying rock, to the conductivity of the rock, and to the form of the mountain surface. The maximum hitherto reached on the north side is 68° F., while on the south side, although a smaller distance has been traversed, it attains to 79° F., due to the more rapid increase in depth. Moreover, the temperature of the rock is observed at the permanent stations, 550 yds. from the entrances, in its relation to that of the tunnel and outside air, and though on the north side that of the rock varies almost as quickly as that of the tunnel air, on the south it is influenced very much less.
A few statistics may be of interest with regard to the progress of the last three months (taken from the trimestrial report of January, 1900). At Brigue, where there are three drilling-machines in No. 1 and two in the parallel heading, the total length excavated was 995 yds. or 6409 cu. yds. in 89 working days, the average cross-sectional area being 57 sq. ft. This required 507 attacks and 3066 holes, which had a total depth of 26,600 ft. and 14,700 re-sharpenings of the drilling-tool, with 44,000 lbs. of dynamite.
The average time occupied in drilling was 2 hrs. 45 mins., while charging, firing, and clearing away the débris took 6 hrs., 35 mins. At Brigue 648 men and 29 horses were employed at one time in the tunnel. At Iselle the numbers were 496 men and 16 horses, working in shifts of 8 hrs. Outside the tunnel, in the shops, forges, etc., the men work 8 hrs. to 11 hrs. per day, the total being 541 men at Brigue and 346 men at Iselle. On the Italian side, where the rock is very much harder, there were three drilling-machines in each heading; the total length excavated, with a cross-sectional area of 62 sq. ft., was 960 yds. or 6700 cu. yds. in 91 working days. This required 61,293 re-sharpened tools, 758 attacks, 7940 holes with a total depth of 33,000 ft., and 56,000 lbs. of dynamite. The average time spent in drilling was 2 hrs. 55 mins., and in charging and clearing 2 hrs. 36 mins. Thus, in the hard gneiss, to excavate 1 cu. yd. of rock required 81⁄2 lbs. of dynamite, and each tool pierced 61⁄2 ins. of rock before it required re-sharpening.
THE MURRAY HILL TUNNEL.
The drift method of excavating tunnels was followed in Section IV of the New York Subway, under Park Avenue between 33rd and 41st Streets. At this point the four tracks of the subway pass under a rocky elevation, known as Murray Hill, in two double track parallel tunnels, 43 ft. apart, center to center. Here already existed a double track tunnel which was built many years ago by the New York Central and Hudson River R.R., and is now used by the Madison Avenue surface cars. The two subway tunnels were driven close below the existing tunnel and also very near the foundations of expensive residences along Park Avenue, particularly on Murray Hill, one of the best residential sections of the city.
Material Penetrated.
—The material penetrated by the excavation consisted chiefly of a surface outcrop of the mica-schist rock which underlies Manhattan Island. The rock was for the most part in compact strata, dipping at about 45° from East to West, but at intervals an unstable stratum was encountered which when free slid on the underlying stratum. Troubles from such slides were experienced during the construction of the tunnel.