AMERICAN PORTAL, ST. CLAIR TUNNEL. NORTH OF DETROIT, MICH.
AMERICAN PORTAL, ST. CLAIR TUNNEL. NORTH OF DETROIT, MICH.
Tunnel Surveying.—The tunnel surveying developed during this century is one of the marvels of surveying work. If a tunnel is to be several miles in length, not only is the excavation commenced at each end, but one or more intermediate shafts are frequently sunk to the level of the tunnel, and excavation is extended in each direction from the shafts. It is extremely important that these sections of the tunnel should “meet” exactly. If they should fail to do so by any appreciable amount, the necessary modifications are frequently costly and therefore justify the most elaborate precautions in the surveying work, especially since the surveying costs much less than the consequences of such a blunder. The Hoosac tunnel is over 25,000 feet long. The heading from the east end met the heading from the central shaft at a point 11,274 feet from the east end and 1563 feet from the shaft. The error in alignment was five sixteenths of an inch, that of levels “a few hundredths,” error of distance “trifling.” The corrected alignment was then carried on toward the heading from the west end, which it met at a point 10,138 feet (nearly two miles) from the west end and 2056 feet from the shaft. Here the error of alignment was 9/16 of an inch and that of levels about 1-5/8 inches. The surveying work of the spiral tunnels on the St. Gothard Railway (to be described later) is another example of marvelously accurate work under peculiarly unfavorable circumstances.
St. Gothard Tunnel.—To appreciate the magnitude of the problem involved, of which this great tunnel is the crowning feature, some idea should be obtained of the Alpine topography lying between Silenen, in Switzerland, and Bodio, in Italy, less than forty miles apart. The idea of connecting Switzerland and Italy by a railroad passing over or through the Alps, by utilizing the St. Gothard Pass as far as possible, dates back to 1850, or even earlier. An enterprise of such magnitude could be consummated only after years of discussion, planning, surveying, negotiations, and even international agreements. In 1871 a treaty was finally ratified between Germany, Italy, and Switzerland, by which the construction and financiering was duly authorized.On August 7, 1872, the contract for the construction was signed, with a proviso that the work must be completed within eight years. On April 30, 1880, the advance headings met, and soon thereafter the mails were regularly carried through, although the tunnel was not actually completed in the specified time.
The route adopted was bold enough to stagger the financier, if not the engineer. Starting from Silenen, Switzerland, it required a climb of nearly 2000 feet to reach Göschenen, the adopted northern portal of the tunnel. This would require anaveragegrade of 200 feet per mile in the ten miles of distance, or an actual grade of 370 feet per mile in the upper part of the line, if the river valley were followed. The line was therefore “developed,” that is, the distance was purposely increased by adopting an indirect line, in order that the grade might be less. It was found possible to run the line from Silenen to Pfaffensprung, a distance of about six miles, on the comparatively low grade of 137 feet per mile. At this point the line suddenly plunges into the mountain, and curves around in a circle, which is, roughly, 2000 feet in diameter, while it continues an upward grade of 121½ feet per mile. After traversing 4845 feet of such tunnel, the line again emerges into the open air, having turned nearly three fourths of a circle in the solid rock. About 2000 feet farther on the line actually crosses itself, the upper line there being 167½ feet higher than the lower line, which is at that point within the tunnel. By this device, which is called a spiral, the line is run at a practicable grade, and an elevation of 167½ feet is surmounted by introducing 6986 feet of “development.” Near the entrance of the Leggistein tunnel, the line is less than 500 feet away (horizontally) from a lower part of the line, which is about 350 feet lower in elevation. Space forbids a further description of this climb of 2000 feet to Göschenen, where the line plunges into the bowels of the earth, and does not again emerge until it has traversednine and one quarter miles, and has reached the southern slope of the Alps. Even here the portal is 3755 feet above sea level, and the valley down to Bodio is steeper in places than the valley of the Reuss. Four spirals are used in descending about 2650 feet in an air line distance of less than 19 miles. In one place even the upper line, where it crosses the lower line, is in solid rock. Imagine standing in the gloom of a tunnel and considering that vertically beneath your feet—more than 100 feet further down in the bowels of the earth—there is another tunnel belonging to the same line of road. The great majority of tunnels are straight. A few have curves at one or both ends, but nowhere else in the world can be found such examples of spiral tunnels carved out of the living rock.
INTERIOR OF ST. CLAIR TUNNEL, NORTH OF DETROIT, MICH.
INTERIOR OF ST. CLAIR TUNNEL, NORTH OF DETROIT, MICH.
St. Clair Tunnel.—A glance at a map of lower Canada and Michigan will show that all the rail traffic of lower Canada, and even that from Montreal and Quebec, that passes as far west as Chicago, must either cross the Detroit River at Detroit or the St. Clair River, at or near Port Huron. Plans for bridging the river have been frequently made, but the Canadian government has steadily refused permission. The traffic along the river in 1896 amounted to over 35,000,000 tons, or more than was shipped at the ports of either New York, London, or Liverpool, and greatly in excess of that which passed through the Suez canal. Such traffic must not be impeded even by a drawbridge; and therefore a tunnel was the only alternative. The problem was in many respects unique. Borings showed that the tunnel must pass through clay and occasional pockets of quicksand, and therefore it would be necessary to employ a pneumatic method. Brunel had used a “shield” on the Thames tunnel half a century before; but all of the earlier tunnels constructed by this method were much smaller, and the difficulty and danger increase very rapidly as the size increases.
In 1886 the “St. Clair Tunnel Company,” virtually a creature of the Grand Trunk Railway Company, was organized, and in 1888 work was begun. After a false start, made by sinking shafts which were afterwards abandoned, open cuttings were commenced at each end, which were extended to points 6000 feet apart, between which the tunnel was excavated and lined. The circular lining, having an outside diameter of 21 feet, is of cast iron, made in segments which are bolted together, having strips of wood three sixteenths of an inch thick placed in the joints. Liquid asphalt was freely used as a preservative and to make tight joints. The tunnel was excavated for nearly 2000 feet on each side as an ordinary open tunnel until the excavation was actually under the river; then a diaphragm with air locks was built on each side, and that part of the tunnel lying under the river—2290 feet in length—was constructed under air pressure. Several curious facts were developed during the construction. The material excavated outside of the shields was thrown inside, loaded on to cars, and hauled by mules to the diaphragm. It was found that horses could not work in compressed air. Mules could do so, but even they were sometimes affected by “the bends,” a disease akin to paralysis, which frequently occurred among the men. The shields were forced forward by twenty-four hydraulic rams, each having a capacity of 125 tons, or 3000 tons for each shield. Usually a force of 1200 to 1500 tons was sufficient. Much gas was encountered, which, on account of its explosiveness, prevented the employment of blasting to break up the boulders which were frequently found. The advantages of electric lighting in compressed air work were exemplified in this tunnel. In August, 1890, about one year after the shields were placed on each side of the river, they met near the centre. The progress of each shield averaged nearly ten feet per day. Considering the frequency with which the cost of great engineering work exceeds the original estimate, it is remarkable to note that in this case the actual cost ($2,700,000) was less than the original estimate, which was about $3,000,000.