Popular Science Monthly/Volume 45/August 1894/The Story of a Great Work
|THE STORY OF A GREAT WORK.|
By J. JONES BELL.
ON the 19th of September, 1891, Sir Henry Tyler, President of the Grand Trunk Railway Company, presided at the inauguration of one of the greatest engineering achievements of the present day, bold in conception, new in design, and novel in many of the methods adopted in its construction. Without the St. Clair Tunnel the immense stream of traffic from the East, which during last summer flowed to the World's Columbian Exposition at Chicago, could not have been successfully handled.
Previous to the construction of the tunnel, connection between the Grand Trunk Railway and the Western roads with which it exchanges traffic was maintained by a ferry, the loaded cars being carried across on the deck of a powerful steamer, specially built for the purpose. Adopted for want of a better, this service was never satisfactory. Though the swift current, where Lake Huron pours its entire volume through a narrow outlet, prevents the river freezing in winter, ice blocks occasionally occurred, and a single day's interruption to traffic involved serious inconvenience and loss. A bridge had often been suggested, but it was always successfully opposed by the vessel interest. A larger number of vessels, with a greater tonnage, pass up and down the St. Clair River during the season of navigation than through the Suez Canal in a year. A high-level bridge is impossible, and a draw would be attended with great interruption to traffic, and danger to vessels on account of the current. The only alternative seemed to be a tunnel. Its completion not only affords a better crossing, but establishes the possibility of such a work being successfully and economically built and worked where favorable conditions exist. The story of its construction is an interesting one.
The tunnel is really a large iron tube, twenty feet in diameter and six thousand and twenty-six feet long, buried under the river, but considerable ingenuity was required to place it there. In 1884 Mr. Joseph Hobson, the chief engineer of the work, and Mr. Hillman, his assistant, made a survey of the river, one mile low the towns of Sarnia and Port Huron. Though not so narrow as where the cars were ferried, the nature of the bed of the river seemed to be more favorable at that point. Borings were made to the rock, eighty-six feet below the level of the river. The greatest depth of water was 40·47 feet. The bed of the stream was found to consist of the following layers: two feet of common yellow sand like that of the seashore, twelve feet of a mixture of
Fig. 2.—The Old Railroad Ferry.
The tunnel could not have been built without this shield. The credit of its invention appears to be due to Mr. Alfred E. Beach, of New York, who designed it in 1868 for use in the construction of the tunnel under Broadway. It was subsequently used in Buffalo, Chicago, at the Hudson River Tunnel, and other places. The use of the shield in tunneling was first introduced by Sir Mark I. Brunei in 1825, and it was afterward employed by Mr. Greathead, in the Thames Tunnel and other works; but the St. Clair shield differs in some important respects from any before employed. It is a cylinder of iron, twenty-one feet and six inches in diameter and sixteen feet long, built of steel one inch thick, and with a sharp cutting edge in front. It is divided into twelve compartments by two horizontal and three vertical stays. It weighs fifty tons, and was built on the spot, the material having been prepared in the workshops at Hamilton. Against the rear end of the shield were ranged twenty-four hydraulic rams, eight inches in diameter and having a stroke of twenty-four inches. These forced the cutting edge forward into the clay, which was then excavated within the shield. By means of a Worthington pump, a pressure of five thousand pounds per square inch, or three thousand tons in all, could be exerted. The greatest pressure used was seventeen hundred pounds per square inch, or a thousand and sixty tons in all. The pressure could be exerted on any or all of the rams so as to preserve the true direction of the shield. The keeping of this direction was one of the interesting engineering feats of the work. It was done by means of a specially made London transit, set on masonry, a series of disks and cross-wires indicating the slightest deviation. Observations were made every day and the results marked on a diagram. The deviation was rarely found to exceed a quarter of an inch, and any error was corrected by adjustment of the hydraulic jacks. When the shields came together they were found to be exactly in line.
Fig. 3.—Line of Excavation of the St. Clair Tunnel.
At one time it was feared the work would have to be abandoned. When the tunnel from the Canadian end reached the bed of the river,, quicksand and water caused much trouble, but by the use of compressed air the difficulty was surmounted. At the line of the river on each side a bulkhead of brick and cement was built across the tunnel, with two air chambers, provided with airtight doors. The greatest atmospheric pressure necessary to prevent an inroad of sand and water was thirty-seven pounds per square inch, and under this pressure, after a short experience, the workmen found no difficulty in pursuing their task, in half-hour shifts. The use of compressed air had to be resorted to at two points.
The completed tunnel, as already stated, is an iron tube. This tube is built up of rings, eighteen inches in width, one of which was put together within the shield each time it was moved forward. Each ring consists of thirteen sections and a key piece, flanged to enable them to be bolted together. The body of the section is two inches thick, and the flanges are six inches wide. Each section weighs about one thousand pounds. The pieces were lifted and placed in position by a revolving crane, a complete ring being put up in about one hour. To ease the pressure and make the joints watertight, the edges were planed and strips of oak and tar canvas inserted. The sections were also heated and dipped in pitch. The tube being only twenty-one feet in diameter, while the shield was twenty-one feet. THE STORY OF A GREAT WORK. 467
and a half, the space under the tube when the shield moved for- ward was filled with cement. The clay was allowed to settle down on the upper part. When the shields met, the tube was built up within them to the junction and the shells of the shields allowed to remain. The inside of the tube is finished with a preparation to keep it from rusting.
On Sunday, August 24, 1890, the two excavations had ap- proached so nearly that an opening was made with an earth auger, and the workmen talked and passed articles to each other. The earth was soon removed, and Mr. Hobson, the chief engi- neer, and others connected with the tunnel company, stepped through. Six days later the shields came together and the suc- cess of the great undertaking was assured. In its construction about seven hundred men were employed, of more than average intelligence, who took great interest in the work.
The actual length of the tunnel, from portal to portal, is six thousand and twenty-six feet. Of this, two thousand three hun- dred and ten feet is under the river, one thousand nine hundred and eighty -two feet under dry ground on the Canada side, and seventeen hundred and thirty-four feet under dry ground on the United States side. The open excavation to reach the ground level .on the Canadian side is three thousand and sixty-one feet, and on the United States side two thousand four hundred and sixty-six feet. The grade is one in fifty, except under the river, where it is practically level, only sufiicient incline one tenth per cent being given toward the Canadian side to provide for drain- age. The depth of the lowest part under the mean level of the river is 77"83 feet. The minimum depth between the top of the tube and the bottom of the river is fifteen feet, the average being twenty-five feet. It was necessary to place it as far down as pos- sible in the clay, consistent with the grade, so as to overcome the tendency of a tube filled with air to rise to the surface in water or mud. The bottom is about nine feet above the rock which underlies the clay. On the Canada side the bottom is sixty feet below the surface of the ground at the portal, on the United States side it is eight feet less. The bottom of the tunnel at its lowest point is one hundred feet below the railway track on the level, which indicates the total ascent and descent which trains have to make in passing through. Ventilation is secured by the motion of the trains, which is found to be ample for the purpose.
The trains are drawn through the tunnel by powerful locomo- tives belonging to the tunnel company, specially built for the purpose. They take eighteen loaded cars at a trip.
The track in the tube is supported on solid brickwork, as shown in the accompanying cross-section. It was at first proposed to build the tunnel wide enough for two tracks, but it was found
�� � that two single-track tunnels would be cheaper, and one of them would sooner be available for traffic. Experience has proved that a second tunnel will not be required for a long time. The largest number of freight cars passed through in twenty-four hours
Fig. 4.—Section of the Tunnel and River-bed.
during the two years the tunnel has been in use was one thousand and fifteen, while twenty-five hundred could be handled if occasion required. The average number is seven hundred in winter and five hundred in summer. This is in addition to passenger trains.
The estimated cost of this great work was between two and a half and three million dollars, but its actual cost was considerably less, a rather remarkable fact in connection with such works. Owing to the great risk any contractor would have to assume, and the large sum required to cover that risk, the work
Fig. 5.—Shield used in the Tunnel Excavation.
was performed by the company, only the material being contracted for.
The opening ceremonies were attended with much éclat, as became the completion of such a work, uniting not simply two towns but two nations, and rendering possible a greatly increased international trade when the tariff barriers which now stand in the way are removed. It was proposed to spread the banquet in the tunnel, beneath the waters of the St. Clair, with the Governor-General of Canada seated on one side of the international boundary line and the President of the United States on the other, but this part of the programme had to be abandoned. The banquet, to which three hundred guests sat down, after they had passed through and formally opened the tunnel, brought together a greater number of notable men in the world of science, literature, and politics than had ever before gathered in a similar manner in Canada. Speaking of tariff barriers recalls the fact that the sections for the ends of the tube were made in different—places those for the Canada end in Hamilton, and for the United States end in Detroit—so as to avoid the payment of duty.
To Joseph Hobson, a native Canadian, is due, more than to any other man, the successful completion of this great work. He was its architect, designer, and builder, and though his proposals did not, at the outset, meet with much encouragement from engineers, the result fully justifies the confidence reposed in him by Sir Henry Tyler, President of the Grand Trunk; Sir Joseph Hickson, its former general manager; and Mr. Seargeant, Sir Joseph's successor, all of whom ably seconded Mr. Hobson. It is a fact worthy of note that Mr. Hobson received all his professional
Fig. 6.—Entrance to Tunnel.
training on the continent of America, never having been farther east than the city of Quebec. He is a member of the Institutes of Civil Engineers of England, America, and Canada, and has established his right to rank among the first engineers of the world.
The successful completion of the St. Clair Tunnel will doubtless be followed by the construction of many similar works. In 1872, when the Great Western Railway of Canada—now a part of the Grand Trunk—was an independent line, tests were made for a tunnel under the Detroit River, and a drainage tunnel excavated for some distance. Quicksand was met, and, the shield and iron tube not having been adopted for tunnel work, it had to be abandoned. The project has been revived, and if, on fuller investigation, the conditions are found favorable and the work carried out, there will be a tunnel over twelve thousand feet long and twenty-seven feet in diameter, to accommodate two tracks. The Michigan Southern has also been making tests at its crossing, a short distance below Sarnia, but the strata are not favorable for tunnel construction.
Fig. 7.—Interior of Tunnel.
One of the remarkable features in connection with the St. Clair Tunnel is the rapidity with which it was constructed. The average advance was 455·4 feet per month. Contrast this with the Thames Tunnel, three thousand six hundred feet long, which was commenced in 1825 and not completed until 1843, though work was, it is true, suspended for a time. A curious incident, bearing on the rapidity of construction, is related. A cooper, who could not obtain work at his own trade, applied for employment, and was put with the excavators in the shield. He was not accustomed to the use of the spade or shovel, the drawknife being his tool. It was hard work digging the tenacious clay with a spade, the only effective tool in its removal being a long, narrow spade, such as tile-ditchers use in England. The next day the cooper appeared with a drawknife of semicircular form, about six inches across, and, despite the jokes of his fellow-workmen, set to work with it. It was soon found that he could shave away the clay much more rapidly than it could be dug out. All the workmen were soon provided with drawknives, and it is probable that tool has come to stay as a means of tunneling in sticky clay.
The accompanying illustrations will give an idea of the character, progress, and appearance of the work after completion.