Slow work that. The Northern Pacific finding its way through the crest of the Cascade Mountains by means of the great Stampede Tunnel, nearly two miles in length, demanded that the contractor work under pressure and make 13-1/2 feet of tunnel a day. The contractor, working under the bonus plan, did better. With his army of 350 "hard-rock men," "muckers," and their helpers, and his tireless battery of 36 drills he sometimes made as high as eighteen feet a day from the two headings. On a three-year job he beat his contract time by seven days. The Northern Pacific paid the price, $118 for each lineal foot of tunnel. That was a high price, occasioned largely by the fact that the work was carried forward in what was then an almost unbroken wilderness. The Wabash finding its way through the great and forbidding hills of Western Pennsylvania to Pittsburgh a dozen years later was able to dig its succession of tunnels at an average cost of $4,509 for 100 feet. Of that amount $2,527 went for labor; and $260 was the price of a ton of dynamite.
When the tunnel engineer finds that his bore is not to pierce hard-rock, of whose solidity he is more than reasonably assured, he prepares to use cutting-shields. These shields, proceeding simultaneously from the portals and from the footings of the shafts, are steel rings of a circumference only slightly greater than that of the finished tunnel. With pick and with drill and dynamite, they constantly clear a path for it, whereupon it is pressed forward in that path. Dummy tracks follow the cutting-shield; and dummy locomotives--more likely electric than steam in these days--are used in removing the material. Electricity has been a boon to latter-day tunnel-workers. Its use for light and power keeps the tunnel quite clear of all gases during the work of boring.
In rare cases, the rock through which the shield has been forced is strong enough to support itself; in most works the engineers prefer to line the bore, with brick and concrete, as a rule. This lining is set in the path of the cutting-shield before its protection is entirely withdrawn; and so the heavy roof-timbering which was formerly a trade-mark of the successful tunnel engineer is no longer used.
Tunnel-boring becomes doubly difficult when the railroad is to be carried under a river or some broad arm of the sea. Men work in an unnatural environment when they work below the surface of great waters, and the record of such work is a record of many tragedies. At any instant firm rock may cease, silt or sand or an underground stream may make its appearance and the helpless workmen find a ready grave. In work where there is even the slightest expectation of such a contingency the air-lock, with its artificial pressure to hold back the soft earth and moisture is brought into use. In another chapter we shall see how the caisson is operated. Suffice it to say now that the necessity of "working under the air," brings no comfort to any one. It vastly hinders and complicates the work of construction, and adds greatly to the expense.
Moreover, it has its own record of tragedies. Still it remains, to the infinite credit of a national persistence, that there is no record in the annals of American engineering where the workers have finally given up a tunnel job. Lives have been sacrificed, good-sized fortunes swept away, but in the end the resistless railroad has always found its underground path.
The tunnel-workers can tell you of the accident when the subway was being driven under the East River from Manhattan to Brooklyn, three years ago.
The cutting-shield, which was advancing from the Brooklyn side, suddenly slipped out from the rock into the unprotected soft mud of the river bottom. The heavily compressed air shot a geyser straight up to the surface of the river some fifty feet above. A workman shot through the geyser, pirouetted gayly for a fraction of a second above the river, then dropped, to be picked up by the crew of a passing ferryboat. In a week he was back at work again inside the cutting-shield. His fortune was the opposite of that which generally awaits a man caught in a tunnel accident.
"It ain't as bad as it used to be," one of them informs you. "When I first got into this profession, they didn't have the electricity for lights or moving the cars or nothing. We used to try and get along with safety lamps an' near choke to death. It was more like hell then than it is now."
[Illustration: "SOMETIMES THE CONSTRUCTION ENGINEER ... BRINGS HIS LINE FACE TO FACE WITH A MOUNTAIN"]
[Illustration: FINISHING THE LINING OF A TUNNEL]
[Illustration: THE BUSIEST TUNNEL POINT IN THE WORLD--AT THE WEST PORTALS OF THE BERGEN TUNNELS, SIX ERIE TRACKS BELOW, FOUR LACKAWANNA ABOVE]
[Illustration: THE HACKENSACK PORTALS OF THE PENNSYLVANIA'S GREAT TUNNELS UNDER NEW YORK CITY]
But your interest in the man who was blown from the tunnel to the surface of the river and escaped with his life is not entirely satiated, and you ask more questions. What do they do when they strike soft mud like that?
"We get down and pray," he of the experience in this weird form of construction engineering tells you. "We try to get the boys safely back through the air-lock, and then we quit boring till we can fix things up from outside. If it's a real bad case we've got to make land to bore through. It's generally done by dumping rock and bags of sand from floats just over where she blows out. It's a pretty rough way of doctoring her up, but it has to go, and generally it does. All we want is to get it to hold until we can set the rings of the tunnel.
"That ain't always the worst. I've been driving a bore under water this way, when we struck stiff rock overhead and soft mud underneath the edge.
That's something that makes the engineers hump. You can't rest a cast-iron tunnel like this on mud and you get a wondering if you've got to quit after all this work under the durned old river, and let the boss lose his money.
"The last time we struck a snag of that sort, the boss didn't give up. He wasn't that kind. He had a chief engineer that was brass tacks from beginning to end. What do you suppose that fellow did? He bored holes in the bottom of the lining and drove steel legs right down to the next ledge of solid rock below. There's that tunnel to-day, carrying 32,000 people between five and six o'clock every night perched down there seventy feet underground like a big caterpillar sprawled under the wickedest ledge o'
rock you ever see."
It takes a real genius of an engineer for this sort of work. He who drives his bore into the unknown must be on guard for the unexpected. Emergencies arise upon the minute, and the tunnel engineer must be ready with his wits and ingenuity to meet them. Finally the day does come when the bores from either shore are hard upon one another. If there has been blasting under the bed of the river it is reduced to a minimum. The drills work at half-speed, the fever of expectancy hangs over the men. Those who are close at the heading catch faint sounds of the workmen on the other side of the thin barrier--the last barrier of the river that was supposed to acknowledge no conqueror.
The first tiny aperture between the two bores is greeted with wild cheers.
On the surface far above, the whistles of the shaft-houses carry forth the news to the outer world; it is echoed and reechoed by the noisy river craft. The aperture grows larger. It is large enough to permit the passage of a man's body; and a man, enjoying fame for this one moment in his life, crawls through it. The men knock off work and have a rough spread in the tunnel. At night the engineers and contractors banquet in a hotel. "Not so bad," the chief engineer says quietly. "We were 3/8 of an inch out, in 8,000 feet." It was not so bad. It spoke wonders for his profession. To carry forth two giant bores from the opposite sides of a broad river, and have them meet within 3/8 of an inch of perfect alignment, was an achievement well worth attention.
After that, the last traces of the rough rock and silt are removed, the iron rings of the tunnel made fast together, the air pressure released, the cutting-shields, that formed so essential a feature of the construction, removed. Then there remains only the work of installing conduits and wiring and laying the tracks before the tunnel is ready for the traffic of the railroad.
The Michigan Central has recently finished a tunnel under the busy Detroit River, at Detroit, which eliminates the use of a car-ferry at that point.
The tunnel was built in a manner entirely new to engineers. The river at Detroit is about three-quarters of a mile wide, and its bed is of soft blue clay, making it difficult to bore a tunnel safely and economically.
To meet this obstacle a new fashion of tunnel-building was created.
The tunnel itself consists of two tubes, each made from steel 3/8 of an inch in thickness and reinforced every twelve feet by outer "fins." The channel was dredged and a foundation bed of concrete laid. The sections of the tunnel, each 250 feet long, were then put in position one at a time.
The section-ends were closed at a shore plant with water-tight wooden bulkheads. They were then lashed to four floating cylinders of compressed air and towed out to position. After that it was merely a matter of detail to drop the sections into place, pour in more concrete and make the new section fast. The wooden bulkheads next the completed tube were then removed and the structure was ready for the track-layers. The sub-aqueous portion of the new Detroit Tunnel is 2,600 feet long; it joins on the Detroit side with a land tunnel 2,100 feet long, and on the Canadian side with a land tunnel of 3,192 feet.
It takes more than a river, carrying through its narrow throat the vast and growing traffic of the Great Lakes--a traffic that is comparable with that of the Atlantic itself--to halt the progress of the railroad.
CHAPTER V
BRIDGES
BRIDGES OF TIMBER, THEN STONE, THEN STEEL--THE STARUCCA VIADUCT--THE FIRST IRON BRIDGE IN THE U. S.--STEEL BRIDGES--ENGINEERING TRIUMPHS--DIFFERENT TYPES OF RAILROAD BRIDGE--THE DECK SPAN AND THE TRUSS SPAN--SUSPENSION BRIDGES--CANTILEVER BRIDGES--REACHING THE SOLID ROCK WITH CAISSONS--THE WORK OF "SAND-HOGS"--THE CANTILEVER OVER THE PEND OREILLE RIVER--VARIETY OF PROBLEMS IN BRIDGE-BUILDING--POINTS IN FAVOR OF THE STONE BRIDGE--BRIDGES OVER THE KEYS OF FLORIDA.
When the habitations of man first began to multiply upon the banks of the water courses, the profession of the bridge-builder was born. The first bridge was probably a felled tree spanning some modest brook. But from that first bridge came a magnificent development. Bridge-building became an art and a science. Men wrought gigantic structures in stone, long-arched viaducts, with which they defied time. Then for two thousand years the profession of the bridge-builder stood absolutely still.
With the coming of the iron and steel age it moved forward again. The development of a fibre of great strength and without the dead weight of granite gave engineers new possibilities. They began in simple fashion, and then they developed once again, with marvellous strides. Steel, the dead thing with a living muscle, could span waterways from which stone shrank. Steel redrew the maps of nations. Proud rivers at which the paths of man had halted, were conquered for the first time. Routes of traffic of every sort were simplified; the railroad made new progress; and economic saving of millions of dollars was made to this gray old world.
The earliest of the very distinguished list of American bridge-builders erected great timber structures for the highroads and the post-roads. Some of them went back many centuries and came to the stone bridge, in many ways the most wonderful of all the artifices by which man conquers the obstructive power of a running stream. But the building of stone bridges took time and money, and time and money were little known factors in a new land that had begun to expand rapidly.
So at first the railroad followed the course of the highroad and the post-road, and took the timber bridge unto itself. In some cases it actually fastened itself upon the highroad bridge, as at Trenton, N. J., where a faithful wooden structure built by Theodore Burr in 1803 was strengthened and widened in 1848 to take the first through railroad route from New York. It continued its heavy dual work until 1875 when it was superseded by a steel bridge. A dozen years ago the railroad tracks were moved from that structure to a magnificent and permanent stone-arch built near-by. Thus the railroad crossing the Delaware at Trenton has, in this way, typified step by step every stage of the development of American bridge-building.
The timber bridges developed the steel truss bridge, the typically American construction, of to-day. In an earlier day the timber bridges were the glory of the engineer. Sometimes you see one of these old fellows remaining, like the long structure that Mr. Walcott built across the Connecticut River at Springfield, Mass., in 1805, and which still does good service; but the most of them have passed away. Fire has been their most persistent enemy. Within the past two years fire destroyed the staunch toll-bridge at Waterford on the Hudson, just above Troy. The bridge was a faithful carrier for one hundred and four years. In many ways it was typical of those first constructions. It consisted of four clear arch spans--one 154 feet, another 161 feet, the third 176 feet, and the fourth 180 feet in length. It was built of yellow pine, wonderfully hewn and fitted, hung upon solid pegs; and save for the renewal of some of the arch footings, the roof, and the side coverings, it was unchanged through all the years--even though the heavy trolley-cars of a through interurban line were finally turned upon it.
About the same time, the once-famed Permanent Bridge across the Schuylkill River at Philadelphia was built. It had two arches of 150 feet each and one of 195 feet. In its day it was regarded as nothing less than a triumph. A very old publication says:
"The plan was furnished by Mr. Timothy Palmer, of Newburyport, Mass., a self-taught architect. He brought with him five workmen from New England. They at once evinced superior intelligence and adroitness in a business which was found to be a peculiar art, acquired by habits not promptly gained by even good workmen in other branches of framing in wood.... The frame is a masterly piece of workmanship, combining in its principles that of king-post and braces or trusses with those of a stone arch."
In after years, the Permanent Bridge was also entrusted with the carrying of a railroad. It has, however, disappeared these many years.
The early railroad builders did not neglect the possibilities of the stone bridge. Two notable early examples of this form of construction still remain--the Starrucca Viaduct upon the Erie Railroad, near Susquehanna, Pa., and an even earlier structure, the stone-arch bridge across the Patapsco River at Relay, Md., which B. H. Latrobe, the most distinguished of all American railroad engineers, built for the Baltimore & Ohio Railroad, in 1833-35. The Thomas Viaduct, as it has been known for three-quarters of a century, was the first stone-arch bridge ever built to carry railroad traffic. It was erected in a day when the railroad was just graduating from the use of teams of horses as motive-power. In this day, when locomotives have begun to reach practical limits of size and weight, that viaduct is still in use as an integral part of the main line of the Baltimore & Ohio. It is built on a curve, and consists of 8 spans of stone arches, 67 feet 6 inches, centre to centre of piers, which, together with the abutments at each end, make the total length of the structure 612 feet. It is in as good condition to-day as upon the day it was built.
When the Erie Railroad was being constructed across the Southern Tier counties of New York in 1848, its course was halted near the point where the rails first reached the beautiful valley of the Susquehanna. A side-valley, a quarter of a mile in width, stretched itself squarely across the railroad's path. There was no way it could be avoided, and it could be crossed only at a high level. For a time the projectors of the Erie considered making a solid fill, but the tremendous cost of such an embankment was prohibitive. While they were at their wits' ends, James P.
Kirkwood, a shrewd Scotchman, who had been working as a civil engineer upon the Boston & Albany, appeared. Kirkwood spanned the valley with the Starucca Viaduct, one of the most beautiful bridges ever built in America.
He opened quarries close at hand and by indefatigable energy built his stone bridge in a single summer. It has been in use ever since. The increasing weight of its burdens has never been of consequence to it, and to-day it remains an important link in a busy trunk-line railroad. It is 1,200 feet in length and consists of 18 arches of 50 feet clear span apiece.
But stone bridges even then cost money, and so the timber structure still remained the most available. Many men can still remember the tunnels, into whose darkness the railroad cars plunged every time they crossed a stream of any importance whatsoever. They have nearly all gone. The wooden bridge was ill suited to the ravages of weather and of fire--ravages that were quickened by the railroad, rather than hindered. A substitute material was demanded. It was found--in iron.
The first iron bridge in the United States is believed to be the one erected by Trumbull in 1840 over the Erie Canal at Frankfort, N. Y. Record is also held of one of these bridges being built for the North Adams branch of the Boston & Albany Railroad, in 1846. About a year later, Nathaniel Rider began to build iron bridges for the New York & Harlem, the Erie, and some others of the early railroads. His bridges--of the truss type, of course, that type having been worked out in the timber bridges of the land--were each composed of cast-iron top-chords and post, the remaining part of the structure being fabricated of wrought-iron. The members were bolted together. Still, the failure of a Rider bridge upon the Erie in 1850, followed closely by the failure of a similar structure over the River Dee, in England, influenced officials of that railroad to a conclusion that iron bridges were unpractical, and to order them to be removed and replaced by wooden structures. For a time it looked as if the iron bridge were doomed. That was a dark day for the bridge engineers. A contemporary account says:
"The first impulse to the general adoption of iron for railroad bridges was given by Benjamin H. Latrobe, chief engineer of the Baltimore & Ohio Railroad. When the extension of this road from Cumberland to Wheeling was begun, he decided to use this material in all the new bridges. Mr. Latrobe had previously much experience in the construction of wooden bridges in which iron was extensively used; he had also designed and used the fish-bellied girder constructed of cast and wrought-iron."
Under the influence of the really great Latrobe, an iron span of 124 feet was built in 1852 at Harpers Ferry. In that same year, the B. & O. built its Monongahela River Bridge, a really pretentious structure of 3 spans of 205 feet each, and the first really great iron railroad bridge in all the land. The path was set. The conquest of iron over wood as a bridge material was merely a problem of good engineering. The iron bridge quickly came into its own. The Pennsylvania Railroad began building cast-iron bridges of from 65 to 110 feet span at its Altoona shops for the many creeks and runs along the western end of its line. The other railroads were following in rapid order. Squire Whipple, Bollman, Pratt--all the others who could design and build iron bridges--were kept more than busy by the work that poured in upon them.
And in the day when the iron bridge was coming into its own, Sir Henry Bessemer, over in England, was bringing the steel age into existence, first making toy cannon models for the lasting joy of Napoleon III, and then making a whole world see that steel--that dead thing with the living muscle--was no longer to be limited for use in tools and cutting surface.
Steel was to become the very right-hand of man. And so steel came to the bridge-builders, at first only in the most important wearing points such as pins and rivets, finally to be the whole fabric of the modern bridge.
The transition was gradual. The early engineers began using less and less of cast-iron and more and more of wrought, until they had practically eliminated cast-iron as a bridge material. Then there came a quick change; there was another dark day for the railroad bridge engineers of America.
In 1876--that very year when the land was so joyously celebrating its Centennial--a passenger train went crashing through a defective bridge at Ashtabula, Ohio. There was a great property loss--thousands and thousands of dollars, and a loss of lives that could never be expressed in dollars.
An outraged land asked the bridge-builders if they really knew their business.
Out of that Ashtabula wreck came the scientific testing of bridges and bridge materials, and the abolition of the rule-of-thumb in the cheaper sorts of construction. Out of that miserable wreckage came also the use of steel in the railroad bridge. Steel had found itself; and how the steel bridges began to spring up across the land! They spanned the Ohio, and they spanned the Mississippi, and they spanned the Missouri; a great structure threw itself over the deep gorge of the Kentucky River. When the day came that fire destroyed the famous wooden viaduct of the Erie over the Genesee River at Portage, N. Y. (you must remember the pictures of that tremendous structure in the early geographies), steel took its place.
All this while the bridge engineer attempted more and more. He built over the deep gorge of the Niagara. He conquered the St. Lawrence in and about Montreal. He laughed at the mighty Hudson and flung a dizzy steel trestle over its bosom at Poughkeepsie. He built at Cairo, at Thebes, and at Memphis, on the Mississippi, and again and again and still again at St.
Louis. The East River no longer halted him or compelled him to resort to the alternative of the very expensive types of suspension bridge. He has finally thrown a great cantilever over it, from Manhattan to Long Island.