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Bridging the Missouri

Among the many interesting problems now presented to the engineering profession, none is of more importance or greater interest than the building of a great bridge. The rapid extension of railway traffic demands an unbroken continuity of lines from ocean to ocean, while it is equally important that our great rivers shall not be bridged in such a way as to impair their usefulness as navigable streams. The Ohio is already spanned by the high railroad bridge at Steubenville and by Mr. Roebling’s magnificent suspension bridge at Cincinnati, while the Louisville bridge, a mile in length, is approaching completion. The Mississippi is crossed at Rock Island, Clinton, Burlington, Quincy, and Dubuque, by low bridges with draws, the pivot-draws of the three last-named being the longest in the world, each affording two boatways 100 feet wide in the clear. But the greatest and most unmanageable river of all has, till lately, been left untouched; the wild Missouri, which, sweeping down with its rapid course from the mountains to form the chief tributary of the great river be- low, affords within its own limits no less than 3200 miles of unbroken inland navigation, has been justly regarded as the most difficult of all streams to bridge. In ordinary times no river will disappoint a stranger more than the Missouri; in many places barely 1000 feet wide, and in some even less, with its muddy water, its crooked channel winding through the bottom-lands from bluff to bluff, and its sandy banks often forming desert expanses hundreds of acres in extent, the Missouri seems equally devoid of beauty and of grandeur. But these very attributes which do most to conceal its magnitude are the real indications of its power; the muddy water tells of its distant source among the disintegrating rocks of the Yellow- stone and the upper river; the narrow channel is due to the rapid current, seldom ,less than three, and at times as much as eight or ten miles a hour; while the great wastes of sand-bar are but the vestiges of its work in time of flood. The bottom consists of a light, shifting sand, rapidly washed out in a current, and continually deposited in slack water, sand-bars being often scoured out to a depth of 25 or 30 feet in two or three days, and again filled up in about as short a time. The rapid current is continually abrading the banks, and, as the governing form of the shore is changed, causes the channel to shift from shore to shore, thus entirely altering the local topography of the stream. The abrasions are attended by corresponding deposits on the opposite side, and hence arise the great sand-bars, submerged only during floods, dry and dusty through the fall and winter, but gradually raised by successive deposits until they reach above the ordinary high water, then, in time, covered with a growth of trees, and formed into a part of the wooded bottom-lands. The changes of channel are mostly rapid, occurring during a sudden rise of the water. A single flood will sometimes cut 500 feet in to the bank, while the farmer on the river-shore knows, to his sorrow, -by the loss of his farm, the truth of the local saying that the Missouri has a standing mort- gage on the entire bottom, from bluff to bluff. These changes have been severely felt by some of the river towns. Weston, Mo., fifteen years ago the principal port of that part of the country, can now be reached by steamboats only during the highest water. Forrest City, a hundred miles above, shared the same fate in 1866, and the same flood, by a sudden cut-off in Iowa, shortened the river nearly a dozen miles. Ten years ago, a bridge over the Missouri was regarded in a similar light as the abolition of slavery— as a great thing, which, as yet, must be hoped for rather than tried. The problem is a double one; foundations must be put in which will stand when the sandy bottom is washed away, and the channel must be fixed; or the unconfined river, even if it does not leave the bridge, may change its direction so greatly as to endanger navigation by throwing the current obliquely against the piers. The ordinary crib and pile foundations are not to be thought of; a crib is useless on an unstable bottom, and piles, when ex- posed to such a scour, are little better. Work can be prosecuted in the main channel only during the low- water season, when the deposit of sand is the greatest; and piles driven to refusal at that time might be washed out in a day by the next flood—a catastrophe which, though it might be somewhat retarded, could scarcely be prevented by the generous use of rip-rap stone, as the strongest current has been known to wash away 5 or 10 feet of broken stone. The only safe course for the channel piers is to carry solid foundations down through the sand to the underlying strata of rock, or, in case no rock is to be found, to a depth somewhat below the greatest possible scour. The location of the structure and protection of the banks, though less recondite, is equally important. Wherever the river impinges upon a rocky bluff, the channel is naturally confined in one direction, while the addition of a concave, rocky shore above would render the course of the river practically stable. These two conditions, however, can seldom be found united, the course of the bluffs being generally constant and rarely presenting a concave face to the river; hence it becomes necessary to place a bridge where it shall have the advantage of one such condition, and to secure the other by artificial protection. Three bridges are now building’ across the Missouri: at Kansas City, St. Charles, and Omaha. That at Kansas City is the pioneer bridge of the river, and Mr. 0. Chakute, the chief-engineer, will have the honor of being the first man to erect a permanent structure over the Missouri. This bridge, on which work was actually begun in September, 1867, is being constructed by the Kansas City and Cameron Railroad Co., (now practically consolidated with the Hannibal and St. Joseph Railroad Co.,) for the purpose of bringing the railroad into the city, and thereby effecting close connections with the important roads running West and South from that point. It is located about a mile east of the Kansas and Missouri State line, and just below a long bend in the river. The shore at the south abutment is a rocky bluff; and the channel is held close to that bluff by strong rip-rap protection on the concave shore above. The bridge consists of five fixed river spans, the largest being 250 feet long, and a short shore span over the Missouri Pacific Railroad, and a pivot-draw of the same dimensions as those used on the last built Mississippi bridges. The total length of the bridge proper is 1,395 feet, to which is to be added a trestle 2,300 feet long in the northern approach, making the effective winter-way in time of extreme flood, when the bottom-lands are overflowed, 3700 feet, or a little less than three quarters of a mile. The four channel piers rest directly upon the rock. which is found at depths varying from 15 to 45 feet; below low-water mark. The three other piers, located upon the sand-bar, rest upon piles driven in excavated pits, and cut off at a safe distance below the surface;: the excavation of the pier nearest the channel being: carried thirty feet lower than the sand, and the piles then driven to rock. The stone used in the masonry is quarried in the neighborhood, and differs somewhat in character, the ashlar courses being for the most part of blue limestone, though considerable quantities of white oolite and other varieties of limestone are used for backing and below the water-line. Though designed primarily for a railroad bridge, the structure is intended to serve also for the accommodation of high- way traffic; the floor is laid with Nicolson pavement, over which a heavy street-rail is carried, and carriages will be allowed to cross at all times, except when trains are crossing or approaching. The superstructure is n composite truss of wood and iron, excepting the draw,, which is to be entirely of iron; and fitted with a steam- engine to turn it. The St. Charles bridge, of which Mr. S. SHALER SMITH is the chief-engineer, is intended for the accommodation of the North-Missouri Railroad, and was at Kansas C begun during the past fall. Here, as at Kansas City, rock is found near the surface on one side of the river, and only at considerable depths n the other. The bridge is to consist of five fixed spans, each over 300 feet long, and will be approached by a long iron trestle. The lower chord of the three central spans is to be 90 feet above low-water mark, thus affording ample height for the passage of steamboats at fill stages of the water. The piers are to be of Grafton stone, surmounted by iron columns, and carried down to the rock. The bridge at Omaha, building in the interest of the Union Pacific Railroad, is just begun. Though 350 miles further up the river than either of the others, its location is such that it will considerably surpass them in size. It is to be a high bridge, with- out a draw, and to consist of ten spans, each 250 feet long. Each pier will be formed of two cast-iron columns, 8 1/2 ft. in diameter, sunk to rock by the pneumatic process, and carried up 70 feet above low water to the bridge-seat, the entire bridge, except the fillings of the columns and one abutment, being of iron. The contract for the iron-work, including the sinking of the columns, has been taken by Mr. BOOMER, of Chicago, and it is intended that the work shall be driven with the same energy which has marked the whole progress of the great railroad of which this bridge will form so important a part.