The Central viaduct, now under construction in the city of Cleveland, is probably the longest structure of the kind devoted entirely to street traffic. The superstructure is in two distinct portions, separated by a point of high ground. The main portion, extending across the river valley from Hill street to Jennings avenue, is 2,840 feet long on the floor line, including the river bridge, a swing 233 feet in length; the other portion, crossing Walworth run from Davidson street to Abbey street, is 1,093 feet long. Add to these the earthwork and masonry approaches, 1,415 feet long, and we have a total length of 5,348 feet. The width of roadway is 40 feet, sidewalks 8 feet each. The elevation of the roadway above the water level at the river crossing is 102 feet. The superstructure is of wrought iron, mainly trapezoidal trusses, varying in length from 45 feet to 150 feet. The river piers are of first-class masonry, on pile and timber foundations. The other supports of the viaduct are wrought iron trestles on masonry piers, resting on broad concrete foundations.
The pressure on the material beneath the concrete, which is plastic blue clay of varying degrees of stiffness mixed with fine sand, is about one ton per square foot.
The Cuyahoga valley, which the viaduct crosses from bluff to bluff, is composed mainly of blue clay to a depth of over 150 feet below the river level. No attempt is made to carry the foundation to the rock. White oak piles from 50 to 60 feet in length and 10 inches in diameter at small end are driven for the bridge piers either side of the river bed, and these are cut off with a circular saw 18 feet below the surface of the water. Excavation by dredging was made to a depth of 3 feet below where the piles are cut off to allow for the rising of the clay during the driving of the piles. The piles are spaced about 2 feet 5 inches each way, center to center. The grillage or platform covering the piles consists of 14 courses of white oak timber, 12 inches by 12 inches, having a few pine timbers interspersed so as to allow the mass to float during construction. The lower half of the platform was built on shore, care being taken to keep the lower surface of the mass of timber out of wind. The upper and lower surfaces of each timber were dressed in a Daniels planer, and all pieces in the same course were brought to a uniform thickness. The timbers in adjacent courses are at right angles to each other.
The lower course is about 58 feet by 22 feet, the top course about 50 by 24 feet, thus allowing four steps of one foot each all around. The first course of masonry is 48 feet by 21 feet 8 inches; the first course of battered work is 41 feet 8½ inches by 16 feet 3 inches. Thus the area of the platform on the piles is 1,856 square feet, and of the first batter course of masonry 777.6 square feet, or in the ratio of 2.4 to 1. The height of the masonry is 78 feet above the timber, or 73½ feet above the water. The number of piles in each foundation is 312. The average load per pile is about 11 tons, and the estimated pressure per square inch of the timber on the heads of the piles is about 200 pounds.
To prevent the submersion of the lower courses of masonry during construction, temporary sides of timber were drift-bolted to the margin of the upper course of the timber platform, and carried high enough to be above the surface of the water when the platform was sunk to the head of the piles by the increasing weight of masonry.
The center pier is octagonal, and is built in the same general manner as to foundations as the shore piers, but the piles are cut off 22 feet below water, and there are eighteen courses of timber in the grillage. The diameter of the platform between parallel sides is 53 feet, while that of the lower course of battered masonry is but 37 feet. The areas are as 2,332 to 1,147, or as 2 to 1 nearly. The pressure per square inch of timber on the heads of the piles is about the same as stated above for the shore piers. The number of piles under the center pier is 483.
The risks and delays by this method of constructing the foundations were much less, and the cost also, than if an ordinary coffer dam had been used. Also the total weight of the piers is much less, as that portion below a point about two feet below the water adds nothing to their weight.
The piles were driven with a Cram steam hammer weighing two tons, in a frame weighing also two tons. The iron frame rests directly upon the head of the pile and goes down with it. The fall of the hammer is about 40 inches before striking the pile. The total penetration of the piles into the clay averaged 27 feet. The settlement of the pile during the final strokes of the hammer varied from one quarter to three quarters of an inch per blow.
There are 122 masonry pedestals, of which eight are large and heavy, carrying spans of considerable length. They will all be built upon concrete beds, except a few near the river on the north side, where piles are required.
The four abutments with their retaining walls are of first-class rock-faced masonry. The footing courses are stepped out liberally, so as to present an unusually large bottom surface. They rest on beds of concrete 4 feet thick. The foundation pits are about 50 feet below the top of the bluffs, and are in a material common to the Cleveland plateau, a mixture of blue sand and clay, with some water. The estimated load of masonry on the earth at the bottom of the concrete is one and seven tenths tons to the square foot. Two of the large abutments were completed last season. They show an average settlement of three eighths of an inch since the lower footing courses were laid.
The facts and figures here given regarding the viaduct were kindly furnished by the city civil engineer, C.G. Force, who has the work in charge. - Jour. Asso. of Eng. Societies.