In an extensive scheme of reconstruction, the Pennsylvania, USA, between 1904 and 1914, a daring step was taken when it was decided to use concrete for most of the work. Tunkhannock Viaduct is claimed to be the largest concrete viaduct in the world
TEN CONCRETE SPANS of 180 feet and two of 100 feet carry the Delaware, Lackawanna and Western Railroad across Tunkhannock Creek, near Scranton, Pennsylvania. The viaduct has a total length of 2,375 feet, including approaches, and it towers 240 feet above the river. The viaduct contains 167,000 cubic yards of concrete and 1,000 tons of reinforcing steel.
CONCRETE has been in use for building construction since the time of the Romans. The drawback of plain concrete is that it is defective in strength under certain stresses, but in recent years this has been remedied by embedding steel bars in the concrete. The resulting material, reinforced concrete, is fast supplanting brick and stone for all kinds of structures.
An interesting example of what the engineer can do with concrete is afforded by the rebuilding work carried out by the Delaware, Lackawanna and Western Railroad, in U.S.A. To eliminate curves, reduce grades and so lower operating expenses over forty miles of its system near Hallstead, Pennsylvania, the company undertook a daring scheme of improvement.
This scheme involved the erection of long viaducts as well as the building of several road bridges, new stations and culverts. All this work was to be carried out in reinforced concrete. At the time the work was executed, between 1904 and 1914, concrete had never before been so extensively used. Over a stretch of forty miles some eighty large and small structures were built. Even the signal boxes were made of concrete. The most notable of these structures are two imposing and graceful viaducts, spanning rivers known as St. Martin’s Creek and Tunkhannock Creek respectively, flowing between steep hills, in fairly wide valleys. St. Martin’s Viaduct is 1,500 feet long, stands 150 feet above the bed of the stream and carries three tracks. It contains 78,000 cubic yards of concrete and 500 tons of steel reinforcing rods. Tunkhannock Viaduct, twenty-two miles west of Scranton, Pa., is claimed to be the largest concrete bridge in the world. It comprises ten spans of 180 feet and two spans of 100 feet, surmounted by small superimposed arches upon which the solid concrete surface of the viaduct and railway track is carried. The viaduct has a total length of 2,375 feet, including approaches, and towers 240 feet above the river. The viaduct contains 167,000 cubic yards of concrete and 1,000 tons of reinforcing steel.
Apart from the fact that the viaduct is built of concrete, it represents a daring and clever piece of work. Obtaining the foundations for the piers proved no light task. They were carried right down to solid rock, which was reached at a depth of from 10 feet to 95 feet below ground surface, making a difference of 300 feet between bedrock and the top of the parapet at the deepest pier.
The piers are 36 ft. 5 in. by 43 ft. 5 in. above and 40 feet by 40 feet below the ground. The foundation for six of the piers was excavated, three to the water line and three to rock by a steam shovel. The shovel, capable of removing a cubic yard of material at one bite, dug its way through the sand and gravel from pier to pier. The material excavated was used for the bed of a narrow-gauge construction track.
Much more elaborate arrangements were necessary for the foundations of the other piers, because of their greater depth. This was because the concrete for the viaduct was laid in lifts. Having been poured in great wooden moulds to a depth of 4 feet, the concrete was allowed two to three days to set before the next pouring. This was to allow the concrete to cool. In large concrete structures to-day engineers do not wait for the concrete to cool in a natural way. It is accomplished by artificial means.
The mould into which the concrete was poured at Tunkhannock was in effect an immense cofferdam sunk on the site to bedrock. As the cofferdam was sunk it was protected by interlocking sheet piling. The deepest foundation required two lengths of 30-feet sheet piling. The piling was driven down into the river bed by a powerful steam hammer. The engineers were anxious to prevent water from reaching the concrete during the pourings. Five feet beyond the foundations, therefore, another enclosure was made by sinking further sheet pilings. Any water collected in the inner or true foundation was carried by pipes to a sump in the outer enclosure. While the pourings were being made, tar paper was placed against the steel sheeting and the cofferdam. When the concrete base rose above the cofferdam the sump was then pumped out and filled with concrete.
A HUGE TIMBER TOWER, 260 feet high, was erected in the centre of Tunkhannock Creek to support the cableway used for handling the building material. The two terminal towers of the cableway were 3,028 feet apart. The central tower measured 40 feet by 60 feet at the base
To handle the material expeditiously the creek was spanned by a cableway. A huge timber tower, 260 feet high and 40 feet by 60 feet at the base, was erected in the centre of the creek to support the cableway. The two terminal towers were 150 feet and 165 feet high respectively and 3,028 feet apart.
The pier forms, consisting of massive timbering, were built in sections 17 ft. 9 in. high. They enclosed the steel reinforcing rods. On their inner face were strips of moulding at 4-feet intervals to form construction joints and the horizontal scorings round the pier. Into these forms concrete was poured in 4-feet lifts — that is to say, the concrete was run into the moulds to a depth of 4 feet, after which it was allowed two to three days to set before any further concrete was added. Each 4-feet lift contained 235 cubic yards of concrete, run in one operation.
After the piers had been built the arches were set up. They are divided into two ribs 14 feet wide and 6 feet apart. The crown thickness is 8 feet. The width of the pier at the springing line is 28 feet. At a distance of 17 ft. 5 in. below the springing line the width of the pier is increased 4 ft. 3 in. on either side.
The centering (framework on which an arch is built) was supported on this ledge. The falsework necessary for carrying the arches consisted of four spans of arched trusses, which were built in sections at the shop, transported to the site and erected by means of the cableway.
The reinforcement for the arches was a light steel girder in the form of a curve, stretched from arch to arch, its two ends being bolted into the masonry of the piers. The steelwork was then encased in wooden moulds into which the concrete was poured and allowed to set. Thus was formed not only what is virtually a solid piece of concrete spanning the entire opening and further strengthened by steelwork, but also an arch built in the form of a “key”. That is to say, the under portions of the blocks of the arch, being smaller than the top portions, not only held them in place, but also gave added strength to the span. The superstructure above the arch consists of eleven arches. Across these is the roadway carrying the railway tracks.
By building this viaduct a distance of just over two miles was saved and some heavy gradients were eliminated. Formerly it took five engines to move a heavy freight train over this route, but the work can now be done at the same speed by only two engines. The time for fast passenger trains was reduced by about twenty minutes.
The two great viaducts, bridges, culverts and other work carried out in connexion with the scheme called for the use of 4,000,000 cubic yards of concrete, and demanded an expenditure of £3,000,000, which was fully justified by the greatly improved operating conditions.
PARTLY COMPLETED CONCRETE ARCHES of Tunkhannock Viaduct. The falsework for carrying the arches during construction consisted of four spans of arched trusses. On top of each completed arch, a superstructure of eleven smaller arches carries the railway tracks.