Athlone Railway Bridge
Opened in 1851 by the Midland Great Western Railway to carry the Dublin-Galway line across the River Shannon at Athlone.
Chief Engineer George Willoughby Hemans, Contractor William Dargan. Ironwork by Fox, Henderson and Co.
Total length 542 ft. There are two wrought iron bowspring spans, each 166 ft long, and between these was a swivelling span (now fixed) to allow the passage of tall vessels. The iron girders were supported on twelve 10 ft diameter cast iron columns. It was initially intended to sink the columns by the vacuum method described below, but this was thwarted by the presence of large boulders in the clay, so the more conventional method was adopted of pressurising with air and manually excavating material from within. Despite this, the work was completed within just 18 months, and the bridge was opened to commercial trains on 1 August 1851. Apart from replacing the cross girders in 1929, little repair work has been required[1]
Sinking the Columns
1850 'INTERESTING EXPERIMENT ON THE NEW IRON BRIGE OVER THE SHANNON.
A very interesting and highly satisfactory experiment was made yesterday on the sinking of one of the cylinders of the new Iron bridge erecting across the Shannon by the Midland Great Western Railway Company, in the presence of G. W. Hemans, Esq., Chief Engineer, J. E. Butler, Esq., District, Charles R. Atkinson, Esq. Resident, and other Engineers connected with the Railway. These cylinders had been previously sunk by excavating and removing the interior, and forced down by their own weight and such additional weight as was found necessary ; but in this instance a method which has been found successful by Messrs. Fox, Henderson and Co., (contractors for the bridge) on many occasions, but which had not hitherto appeared applicable to the structure in question, was resorted to. The well-known property of atmospheric air to press upon a vacuum with a weight dependent on the comparative perfection of that vacuum was the means used in this experiment. Its application may be simply explained by stating that a vessel or air-tight chamber was adapted to the hollow cylinder pile to be sunk there being a means of communication between the chamber and the pile, closed or opened at pleasure. The air, by means pumps, was exhausted from the chamber, and when the internal air was of sufficient rarity to allow the atmosphere to raise a column of mercury 26 inches high, equivalent to a pressure about 13lbs. per square inch, the communication before alluded as existing between the chamber and the cylinder was opened. The air contained within the cylinder or pile then rushing into the attached chamber formed a vacuum below, leaving the atmosphere to press with its due weight on the upper surface of the cylinder. The effect was as though many tons weight had suddenly fallen on it, for the whole rapidly descended between five and six feet into the ground until checked by the obstruction of a piece of timber. The sinking of this cylinder 10 feet in diameter through hard yellow clay did not occupy more than a few seconds. The experiment was made under the direction of J. Milner, Esq., the Contractor's Engineer. The great progress made with this work during the last three months enables a casual spectator to form an idea of the completed structure, and although the light and elegant wrought iron beams are not yet thrown across the wide space between the pillars, shows how much the facilities of navigation have been considered in the design. The situation of the intended new road into this town is now distinctly marked out, bounded on one side by a pier of masonry, which reflects no less credit on Mr. Dargan, the Contractor, than on the Engineer, and the other side by the western piers of the bridge. We need scarcely remark that the new approach to Athlone will add much to its benefit as well as its appearance.'[2]
A description of the method, originally developed by Dr. Potts - Laurence Holker Potts- may be found in the entry for Rochester Bridge.