Page:Cassier's Magazine Volume XV.djvu/18

6 ultimately the abandonment of the scheme of Sir Thomas Bouch.

After the fall of the Tay Bridge, Mr. Arrol spent some time in examining the old structure and in preparing plans and submitting proposals for rebuilding it. His idea was to surround the old cast iron columns with others of steel, and to connect these new columns securely together by sufficient bracing; but it was found on further examination that the foundations of the old bridge were not so secure as was essential in a structure of this kind. It was, therefore, ultimately decided to discard the



whole of the old structure and to build an entirely new bridge a short distance further up the river. The new scheme was passed through Parliament, under the guidance and direction of Mr. W. H. Barlow as engineer.

While this was in progress Mr. Arrol was constructing a viaduct over the South-Esk at Montrose, and with a view to gaining experience he adopted the novel method of sinking the cylinders from a pontoon carried by four legs resting on the bottom. The pontoon and legs were so arranged that the pontoon could be lowered, the legs drawn up off the bottom, and the whole floated to a new position, where it was the work of a very short time to drop the legs on the bottom again and raise the pontoon sufficiently clear of the water to allow sinking operations to again commence on the site of any new pier.

When Mr. Arrol's tender for the new Tay viaduct was accepted, he immediately made arrangements for starting the work. His experience with the pontoon at Montrose was such as to decide him to adopt pontoons for the sinking of the cylinders of the new structure. These pontoons were very much larger than that used at Montrose, and contained all the plant necessary for the operations performed at each pier. Thus, the cylinders were built and lowered into position by a hydraulic apparatus from the deck of the pontoon. After the cylinders had been placed in position, the diggers were set to work, these being worked by steam cranes, also resting on the pontoon itself, and when the sinking operations were completed, concrete mixers, having for a platform a part of the pontoon, were employed, and the concrete was filled into the cylinders as required. These various operations being finished, the pontoon was lowered and floated to another pier. The general arrangement of the whole is seen in the illustration on page 16.

Several pontoons were adopted in the building of the new Tay Bridge, and in this work a considerable advantage was gained from the wreckage of the old structure being near at hand, as many of the operations in connection with the new structure were conducted from the old, although the main girders were the only portions of the old bridge actually wrought into the new. These girders were transferred from the old bridge by cranes in the case of the smaller girders, and in the case of the larger girders they were lifted by pontoons bodily off the old bridge and floated and lowered into their final position. A roadway was thus made on