Where a low-level viaduct with opening bridge can be adopted, there will be a very great saving of expenditure; and there are numbers of such viaducts in existence, accommodating a large railway and river traffic without inconvenience. Even with a low-level viaduct the height from water-level to the
under side of the girders of the various fixed-spans will generally be sufficient for the passage of barges and small craft, leaving the opening portion to be used by the larger vessels.
The principal openings for these large river viaducts are generally constructed for girders, partly on account of the greater facility of girder work for large spans, and also for the advantage of having one uniform height, or headway, from pier to pier.
For a high-level viaduct across a deep-water river, the cost of the lofty piers forms a very important part of the undertaking. Each pier will require its own cofferdam, caisson, or other appliance for obtaining a suitable foundation. The deeper the water, the more costly the arrangement for foundation; and the higher the pier to rail-level, the greater the amount of material in the construction of the pier. The consideration of these two points will at once show that it is very desirable not to have more of these costly piers than is actually necessary, and in studying out the design it will be a question for calculation how far the spans may be increased so as to dispense with one or more piers.
In every work of this description there is a relative proportion between span and height, which will give the most economical result from a cost point of view; the proportion varying according to the depth of the water and description of ground for foundations. An increase in the span will naturally necessitate an increase in the thickness of the pier; but where a cofferdam, or arrangement for putting in the foundations, must in any case be made, a small addition to its width may not necessarily form a large increase to its cost.
[Figs. 74, 75, 76, and 77] are sketches of high-level railway viaducts which have been constructed with great height, or headway, to allow large vessels to pass under at all times without interruption. This description of work is very costly, not only in the deep-water foundations, but also in the heavy scaffolding and appliances requisite for building piers and girders at such an elevation above the ground-level. The hoisting of the material alone forms an important item where such vast number of pieces have to be lifted to a height of 80, 90, or 100 feet.
[Figs. 78 and 79] are sketches of low-level viaducts constructed with one large opening span, or swing-bridge, for the passage of vessels. The girders and roadway of such opening span are