RAILWAY CONSTRUCTION. 83 



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 under- 

 taking. 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 pro- 

 portion 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-Lvel 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 



