RAILWAY CONSTRUCTION. 93 



to continue onward the rail-level at the height already attained. 

 A decision to adopt the latter course brings forward the con- 

 sideration as to the method of carrying out the work. To 

 form a high embankment across such a valley would entail 

 an enormous expenditure for earthwork, and several openings, 

 or bridges, would have to be made in the embankment for 

 streams, rivers, and roadways. Instead, therefore, of making 

 this part of the line entirely of embankment, it is usual to 

 carry the earthwork forward until the height is about 25 or 

 30 feet, and to form the remainder of the opening of arching, 

 as shown in Fig. 91. 



This arrangement is not only less costly than an embank- 

 ment of such height, but has also the great advantage that 

 any or all of the arches are available for the passage of streams, 

 rivers, roads, and accommodation works. 



The character of the work to be carried out in the con- 

 struction of bridges or viaducts over rivers or valleys must 

 greatly depend upon the description of materials at command. 

 Where good building-stone is plentiful, and the price of labour 

 moderate, works of masonry should be adopted as far as 

 practicable. Brickwork is an excellent substitute for masonry, 

 provided that specially selected bricks are used for all facework, 

 or parts exposed to the weather. For water-washed piers and 

 abutments, the lower portion should be faced with good hard 

 stone. 



Bridges and viaducts consisting of arches of masonry or 

 brickwork form the most substantial and permanent works of 

 construction for railway purposes ; once properly built, the 

 expenditure on future maintenance or repairs is merely nominal. 

 For viaducts the span of the arching must be regulated by 

 the height of the viaduct. The greater the height the larger 

 the span. In one case 30-feet spans may be suitable, whereas 

 in another it may be more economical to introduce spans of 

 60 feet or more, and so reduce the number of lofty piers. From 

 a cost point of view there is, however, a limit to the span 

 of arching, and, except for special cases, where expenditure is 

 of secondary importance, large spans are very rarely adopted. 

 Arches of large spans, no doubt, have been built both in masonry 

 and brickwork, and have been a complete success in every 

 way except expense. Unfortunately, the quantity and weight 

 of materials in arching, and the corresponding cost, increase 



