RAILWAY CONSTRUCTION. 129 



and employ divers to cut off the tops a little above the bed of 

 the river. 



In preparing the design for a large foundation it is absolutely 

 necessary to first ascertain by careful borings the description of 

 material upon which that foundation must be placed, so as to 

 proportion the area of bearing surface to the weight to be 

 sustained. Some materials will naturally carry more weight 

 than others, and although the engineer cannot always select the 

 material he would prefer, he can, however, control the superficial 

 area of the foundations. Much valuable information has been 

 obtained both from experiments and from comparisons of actual 

 practice, and the following memoranda may be useful for 

 reference, as indicating the pressures per superficial foot which 

 may be safely put on various materials : 



Moderately stiff clay 2 tons. 



Chalk 4 



Solid blue clay 5 



Compact gravel and close sand 6 



Solid rock 12 



Doubtless the above weights have been exceeded in many 

 cases, but it is better to be on the safe side, and leave a good 

 margin for stability. 



Large subaqueous foundations for heavy piers and abutments 

 are costly and tedious, and especially so when the pneumatic 

 process has to be adopted. Special appliances and well-trained, 

 experienced workmen are requisite, and if all the men and 

 materials have to pass through the air-locks, the progress of the 

 work must necessarily be slow. When the foundations have 

 been completed up to the level of the water, the construction 

 can be pushed on more rapidly, as the work of scaffolding, 

 hoisting, and building, can all be carried on in the open air. 



Amongst the very many types of arch-work and girder- work 

 adopted for railway purposes, the following examples from actual 

 practice may be useful for reference : 



Fig. 126 represents small 24-foot span, low viaduct arching 

 suitable for a line passing through towns or villages, where 

 ground is valuable and the area to be covered must be kept as 

 small as possible. The arches may be utilized for stables, stores, 

 or roads of communication between the lands and properties 

 intersected by the railway. The segmental form gives a better 

 headway underneath than the semicircular, besides containing 

 less material in the arching proper, and requiring a smaller 



K 



