RAILWAY CONSTRUCTION. in 



similar in character to the public road bridges, but of much less 

 width for the roadway. Those over the railway must have the 

 standard span and height adopted as a minimum for the other 

 over-line bridges, and those under the railway must have the 

 full width on the top for the lines of rails, but will have less 

 width between the abutments for the roadway. 



Foundations. So much depends upon the soundness and 

 security of the foundations of any bridge, viaduct, or large 

 building, that it would be almost impossible to devote too much 

 care to the selection and treatment. Unless the foundation be 

 firm, the entire structure will be exposed to the risk of failure, 

 etther in subsidence of masonry, giving way of arches, or 

 depression of girders. A small matter overlooked during the 

 construction of this part of the work will be most difficult to 

 correct or adjust afterwards. 



The insistent weight of all structures built of masonry or 

 brickwork will cause the mass to settle to a certain extent, 

 according as the joints of mortar or cement become compressed 

 by the number of superincumbent courses. In a similar manner 

 the gravel and clay of a foundation will compress more or less 

 according to its compactness and the weight of the structure. 

 No inconvenience will, however, arise if the settlement or com- 

 pression be uniform throughout the entire area. 



In ordinary average, dry, solid ground, a good foundation 

 can usually be obtained at a moderate depth. The removal of a 

 few feet of the surface layers will generally lead to a good hard 

 stratum of natural material sufficiently firm to carry the abut- 

 ments and piers of railway bridges and viaducts. Two or more 

 footings are usually adopted so as to distribute the weight over 

 an increased area, as shown in Fig. 108. 



Where the weight to be carried is considerable, it is better to 

 increase the number of the footings, and give them a smaller 

 projection, as in Fig. 109, rather than have a lesser number and 

 greater projection, as in Fig. 108. There is greater liability of 

 fracture of the material in the latter than in the former. 



Care must be taken to distinguish between made ground and 

 natural ground. Hollows which have been filled in must not be 

 relied upon to sustain heavy weights ; the material may have 

 been consolidating for years, but it is safer to cut through it 

 and found upon the natural stratum beneath. 



Soils of a clayey nature must be dealt with very cautiously 



