RAILWAY CONSTRUCTION. 75 



reconstructed in a more substantial manner. It is a simple and 

 comparatively inexpensive matter to rebuild a drain before the 

 earth filling is brought forward, but it is a costly work to open 

 out an embankment, and rebuild a culvert afterwards. Unless 

 the seat of an embankment is well drained and kept free from 

 the accumulation of running water, the earthwork will be 

 exposed to washing away of the lower layers, and consequent 

 subsidence. Each watercourse or open drain must be provided 

 for either by a separate culvert of suitable size or, as may be 

 done in some cases, by leading two or more watercourses into 

 one, and thus passing all through one culvert of ample capacity. 

 When fixing the sizes of the culverts they must not be limited 

 to the normal flow of water, but a large margin must be allowed 

 sufficient to meet extraordinary floods. The depth of the bed or 

 invert of a culvert is a very important point. If laid too high, 

 and the stream above should at any time deepen, the high invert 

 would check the flow of the water, and would also incur the risk 

 of being undermined and gradually carried away. If, on the 

 other hand, the invert be laid too low, it will gradually silt up 

 to the level of the stream-bed alongside, and there will be so 

 much of the culvert space lost for all practical purposes. In 

 cases when the invert of a culvert has to be laid at a special low 

 depth to allow for future improvements in drainage, it is advis- 

 able to give extra height from the invert to the crown, or top, so 

 as to provide ample waterway in the event of any silting up in 

 the mean time. Particular care should be taken when building 

 the foundation of a culvert. It has to be laid on the site of the 

 watercourse, or on a new channel which will ultimately form 

 the watercourse, and it should be built sufficiently deep into the 

 ground to avert as far as possible the chance of water finding a 

 course through below the foundation. 



The invert may be of stone pitching or brick if the current 

 is not rapid, or liable to bring down stone boulders from its 

 gravelly bed. 



With a stream-course having considerable fall, and which 

 carries with it large stones, roots of trees, and other debris, the 

 invert should consist of strong pitching, composed of large-sized, 

 rough-dressed stones of hard, durable quality, capable of with- 

 standing the pounding of the boulders brought down during 

 floods. A soft description of stone would be quite unsuitable 

 for the invert of such a stream ; the pitching would wear away 



