78 RAILWAY CONSTRUCTION. 



must be cut and dressed to suit the small radius of the arch. 

 The side walls may be of brick of good quality. Occasionally 

 they are built of concrete. The wing walls may either be 

 carried out in the direction of the stream, as in the sketch of 

 the 6-foot culvert, or they may be built transverse, as shown 

 on the 4-foot culvert, whichever arrangement is found to work 

 in the best for the case in question. 



For arch culverts on very steep side-lying ground it is better 

 to build the arch-top in steps, as shown in Fig. 72, instead of 

 forming it parallel to the invert, or slope, of the stream-course. 

 The level portions of the arching give a better hold for the em- 

 bankment than could be obtained on a long inclined surface of 

 brickwork or masonry. 



The writer has built a large number of culverts of this type 

 for mountain streams on steep hillsides, and has found them to 

 prove satisfactory in every way. 



In embankments alongside tidal rivers, or across the corners 

 of estuaries of the sea, culverts have frequently to be so con- 

 structed that they will permit the passage of the drainage water 

 from the land, or high side, without admitting the tidal water. 

 This can be arranged by placing at the lower end of the culvert 

 close-fitting hinged-flap valves opening outwards. When the 

 tide has gone down the weight of the fresh, or land, water 

 swings the flap-valve sufficiently open to allow of a free 

 passage ; and, on the other hand, when the tide rises, the 

 pressure of the water against the face of the flap-valve keeps 

 it tightly closed, and prevents ingress of the salt water. 



Culverts are sometimes fitted with lifting-valves or doors, 

 which can be raised or lowered to serve irrigation purposes. 

 The door, which works in guides, is made sufficiently heavy to 

 fall with its own weight, and the raising is effected by means of 

 a screwed suspension-rod working in a well-secured fixed nut. 



In cases of soft or treacherous ground, timber-piling or wide 

 bed-courses of cement concrete are necessary to form firm 

 foundations for culverts. Drains and streams which are inter- 

 sected by a railway cutting have to be dealt with according to 

 their size and their height above the finished rail level. The 

 water from a small drain or field spring may be conducted in 

 pipes down the slope of the cutting into the water table, or side 

 drain, at formation level, and will be thus carried away to the 

 lower level at the entrance of the cutting. In many cases 



