RAILWAY CONSTRUCTION. 161 



above will depend mainly upon the cost of building materials 

 as compared with the cost of earthwork and land. 



Where land is very valuable, and where residential property, 

 streets, or roads must be interfered with as little as possible, the 

 retaining walls may have to be carried up to the level of the 

 original surface of the ground, as in Fig. 197, which is shown as 

 for a cutting 25 feet deep. The walls may be built of masonry, 

 brickwork, or concrete, or a combination of them, and the dimen- 

 sions or thickness will depend upon the description of material 

 to be supported. Weeping holes, or small pipe drains, should be 

 formed in the walls, a little above formation level, to take away 

 any water which may collect at the back. 



Where the cutting is through soft, wet, treacherous clay, 

 liable to slip or expand, it may be necessary to insert arched 

 thrust girders extending from side to side, as in Fig. 198, so that 

 the outward pressure against the one wall may counteract 

 against the outward pressure of the other. The thrust girders 

 should be placed at from 10 to 15 feet centres, and be well 

 braced together in plan to enable them the better to resist any 

 tendency of bulging out of the walls. 



A similar arrangement of high retaining wall may be intro- 

 duced in embankment to lessen the encroaehment on streets or 

 public roads, as shown in Fig. 199. 



In making a railway through thickly populated towns, it is 

 generally preferable to construct the line on arches rather than 

 on earthwork filling between two high retaining walls. The 

 numerous openings are available for future streets, or means of 

 communication from one side to the other, and the arches them- 

 selves can be profitably utilized for stables, stores, offices, and 

 workshops. 



Fig. 200 shows a narrow rocky pass with deep rapid river on 

 the one hand and high cliffs on the other, the only available 

 ledge being already occupied by a public highway. By building 

 a retaining wall, as indicated on the sketch, and excavating a 

 little out of the cliff, space may be obtained for a line of railway ; 

 or the arrangement may have to be reversed, and the retaining 

 wall for the railway built along the margin of the river, as in 

 Fig. 201. 



In both the cases, Figs. 200 and 201, not only must there be 

 a number of weeping holes left in the lower part of the wall, but 

 there must be sufficient well-built drains and culverts under the 



