RAILWAY CONSTRUCTION. 71 



embankment filling which will be necessary to form a permanent 

 foundation for the line ; and the construction of a high heavy 

 embankment across such a place is one of those undertakings 

 which every engineer is most anxious to avoid. A large quantity 

 of material may be tipped into the bog, and seem to stand fairly 

 well for a time, and then suddenly disappear altogether. More 

 material has to be brought forward, and will most likely dis- 

 appear in a similar manner. The filling material being heavier 

 than the bog on to which it is thrown, falls through, and 

 displacing the soft semi-liquid matter, continues to sink down 

 lower and lower until it is stopped by a harder stratum under- 

 neath. In a measure the operation somewhat resembles the 

 tipping of earth into a lake ; the material will go down until it 

 meets with a solid bottom, and in going down it assumes its own 

 natural slope, and forms for itself a width of base corresponding 

 to its height. It will be readily understood what an enormous 

 amount of filling material will be swallowed up in following out 

 such a process. On a very soft bog, say 20 feet in depth, over 

 which an embankment 20 feet high has to be formed, the extent 

 of the actual earthwork filling will very probably closely 

 approach the outline shown in Fig. 63. The upper portion, 

 A B C D, representing the embankment proper, will contain 

 about 133 cube yards to the yard forward, whereas the lower 

 portion, C D E F, which has displaced the soft boggy matter, 

 will contain about 266 cube yards to the yard forward, or, in 

 other words, the filling which is out of sight will be double the 

 filling which is in view above the section ground line. 



Apart from the large amount of filling consumed in forming 

 this semi-artificial island, the progress of the work itself is very 

 perplexing. A long length of the bank may have been raised 

 again, once or twice, to the proper height, and may have carried 

 rails and earth- waggons for some weeks, and then sink all at 

 once several feet. The sinking, too, may not be uniform, but may 

 produce fissures, depressions, and separation of the earthwork 

 which will necessitate much care when bringing forward fresh 

 filling material. The bog may not be of the same consistency 

 throughout, there may be some layers of harder material, such as 

 imbedded trunks of trees, and these may sustain the filling for a 

 time, and then yield under the increasing weight of the super- 

 incumbent mass. Even when the embankment is finished 

 throughout, and shows no sign of sinking, it should be very 



