RAILWAY CONSTRUCTION. 123 



pier to be constructed. Caissons for heavy work on difficult or 

 treacherous ground require great care, not only in their con- 

 struction, but also in placing them in exact position, and in 

 sinking them correctly to their proper depth. A tilted caisson 

 is a most difficult subject to handle, and entails heavy expendi- 

 ture to restore it to a true vertical position. By making careful 

 borings, the engineer can ascertain very closely the depth to 

 which the caisson will have to be lowered to obtain a good firm 

 foundation. With this information the caisson can be so con- 

 structed that the upper portion, termed the temporary caisson, 

 commencing a few feet above the bed of the river, can be 

 detached, and removed at the completion of the work from the 

 lower or permanent portion sunk below the ground line. 



Fig. 118 gives sketches of a wrought-iron plate-caisson 

 applied to a deep-water river pier, and lowered to its full depth 

 by the pneumatic process ; dotted lines show the air- tubes 

 through which the excavated material is hoisted and emptied 

 into barges alongside. 



Many large and important pier foundations have been 

 constructed on the system of brick cylinders or wells, particu- 

 larly in India, where the foundations for large river viaducts 

 have to be carried down to great depths through thick deposits 

 of soft material. These wells are built upon V-shaped curbs to 

 facilitate the penetration when sinking. Fig. 119 is a section 

 of a well with a wrought-iron curb, and Fig. 120 is a similar 

 well with a wooden curb. The wrought-iron curb is made in 

 segments for convenience of transport, the pieces forming the 

 complete ring being bolted or riveted together at the site of the 

 foundations. The wooden curb is composed of several thick 

 layers of hard wood planking cut to the proper shape, and laid 

 with broken joints, the whole being bound together with suitable 

 bolts and spikes. In some cases the lower or cutting edge of the 

 wooden curb is strengthened or protected by a sheathing of 

 wrought-iron plates. 



Well foundations are usually put in when the rivers are at 

 their lowest, and reduced to a few small channels in the great 

 width of dried-up river bed. This condition enables the greater 

 portion of the curbs to be conveniently and accurately placed in 

 position on dry ground, or on ground which, although soft and 

 muddy, is not covered with water. Should the site of one of 

 the wells occur in one of the small channels, the stream can be 



