132 RAILWAY CONSTRUCTION. 



which the water can be excluded, and a masonry or brickwork 

 pier constructed inside. It may be made all in one piece to 

 correspond to the form of the pier, or in separate pieces to form 

 one whole, each being sunk independent of the other, and con- 

 nected together afterwards. Being built up of plates cut to the 

 proper size and shape, it is a very simple matter to rivet on 

 additional tiers of plates as the caisson is lowered deeper and 

 deeper into the bed of the river. The lower length is made with 

 a cutting edge to penetrate the ground; the exterior is made 

 without any projection larger than the rivet heads, and the 

 interior is strengthened with T-irons or double L-irons at the 

 joints, and strong cross-bracing to resist the pressure of the water. 

 About 7 or 8 feet above the cutting edge a strongly framed iron 

 floor is riveted to the vertical sides, and strengthened by plate- 

 iron under-brackets placed at short distances. The excavators 

 work in the space below the floor, and the excavated material is 

 passed up through openings formed in the floor at convenient 

 points to suit the working. The methods of lowering a caisson 

 are the same as for lowering a cylinder. If the pneumatic 

 system has to be adopted, then two or more air-tight tubes of 

 liberal dimensions (say 5 to 8 feet diameter), according to the 

 size of the caisson, must be attached to the floor, and on the top 

 of each of these tubes air-locks must be secured for the removal 

 of men and materials. The masonry or brickwork of the pier is 

 built upon the iron floor, and a portion of this building work is 

 usually carried on during the sinking of the caisson to obtain 

 weight to assist in the lowering. When down to the proper 

 depth, the space below the floor is properly cleared of debris and 

 water, and then carefully filled in with cement concrete. 



Some caissons are made with vertical sides throughout their 

 entire height; others have an outward taper for 15 or 20 feet on 

 the lower end. The former are not only simpler in construction, 

 but are more easily kept in a vertical position during the 

 sinking. Caissons are usually put together in some convenient 

 place near the edge of the water, and then conveyed on pontoons 

 to the sites of the piers. Great care is required in lowering 

 them into position in the bed of the river, and guide-piles, guy- 

 chains, and other appliances are frequently necessary to keep 

 them vertical during the sinking. 



The form, dimensions, thickness of plates, cross-bracing, and 

 general arrangement will depend upon the size and depth of the 



