114 RAILWAY CONSTRUCTION. 



shelving rock, and a level seating cut into it for the entire width 

 of the foundation, as shown in Fig. 110. 



A thick bed of concrete, as in Fig 109, makes an excellent 

 foundation course. When firmly set it becomes one solid 

 massive base from end to end, and prevents the yielding or 

 dropping of masonry at any intermediate points. 



There are many places in soft, wet ground where instead of 

 attempting to excavate all the soft material down to a harder 

 stratum, it is better to adopt timber pile foundations, as shown 

 in Fig. 111. The size of the piles and their distance from centre 

 to centre must be regulated by the description of material into 

 which they have to be driven and the weight they have to 

 sustain. Double waling pieces should be properly checked and 

 bolted on to the heads of the piles, and trimmed or levelled off to 

 receive a double floor of thick planks. The spaces round the 

 heads of piles and walings should be filled in and levelled up to 

 under side of flooring, with cement concrete. 



For bridges of moderate span, over soft ground or over shallow 

 fresh water, strong cast-iron screw piles can be adopted with 

 great advantage. Fig. 112 shows a very usual form of screw 

 pile, made with an external screw at the lower end and with a 

 sharp cutting edge to facilitate penetration into the ground. 

 The upper portions are made in suitable lengths, and all to one 

 pattern and template, for convenience in carrying out the work. 

 The screwing into the ground is generally effected by means of 

 a capstan or cross-head fixed to the top of the first working 

 length of pile, and which is pulled or turned round by ropes 

 worked from stationary windlasses. In some cases long bars 

 or levers are attached in radiating positions to the capstan-head, 

 and a number of men are employed to walk round and round, 

 pushing the levers, and in this way screwing the pile into the 

 ground. As the pile goes down the capstan-head has to be 

 removed, and additional lengths bolted on, until the pile enters a 

 solid stratum, or is considered deep enough for the duty it has 

 to perform. The last or top length has generally to be cast to a 

 special length to bring the work up to the exact height to receive 

 the girders. The core of excavated material passes up into the 

 interior of the pile, and in some cases becomes so compressed or 

 tight as to require the use of an internal augur to remove a 

 portion of it to enable the screwing to proceed. The pile shown 

 in Fig. 112 is one of a number which were successfully screwed 



