RAILWAY CONSTRUCTION. 113 



If the ground be fairly level, and the material firm, a solid 

 foundation may be obtained, but the excavated portion should 

 be covered up as quickly as possible to prevent any decomposing 

 action taking place upon exposure to the open air. The 

 expansive nature of some clays must be carefully kept in view, 

 so as to guard against any disturbance in the finished foundation. 

 There are some descriptions of shale which when first opened 

 out appear to have the solidity of hard rock, and yet, after a few 

 days' exposure to the atmosphere, are changed to the consistency 

 of soft mud. 



Sand, being composed of such small particles, is almost incom- 

 pressible, and makes an excellent foundation so long as it can be 

 retained in its position. Little or no settlement will take place 

 if the sand remains undisturbed, but so soon as it comes under 

 the influence of running springs, or underground drainage, the 

 fine particles of the sand will be gradually but surely carried 

 away with the water, and the entire foundation be undermined. 

 The opening out of a neighbouring excavation, or the carrying 

 out of some low-level drainage, would endanger a construction 

 which otherwise would be solid and permanent. 



In many cases of soft ground, more particularly abroad, sand 

 piles have been adopted and have given very good results. The 

 system is carried out by first driving a large wooden pile down 

 through the soft material into the more solid stratum below. 

 The timber pile is then carefully withdrawn and the cavity 

 filled with clean sand. The number and distance apart of these 

 sand piles will depend upon the nature of the ground and descrip- 

 tion and weight of structure to be carried. 



Clean, compact gravel is one of the best materials to build 

 upon, being almost incompressible and quite unaffected by 

 exposure to the atmosphere. It is easily excavated and levelled 

 off to the surface required. 



A foundation of rock may be considered in the abstract as 

 the most solid base to be obtained, but it must be treated 

 judiciously, and a proper surface secured. The outer portion of 

 many descriptions of rock consists of blocks or layers of stone 

 partially or entirely separated from the main bed, and these, 

 lying in a loose condition, are deceptive and treacherous as a 

 foundation base. The exposed rock should be carefully examined, 

 and all detached or outlying pieces or layers removed before 

 placing any 4 foundation course. Special care must be paid to all 



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