166 RAILWAY CONSTRUCTION. 



No attempt should be made to carry a tunnel through material 

 which has been disturbed or at all affected by any natural slip or 

 cleavage, as although the strata may be hard and compact in 

 themselves, they have really no solid or fixed foundation. The 

 sliding away, once initiated, is certain to continue, and, accelerated 

 by the tunnelling operations, will most likely, sooner or later, 

 crush in the tunnel and sweep away every vestige of the work. 

 Amongst the great mountain ranges these natural disturbances 

 are by no means rare, and it will be wiser to keep away from 

 their locality, even at the expense of a longer tunnel. Unfor- 

 tunately, instances are on record of tunnels made, or in course of 

 construction, through hillsides which had already commenced to 

 slide away from the more solid rock, and the ultimate results 

 were a further sliding away and total destruction of the work. 



The lower slopes and outlying portions of high mountains are 

 the most exposed to these natural slips, and they should be most 

 carefully studied before commencing any tunnelling operations 

 through them. 



To facilitate drainage, it is essential that a railway tunnel 

 should be laid down with a gradient or gradients falling in the 

 direction of one or both ends of the tunnel. In nearly all tunnels 

 a considerable amount of water finds its way in through the 

 weeping-holes left for that purpose in the side walls, and must be 

 carried away in suitable drains. If the quantity of water be 

 small, ordinary water-tables, one on each side, may be sufficient ; 

 but for large volumes of water it will be necessary to build sub- 

 stantial side-drains, or an ample culvert below the level of the 

 rails. 



The gradients in a tunnel should be moderate, and not by 

 any means excessive, or likely to tax the hauling powers of the 

 locomotives. When an engine is working nearly to the utmost 

 of its power on a steep tunnel incline, and the speed has become 

 very slow, the exhaust vapours or gases from the funnel strike 

 the arching with great force, and are deflected down on to the 

 footplate in such dense volumes as to almost suffocate the driver 

 and fireman. The writer will never forget two or three trying 

 experiences in foreign tunnels, when he and the engine-staff 

 were compelled to leave the footplate and climb forward to the 

 front of the funnel, leaving the engine to work its way alone. 

 Except for very short tunnels it is wiser to have easy inclines, 

 and to restrict the steep gradients to the open line, where 



