164 RAILWAY CONSTRUCTION. 



railways extended further away from the important centres, the 

 probabilities of traffic would become less promising, and efforts 

 would be made to keep down cost of construction, and avoid 

 tunnel work as much as possible. 



It is not easy to define where cutting should end and 

 tunnelling begin. There is no practical difficulty in making a 

 cutting 50, 60, or 70 feet deep, with slopes to suit the material 

 excavated, and the estimated cost per yard forward may even 

 compare favourably with the cost of average tunnel-work. But 

 there are other questions which must be kept in view the time 

 required to form the cutting, the space to be obtained on which 

 to deposit the enormous quantity of excavated material, and the 

 probable difficulty in obtaining the large area of land necessary 

 for the cutting. 



Before deciding the actual position of a tunnel, both as to 

 line and level, it is necessary to obtain the most reliable informa- 

 tion possible regarding the strata through which it has to pass. 

 In addition to the geological indications on the surface and in 

 the locality, borings should be made, and trial holes or shafts 

 sunk along the proposed centre line of the work, and from these 

 an approximately accurate longitudinal section can be laid down 

 on paper, showing the respective layers of material to be cut 

 through, and the angle at which they lie. With these particulars 

 before him, the engineer may, in some cases, consider it more 

 prudent to change the position of the tunnel in preference to 

 incurring specially difficult or tedious work in dealing with some 

 recognized unfavourable material. Occasionally the route may 

 be slightly varied and better material obtained, but very 

 frequently there is little to be gained except by a wide devia- 

 tion from the original line. 



Solid rock, except for the slow progress, is perhaps the most 

 favourable material for tunnelling, as the timbering, side walling, 

 and arching can be almost, if not entirely, dispensed with. 



Loose rock, although more readily removed, necessitates 

 strong timbering to prevent large masses breaking away and 

 falling into the tunnel. 



Some clays are very compact and tenacious, and will stand 

 well with moderate timbering, but even these should not be left 

 long before following up with the side walls and arching. 



Many clays give much trouble by expanding, or swelling out, 

 when the excavation penetrates the layer, and although extra 



