170 RAILWAY CONSTRUCTION. 



Figs. 202 and 204*. Many of the earlier tunnels were constructed 

 on the former system, while of late years the latter method has 

 been very largely adopted. The bottom heading may perhaps 

 in some instances be more efficacious for drainage, but it is very 

 liable to be frequently choked up when taking out the excava- 

 tion to the full size, and the lower surface is much cut up by the 

 movement and conveyance of materials. Another disadvantage 

 arises from the necessity of removing such a large amount of the 

 cutting approaching the tunnel entrance before a beginning can 

 be made to the bottom heading. The top heading has the advan- 

 tage that it requires less removal of open cutting previous to its 

 commencement, and, being high up in position, there is less 

 chance of its being stopped up by falling material, the finished 

 excavations being carried out on the sides and below the 

 heading. 



Where the headings are cut through solid rock, stiff shale, or 

 compact chalk, little or no supports are necessary, but where 

 they pass through clay or loose material, timbering will be 

 required for sides, roof, and floor. Rough round poles, about 6 

 inches in diameter, are generally used for verticals, and are 

 firmly secured to transverse sole-pieces, and on the top of these 

 verticals strong transverse top-sills are fastened by means of 

 rough tenons or checks. Strong boards are inserted at the back 

 of this framework to keep the earth from falling into the work- 

 ing. The distance apart of the verticals will depend upon the 

 description of material excavated ; in very soft places they will 

 have to be placed very close together, but where fairly sound 

 and tenacious they may be placed at about 3-foot centres. 

 The excavated material must be conveyed away to the entrance 

 of the heading in small hand-trucks running on planks or light 

 rails. 



The widening out of the excavation to the full size will be a 

 repetition on a large scale of the work carried out in the heading, 

 with the difference that, the exposed surfaces being of so much 

 greater extent, extra care and precautions must be taken with 

 the framework and shoring of the timbering. 



The form and arrangement of the timbering, as well as the 

 number, sizes, and positions of the pieces, must be determined by 

 the material of the excavation and the contour line of the finished 

 arching or lining. The framework, which would be sufficient to 

 support ordinary soft material, must be largely augmented both 



