146 



TUNNELING 



width of each heading has to be sufficient for the construction 

 of the masonry and strutting, and for the passage of narrow 

 spoil cars alongside them. These drifts are increased in height 

 to the springing line of the arch by taking out the two drifts 

 No. 2. Next the top center heading No. 3 is driven, and 

 iinally the two haunch headings No. 4 are excavated. The 

 center core No. 5 is utilized to support the strutting until 

 the side walls and roof arch are completed, when it is broken 

 down and removed. In case of very loose material-, where the 

 first side drifts cannot be carried as high as one-third the 

 height of the section, it is the common practice to make them 

 about one-fourth the height, and to take out the side portions 

 of the annular gallery in three parts, as 

 shown by Fig. 73. 



The top center heading plan of com- 

 mencing the excavation is usually em- 

 ployed in firm materials or when a vein 

 of water is encountered in the upper part 

 of the section. In the latter contingency 

 a small bottom drift A, Fig. 74, is first 

 driven to serve as a drain ; but in any 

 case the excavation proper of the tunnel 

 consists in first driving the center top 

 heading No. 1, and then by working both 



ways along the profile parts, Nos. 2, 3, 4, and 5 are removed. 

 Part No. 6 is left to support the strutting until the side walls 

 and roof arch are built, when it is also excavated. 



Strutting. When the excavation is begun by bottom side 

 drifts these drifts are strutted by erecting vertical posts close 

 against the sides of the drift and placing a cap-piece trans- 

 versely across the roof of the drift. The side posts are 

 usually supported by sills placed across the bottom of the drift. 

 These frameworks of posts, cap, and sill are erected at short 

 intervals, and the roof, and, if necessary, the sides of the drift 

 between them, are sustained by means of longitudinal poling- 



FiG. 74. Diagram Show- 

 ing Sequence of Excava- 

 tions in Water Bearing 

 Material, German 

 Method. 



