34 



TUNNELING 



be verified, and so that as great an area of rock as possible may- 

 be attacked at the same time in the work of enlarging the 

 profile. In short tunnels, where the ranging of the center line 

 is less liable to error, shorter headings are employed, and in soft 

 .soils they are made shorter and shorter as the cohesion of the soil 

 decreases. When the material has too little cohesion to stand 

 .alone, the tops and sides of the heading require to be supported 

 by strutting. To prevent caving at the front of the heading, 

 the face of the excavation is made inclined, the inclination 

 following as near as may be the natural slope of the material. 



Enlargement of the Profile. The enlargement of the profile 

 is accomplished by excavating in succession several small 



prisms parallel to the heading, and 

 its full length, which are so located 

 that as each one is taken out the 

 cross-section of the original her.ding 

 is enlarged. The number, location, 

 and sequence of these prisms vary 

 in different methods of excavation, 

 and are explained in succeeding 

 chapters where these irethods are 

 described. To direct the excava- 



Pio. 16. Diagram Showing Manner 



of Determining Correspondence of tion SO as to keep it always Within 

 Excavation to Sectional Profile. ,11 i . P , 



the boundaries of the adopted pro- 

 file, the engineer first marks the center line on the roof of the 

 heading by wooden or metal pegs, or by some other suitable 

 means by which a plumb line may be suspended. He next 

 <lraws to a large scale a profile of the proposed section ; and 

 beginning at the top of the vertical axis he draws horizontal 

 lines at regular intervals, as shown by Fig. 16, until they inter- 

 sect the boundary lines of the profile, and designates on each 

 of these lines the distance between the vertical axis and the 

 point where it intersects the profile. It is evident that if the 

 foreman of excavation divides his plumb line in a manner corre- 

 sponding to the engineer's drawing, and then measures horizon- 



