.,K\i:i:\I. Ml.THuPS OF i:.\< AVATION 33 



of the tunnel section are usually designated by the general 

 term of enlargement of the profile. The various operations of 

 excavation may, therefore, be classified either as excavation 

 of the heading or enlargement of the profile. 



Excavation of the Heading. There is considerable confusion 

 among the dilYerent authorities regarding the exact definition 

 of the term "heading" as it is used in tunnel work. Some 

 authorities call a small passage driven at the top of the profile 

 a heading, and a similar passage driven at the bottom of the 

 profile a drift; others call any passage driven parallel to the 

 tunnel axis, whether at the top or at the bottom of the profile, 

 adrift; and still others give the name " heading" to all such 

 iges. For the sake of distinctness of terminology it sem.s 

 preferable to call the passage a heading when it is located at 

 the top of the profile, and a drift when it is located near the 

 bottom. 



Headings and drifts are driven in advance of the general 

 excavation for the following purposes: (1) To fix correctly 

 the axis of the tunnel; (2) to allow the work to go <>n at 

 different points without the gangs of laborers interfering with 

 each other; (3) todetect the nature of material to \tc dealt with 

 and to be ready in any contingency to overcome any trouble 

 caused by a change in the soil : and (4) to collect the water. 

 The dimensions of headings in actual practice vary according 

 to the nature of the soil through which they are driven. As 

 a general rule they should not l>e less than 7 ft. in height, 

 to allow the men to work standing, and have room left for the 

 roof strutting. The width should not be less than ' fi.. to 

 allow two men to work at the front, and to give room for 

 the material cars without interfering with the wall strutting. 

 Usually headings are made X ft. wide. The length of headings 

 in practice varies according to circumstances. In \ei\ long 

 tunnels through hard rock the headings are sometimes ex- 

 cavated from 1000 ft. to 2000 ft. in advance, in order that thev 

 may : -on as possible and the ranging of the center line 



