TUNNELS THROUGH HARD ROCK 123 



tunnel before the upper part of the section is removed, and 

 when the heading method is employed the upper part of the 

 section is completely excavated and lined before any part of 

 the section below is excavated. When the drift method of 

 driving is employed polygonal strutting is usually used, and 

 longitudinal strutting is employed with the heading method of 

 driving. In the drift method the hauling is done by one system 

 of tracks at the same level, while in the heading method two 

 systems of tracks are employed at different levels. 



It is, perhaps, impossible to state without qualification, which 

 method is the better. European engineers unanimously prefer 

 excavation by a drift, especially for long tunnels. An advan- 

 tage that this method affords in long tunnels is, that the water 

 which is usually found in large quantities under high moun- 

 tains is easily collected in the drift and conveyed to the culvert, 

 while in the heading method the water from the advance gallery 

 before being collected into the culvert built on the floor of the 

 tunnel, must pass through all the workings. This may be a 

 serious inconvenience when water is found in large quantities, 

 as, for instance, was the case in the St Gothard tunnel, where 

 the stream amounted to 57 gallons per second. The heading 

 method has an advantage in tunneling loose rock, since it is the 

 more economical in strutting. 



