122 TUNNELING 



heading was driven, and rested upon the bench until it was 

 removed and the side posts inserted. Where niud pockets 

 were met the plank lagging was inserted behind the side posts 

 as well as above the roof-arch ribs, and when the pressures were 

 unusually great a double lining was employed. 



Progress of Work. The rate of progress made in exca- 

 vating the Busk tunnel was as follows : - 



Total time consumed in driving the heading 1,118 clays 



Average daily progress for both headings 8.4 feet 



Greatest progress in one month 337 



Average daily progress, one month, 31 days 10.87 



Greatest progress in one month (28 days) at one end . . 202.5 

 Average progress in one month (28 days) at one end . . 7.23 



Greatest monthly progress on bench 218 



Average daily progress on bench 7.79 



Cost of Work. The cost of the tunnel was calculated on 

 the assumption that the excavation per lineal foot was 10.19 

 cu. yds., and where the section was enlarged for timbering, 

 1379 cu. yds. The contractors' estimate for excavating and 

 timbering the tunnel was as follows : 



Excavation of 9,393.66 lineal feet @ $62,50 .... $587,103.73 



Enlargement for timbering 32,575 cubic yards . . . 81,437.50 



Cost of timber 81,600.00 



Cost of labor on timbering 2,723,000 ft. B. M. @ $12 . 32,676.00 



Total $782,817.25 



This is an average cost per lineal foot of tunnel of $83.14, 

 which is very close to the average cost of single-track timber- 

 lined tunnels in America, which is usually figured at $85 per 

 lineal foot. 



COMPARISON OF METHODS. 



The differences between the drift and heading methods of 

 excavating tunnels through rock, consist chiefly in the excava- 

 tions, strutting, and hauling. When the drift method is em- 

 ployed an advanced gallery is opened along the floor of the 



