COST OF EXCAVATION AND TIMK ItEQUIUED 305 



materials, and driven according to tin- various methods which 

 have been illustrated. 



The Halias tunnel through quicksand, between Dax and 

 Ramoux. France, cost 118.50 per lin. ft. The cost of 

 the Hoston subway was *:>4:2.40 per lin. ft. The Severn 

 and .Mersey tunnels, constructed through rock under water, 

 cost respectively ?:>08.33 and $263 per lin. ft. The First 

 Thames Tunnel, driven by Brunei's shield, cost $1661.6(5 per 

 lin. ft. The Hudson River and St. Clair River tunnels, exca- 

 vated through soft ground by means of shields and compressed 

 air, cost respectively $305 and $315 per lin. ft. The Black- 

 wall double-track tunnel under the River Thames, which is 

 the largest tunnel ever built by the shield system, cost $400 

 per lin. ft. 



In making estimates of the cost of projected tunnel work 

 based on the cost of tunnels previously constructed through 

 similar materials, it is important to keep in mind the date and 

 location of the work used as the basis for calculations. For 

 example, a tunnel excavated in Italy, where labor is very cheap. 

 will cost less than one excavated in America, where labor is 

 dear, all other conditions being the same. Other reasons for 

 variation in cost due to difference of date and location of con- 

 struction will suggest themselves, and should be taken into full 

 consideration in estimating the cost of the new work. 



Time. The time required to excavate a tunnel depends 

 ujHtn the character of the material penetrated and upon the 

 method of work adopted. Tunnels driven through soft ground 

 l>v hand require about the same time to construct as tunnels 

 driven through hard rock by the aid of machinery. Tunnels 

 can be driven through hard rook at about as great a speed as 

 through soft or fissured rock, chiefly U-cause the work of 

 blasting is more efficient in hard rock, and localise no time 

 is required in timlHM-inij. The following table shows the 

 average rate of progress in different parts of the tunnel excava- 

 tion through both hard and soft materials in feet per month: 



