SPECIAL THEAl IlKKolS UKOUND METHOD 17T 



descending grades. During construction, however, special 

 means will have to be provided for removing the water from 

 the excavation, their character depending upon the method of 

 excavation and upon the grades of the tunnel bottom. When 

 the excavation is carried on from the entrances only, unless the 

 tunnel has a descending grade from the center toward each end, 

 the tunnel floor in one heading will be below the level of the en- 

 trance, or, in other words, the descending grade will be toward 

 the point wl-ere work is going on, while at the opposite entrance 

 the grade will be descending from the work. In the latter 

 case the removal of the seepage water is easily accomplished by 

 means of a drainage channel along the bottom of the excavation. 

 In the- former ease the water which drains toward the front is- 

 collected in a sump, and if there is not too great a difference in 

 level between this sump and the entrance, a siphon may be used 

 to remove it. Where the siphon cannot be used, pumps are 

 installed to remove the water. When the tunnel is excavated 

 by shafts the condition of one high and one low front, as com- 

 pared with the level at the shaft, is had at each shaft Gene- 

 rally, therefore, a sump is constructed at the bottom of the 

 shaft; the culvert from the high front drains directly to the 

 shaft sump, while the water from the low-front sump is either 

 siphoned or puni])ed to the shaft sump. From the shaft sump 

 the water is forced up the shaft to the surface by pumps. 



THE PILOT METHOD. 



The pilot system of tunneling has been successfully em- 

 ployed in constructing soft-ground sewer tunnels in America 

 by the firm of Anderson \- Marr, which controls the patents. 

 The most important work <>n which the system has been em- 

 ployed is the main relief sewer tunnel built in Brooklyn, N.Y., 

 in 1892. This work comprised 800 ft of circular tunnel 15 ft 

 in diameter, 4400 ft 14 ft. in diameter, 8200 ft 12 ft in 

 diameter, and 1000 ft. 10 ft in diameter, or 9400 ft of tunnel 



