246 



TUNNELING 



tons, this shield worked successfully, and during several months 

 the construction proceeded at the rate of 2 ft. every 24 hours. 

 There were two irruptions of water and inud from the river 

 during the work, but the apertures were effectually stopped by 

 heaving bags of clay into the holes in the river bed, and cover- 

 ing them over with tarpauling, with a layer of gravel over all. 

 The tunnel was completed in 1843, at a cost of about $5600 

 per lineal yard, and 20 years from the time work was first 

 commenced, including all delays. 



The next tunnel to be built by the shield system was the 

 tunnel under London Tower constructed by Barlow and 

 Greathead and begun in 1869. In 1863 Mr. Peter W. Barlow 



secured a patent in England 

 for a system of tunnel con- 

 struction comprising the use of 

 a circular shield and a cylindri- 

 cal castriron lining. The shield, 

 as shown by Fig. 123, was 

 simply an iron or steel plate 

 cylinder. The cylinder plates 

 were thinned down in front to 

 form a cutting edge, and they 

 extended far enough back at the rear to enable the advance 

 ring of the cast-iron lining to be set up within the cylinder. In 

 simplicity of form this shield was much superior to Brunei's ; 

 but it seems very doubtful, since it had no diametrical bracing 

 of any sort, whether it would ever have withstood the com- 

 bined pressure of the screw-jacks and of the surrounding earth 

 in actual operation without serious distortion, and, probably, 

 total collapse. It should also be noted that Barlow's shield 

 made no provision for protecting the face of the excavation, 

 although the inventor did state that if the soil made it neces- 

 sary such a protection could be used. The patent provided 

 for the injection of liquid cement behind the cast-iron lining 

 to fill the annular space left by the advancing tail-plates of the 



FIG. 123. First Shield Invented by Barlow. 



