. Shield and compressed air tunneling . g through the silt and particularly sowhen the shields driven from opposite sides of the river met in thesoft silt. The hood was arranged, therefore, so that it could beremoved when the shield was about to enter the silt. The hoodprojected 2 ft. 1 in. beyond the cutting edge and covered the topas far down as to the upper platform of the shield. It was builtup in nine sections of the same thicknesses of plates as the skinand was fastened by interior splice plates. At each junction of 12 178 SHIELD TUNNELING the sections of the hood it was supported by bra
. Shield and compressed air tunneling . g through the silt and particularly sowhen the shields driven from opposite sides of the river met in thesoft silt. The hood was arranged, therefore, so that it could beremoved when the shield was about to enter the silt. The hoodprojected 2 ft. 1 in. beyond the cutting edge and covered the topas far down as to the upper platform of the shield. It was builtup in nine sections of the same thicknesses of plates as the skinand was fastened by interior splice plates. At each junction of 12 178 SHIELD TUNNELING the sections of the hood it was supported by brackets extendingback and attached to the steel structure. 85. Face Jacks.—In each of the platforms were placed fourpairs of hjdrauhc jacks acting as face supports and supports forshding platforms, which could be extended 2 ft. 9 in. viz.: 8 in. inadvance of the hood. The platforms were strong enough to carrya load of 7,900 lb. per square foot, this being equivalent to the maxi-mum combined head of water and ground. The plunger head of. The parts of a shield. Rear perspective. each of the jacks was fastened to and supported against the rearpart of the box-girder forming the platform. The jack cjdinderwas square in outside cross-section and could slide through twobearings under the floor. At the outer end of the cylinder wasbolted a casting or nosing which extended up flush with the floorof the platform, and a steel plate attached to the nosings of eachpair of cylinders formed the sliding platform. The platform TUNNEL SHIELDS 170 jacks were single acting, had a diameter of 33^^ in. and a strokeof 2 ft. 9 in. At a hydraulic pressure of 5,000 lb. per square incheach jack could develop a thrust of 48,000 lb. 86. Shield Jacks.—The propelling jacks, of which there were 24and which are shown in Fig. 70, had a plunger diameter of 83^-^ jacks were tested to a pressure of 6,000 lb. per square inchand^werc intended to be used with a maximum working pressure Cutting Eoce
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Keywords: ., bookcentury1900, bookdecade1920, bookidshieldcompre, bookyear1922
