. Engineering and Contracting. tral wall between the tracks. Thiswas constructed in tunnel, with the exceptionof 172 ft. about midway between Tenth andEleventh where the rock dipped belowthe roof of the tunnel, and there the con-struction was made in open cut. These tun-nels were lined with concrete with brick arch- chambers were made for the erection of twosets of shields, about 6,100 ft. apart. The Board of Engineers decided, and it wasso stated in the contract and specifications,that the river tunnels should be constructedby means of hydraulic shields, but bidderswere permitted to pr
. Engineering and Contracting. tral wall between the tracks. Thiswas constructed in tunnel, with the exceptionof 172 ft. about midway between Tenth andEleventh where the rock dipped belowthe roof of the tunnel, and there the con-struction was made in open cut. These tun-nels were lined with concrete with brick arch- chambers were made for the erection of twosets of shields, about 6,100 ft. apart. The Board of Engineers decided, and it wasso stated in the contract and specifications,that the river tunnels should be constructedby means of hydraulic shields, but bidderswere permitted to present to the board anyscheme on wliich they might desire to bid, but, tending beyond the normal line of the cuttingedge, for use in sand, gravel and ballast, to beremoved when the shield reached the silt. Theshields were thrust forward by 24 , capa-ble of exerting a pressure of 3,400 tons at ahydraulic pressure of 5,000 lbs. per sq. into account 30 lbs. air pressure, thispressure was increased to 4,400 tons. The. Til. iKtre trgmcnt willioutthe moveable port is. coveridwith cement mortar in&tecdof concrete. CoAt^steel plug removed ami space filled with sandwhicti mu&t be thoroughly compacted under the cover Fig. 7—Cross-Section of Tunnel as Actually Constructed., February 2, 1910. ENGINEERING-CONTRACTING 105 shield was fitted with a single hydraulic erectorand hydraulic sHding platforms, and whencomplete weighed 194 tons. The shields werehuilt in accordance with the design previouslyreferred to. and proved entirely , the materials passed through wereas follows: Starting out in full face rock,from it into a mixed face of rock and sand,thence into sand and gravel, full face of sand,piles, rip-rap and the Hudson silt: and allwere fully charged with water. The contractfor the river tunnels was let to the ORourkeEngineering Construction Co. Compressed air, at an average gage pressureof about Ihs. and a maximum of 40 lbs. per leased this
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