Tunnels and Tunneling

feet, river, tunnel, inches, rock, air, concrete, pounds, east and lining

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Bergen Hill Tunnels.— These consist of two parallel single-track, concrete-lined tunnels 5,920 feet long between portals. The boring through Bergen Hill was largely in trap rock (diabase), partly in sandstone. Some of the former was so extraordinarily hard as to re quire 10 hours to drill a 10-foot hole in the heading with a number 34 slugger drill at 90 pounds air pressure. Work started May 1905 and completed May 1908. Height of the tunnel inside is 16 feet 2 inches from the top of track rail to crown. The tunnel is lined with con crete variably, according to the soils to be sup ported. The water proofing is of six-ply felt and seven layers of pitch. Bench walls at either side of tunnel carry conduits for tele phone, telegraph, etc.

North River Tunnels.— These extend from 10th avenue, New York City, to the large shaft at Weehawken, N. J., crossing under the Hudson River. They consist of two single track tubes lined with cast-iron rings, and have an outside diameter of 23 feet, and 21 feet 2 inches inside diameter. The subaqueous portion is 6,128 feet long. Work was started April 1904 and completed 1910. The Weehawken shaft was a large piece of work being over the tunnels and including both. Difficulties occurred from the treacherous nature of the rock, which hindered work till it was lined its entire length with concrete. The tunnels under the river were driven with shields. The floors of two plat forms were so constructed that they could be extended 2 feet 9 inches in front of the cut ting edge or 8 inches in front of the hood. The motion to the sliding platforms was given by hydraulic jacks. The land tunnels consist of 977 feet of double tunnel on the New York City side and 230 feet on the New Jersey side. The Manhattan end (from 11th avenue to 32d street) is very complicated, having in its 977 feet nine different styles of cross section caused by a curve; for the curve is effected in straight lengths not curves. Parts of the land tunnels were shield driven but other parts were open cut work and ordinary land tunneling, as great variation was found in the nature of the ma terial met with; some soft ground caused some cut-and-cover work. The cast-iron rings of the tube are bolted together through holes in flanges. A f ter lining with concrete the inside diameter of the tube is 19 feet, but after laying the flat bed there is left 16 feet 7 inches from top of rails to soffit of arch. The concrete lining is reinforced with steel rods. Excavating was done under an average air pressure of 25 pounds when air pressure was needed.

Cross Town Tunnels.— These extend east erly from the Terminal Station to permanent shafts east of First avenue under 32d and 33d streets. For an average of 350 feet west from the First avenue shafts there are four tunnels in sound, fairly dry rock which was drilled and blasted, but this Hudson schist varied consid erably physically and an ancient water-course was struck causing difficulties. Sand also caused much trouble in spots. Some open-cut work on 32d and 33d streets caused much underpinning of houses. On Sixth ave

nue the Elevated Railroad was under pinned to rock. Concrete lining is used on these twin tunnels. These two-track tunnels have their crown about 60 feet below street surface. In the three-track tunnel heavy brick arching was used for tunnel lining and concrete for roof of open-cut work. These tunnels were completed March 1909, but road way restoration required further time.

East River Tunnels.— These connect the cross-town tunnels, by tunnels beneath the East River, with Long Island City. They consist of four single-track, cast-iron and concrete lined tubes with an outside diameter of 23 feet, starting from two permanent shafts on the east side of the river. Each tunnel is about 6,000 feet long, 3,900 feet of which is between the shafts each side of the river and 2,000 feet in Long Isltid City. The work was finished in exactly five years. The East avenue heading needed at start 15 pounds air pres sure for soft ground and considerable water; then rock permitted discontinuance. A shield was soon brought into use but found imprac ticable except for a short distance under the Long Island passenger station. Solid rock ex cavating was done with bottom-heading-and break-up or with top-heading-and-bench meth ods; soft ground in top of rock tunnel was ex cavated in normal air by mining-and-timbering method. In some parts of this work water was struck and tunneling was done in compressed air without a shield by building the iron lining up to face of the full-sized excavation, then top-heading a hole for about 10 feet in ad vance about 3 feet wide by 4 or 5 feet high, then timbering. A permanent shaft was con structed on the Manhattan side over each pair of tunnels. The four river tunnels between the Manhattan-Long Island City shafts were con structed with shields working both ends at once. One difficulty overcome was that caused by the small depth of roof at the deepest part of the river (at the Manhattan pier-head line) and consisting of fine sand. Safer cover to protect against blow-outs was given by adding blankets of clay on the river bed seven to ten feet thick. Air pressure carried in soft ground work were from 30 to 34 pounds ranging up to 37 pounds in tunnels B and D at Manhattan and up to 42 pounds for short periods to remove broken plates. The chief difficulty was in soft sand allowing the shield to settle, thereby breaking plates in the bottom of the rings. Blow-outs of such severity oc curred as to stir up the bed of the river suf ficiently to necessitate shutting down work for three weeks to allow the river bed to consoli date and permit further work Standard cast iron tunnel lining was used, giving 23 feet out side diameter with rings 30 inches wide, 11 segments and a key to each ring. Inside di ameter of tubes is 21 feet 2 inches. There are P/2-inch thick webs in the central portion; flanges are 11 inches deep. In passing from rock to soft ground cast-steel rings were added as reinforcement for a short stretch. The iron tube is line with concrete. For Bibliog raphy see under TUNNELS, GREAT MODERN.

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