AMERICAN SOCIETY OF CIVIL ENGINEERS

INSTITUTED 1852

TRANSACTIONS

Paper No. 1158

THE NEW YORK TUNNEL EXTENSION OF THEPENNSYLVANIA RAILROAD. THE CROSS-TOWN TUNNELS. [A]

BY JAMES H. BRACE AND FRANCIS MASON, MEMBERS, AM. SOC. C. E. 

In this paper, it is proposed to describe the construction of thetunnels extending eastward from the easterly extension of the TerminalStation to the permanent shafts east of First Avenue. 

They were located under 32d and 33d Streets from the station to SecondAvenue, and thence, curving to the left, passed under private propertyand First Avenue to the shafts, as described in a preceding paper. Typical cross-sections of the tunnels are shown on Plate XII. [B]

On May 29th, 1905, a contract was entered into with the UnitedEngineering and Contracting Company for the performance of this work. This contract provided that work on each pair of tunnels should becarried on from two shafts. The first, here referred to as the FirstAvenue Shafts, were located just east of that avenue and directly overthe line of the tunnels; the other two, called the Intermediate Shafts, were located on private property to the north of each pair of tunnels inthe blocks between Fourth and Madison Avenues. It was originallyintended to do all the work of construction from these four shafts. Workings were started both east and west from the Intermediate Shafts, and those to the west were to be continued to the Terminal Station. After the change of plans, described in a previous paper, it was decidedto sink a third shaft on each line. These were known as the West Shafts, and were located between Sixth and Seventh Avenues. Finally, it wasfound necessary to build a portion of the tunnels on each line west ofSixth Avenue in open cut. The locations of the shafts are shown on PlateXIV. [C]

The First Avenue shafts were built by S. Pearson and Son, Inc. , for thejoint use of the two contractors, as described in the paper on thetunnels under the East River. While the shafts were being sunk, thefull-sized tunnels were excavated westward by the contractor for theriver tunnels for a distance of 50 ft. , and top headings for 50 ft. Farther. By this means, injury to the caissons and to the contractor'splant in the shafts by the subsequent work in the Cross-Town Tunnels wasavoided. The west half of the shaft was for the exclusive use of thecontractor for the Cross-Town Tunnels. 

CONTRACTOR'S PLANT. 

The method of handling the work adopted by the contractor was, broadlyspeaking, as follows: Excavation was usually carried on by modificationsof the top-heading and bench method, the bench being carried as close tothe face as possible in order to allow the muck from the heading to beblasted over the bench into the full section. The spoil was loaded into3-yd. Buckets (designed by the contractor and hereinafter described), bysteam shovels operated by compressed air, and hauled to the shafts byelectric locomotives. Electrically-operated telphers, suspended from atimber trestle, hoisted the buckets, and, traveling on a mono-railtrack, deposited them on wagons for transportation to the dock. Arrivingat the dock, the buckets were lifted by electrically-operated stiff-legderricks and their contents deposited on scows for final disposal. Thespoil was thus transported from the heading to the scow without breakingbulk. 

When concreting was in progress, the spoil buckets were returned to theshafts loaded with sand and stone. The concrete materials were depositedin storage bins placed in the shafts, from which they were fed to themixers located at the foot of the shaft about on a level with the crownof the tunnels. The concrete was transported to the forms in side-dump, steel, concrete cars, hauled by the electric locomotives. 

Electrical power was adopted largely on account of the restricted areaat the shaft sites, where a steam plant would have occupied considerablespace of great value for other purposes. The installation of a steamplant at the Intermediate Shafts, which were located in a high-classresidential district, would have been highly objectionable to theneighboring property owners, on account of the attendant noise, smoke, and dirt, and, in addition, the cost of the transportation of fuel wouldhave been a serious burden. Except for the forges and, toward the last, the steam locomotives, not a pound of coal was burned on the work. Theuse of the bucket and telpher also eliminated most of the objectionablenoise incident to the transfer of spoil from tunnel cars to ordinarywagons at the shaft sites. Power plants were installed at the NorthShaft near First Avenue and at the rear of the 33d Street IntermediateShaft. 

_First Avenue Plant. _--Fig. 1, Plate LVIII, is a general view of theFirst Avenue plant. The power-house at the corner of 34th Street andFirst Avenue supplied compressed air for operating drills, shovels, pumps, and hoists in the tunnels driven from the river shafts, and in itthree Laidlaw-Dunn-Gordon compressors were installed. The largest was a32 by 20 by 30-in. , two-stage, cross-compound, direct-connected to aFort Wayne 480 h. P. , 230-volt, direct-current, constant-speed motor runat 100 rev. Per min. This compressor was rated at 2, 870 cu. Ft. Of freeair per minute at a pressure of 100 lb. It was governed by throttlingthe suction, the governor being controlled by the pressure in the airreceiver and the motor running continuously at a constant speed. The twoothers were of similar type, one was 22-1/2 by 14 by 18-in. , rated at1, 250 cu. Ft. Of free air at a pressure of 100 lb. , the other was 16 by10 by 18-in. , rated at 630 cu. Ft. They were fitted with 9-ft. Fly-wheels, and were driven at 150 rev. Per min. By 105-h. P. , GeneralElectric, 220-volt, compound-wound, direct-current motors running at 655rev. Per min. The larger of these two compressors was driven by two ofthe motors belted in tandem, and the smaller was belt-connected to athird motor. The compressors were water-jacketed and had smallinter-coolers, the water supply for which was itself cooled in a WheelerCondenser and Engineering Company's water-cooling tower. The pump andthe blower operating it were electrically driven. 

The telphers, used for hoisting muck from the tunnels and for loweringsupplies, were each hung from single rails on a timber trestle, about40 ft. High, spanning and connecting the two shafts. One machine wasprovided for each shaft, and where their tracks crossed 33d Street theywere separated sufficiently to permit the machines to pass each other. At this point, and covering the street, a large platform was provided, on which the trucks were loaded and unloaded (Fig. 2, Plate LVIII), andfrom which they descended by an incline on First Avenue leading south to32d Street. The platform also covered practically all the yard at theSouth Shaft and materially increased the available working area. Thetelphers were built by the Dodge Cold Storage Company, and were operatedby a 75-h. P. General Electric motor for hoisting and a 15-h. P. NorthernElectric Company motor for propulsion. Their rated lifting capacity was10, 000 lb. At a speed of 200 ft. Per min. 

The carpenter shop and machine-shop, both of which served the entirework, were conveniently located in small buildings on the loadingplatform. In the former the saws were each run independently by smallelectric motors suspended under the platform. The heavy forms and formcarriages used in lining the tunnels with concrete were fabricated andstored on the platform outside. The machine-shop lathes, etc. , were allbelted to one shaft driven by an 8-h. P. General Electric motor. Abovethe machine-shop was a locker-room and below it on the street level wasthe main blacksmith shop for the work. Subsidiary blacksmith shops werelocated at each of the other shafts. The storeroom and additionallocker-rooms were located above the power-plant in the North Shaft yard, and isolated from the other structures was a small oil-house. Additionalstorage space was provided by the contractor on 32d Street just west ofFirst Avenue by renting three old buildings and the yards in the rear ofthem and of the Railroad Company's cement warehouse adjacent. Hereelectric conduits, pipe, castings, and other heavy and bulky supplieswere stored. 

During excavation the headings were supplied with forced ventilationthrough 12-in. And 14-in. No. 16, spiral-riveted, asphalted pressurepipes, canvas extensions being used beyond the ends of the pipes. A No. 4 American Blower, located at the top of each shaft and driven by a15-h. P. General Electric motor, supplied the air. 

[Illustration: PLATE LVIII, FIG. 1. --VIEW OF FIRST AVENUE PLANT. ]

[Illustration: PLATE LVIII, FIG. 2. --TELPHER STRUCTURE AND LOADINGPLATFORM, FIRST AVENUE SHAFT. ]

[Illustration: PLATE LVIII, FIG. 3. --HEADWORKS AT 33D STREET:INTERMEDIATE SHAFT. ]

[Illustration: PLATE LVIII, FIG. 4. --LOADING SPOIL ON BARGES, 35THSTREET PIER. ]

A concrete-mixing plant was placed in each shaft, the mixer beinglocated high enough to discharge into cars at about the level of thespringing line of the arch. Above the mixers were the measuringhoppers set in the floor of a platform which was large enough to carryhalf a day's supply of cement. At the South Shaft the cement wasdelivered to this floor from the loading platform through a spiral steelchute; at the North Shaft it was lowered in buckets by the telpher. Thesand and stone were drawn into the hoppers through short chutes from thebase of the storage bins which occupied the remaining height of theshaft--about 50 ft. At the South Shaft the bins were of concrete andsteel, about 6 by 12 ft. In section, and attached to the central wall ofthe caisson. Sand and stone were delivered into them from dump-wagons onthe loading platform. At the North Shaft steel-plate bins were used, andwere supplied with material by the buckets handled by the telpher. Themixers were No. 5 Smith, belt-connected to 25-h. P. Motors, and about 0. 8cu. Yd. Of concrete was mixed at a batch. The concrete cars were steelside-dumpers of the Wiener or Koppel type. 

In order to be able to continue concreting during the winter, whenneither sand nor stone could be obtained by water, practically all thespace under the loading platforms in the South Shaft yards not occupiedby the blacksmith shop was filled with these materials, which wereplaced in storage in the late fall. 

_Intermediate-Shaft Plant. _--The air-compressing plant was located atthe rear of the 33d Street Intermediate Shaft, and supplied air fordriving the tunnels east and west from the Intermediate Shafts on both32d and 33d Streets. Two compressors, the same as the largeLaidlaw-Dunn-Gordon machine at First Avenue, were installed here, with asimilar water-cooling tower. 

Both shafts were on private property, owned by the Railroad Company, onthe north side of the streets, and each was equipped with two telpherssupported on timber trestles, similar to those at First Avenue. Here, however, the buckets were placed on wagons standing at the curb, asshown by Fig. 3, Plate LVIII. 

Blowers for ventilation were installed at each shaft, as at FirstAvenue, and, after the excavation had proceeded some distance, smallblacksmith shops, for sharpening drill steel and making minor repairs, were located in the tunnels near the shafts. 

The concrete plant in each shaft was similar in arrangement to those atFirst Avenue, but the storage bins had wooden walls made of 2 by 4-in. And 2 by 6-in. Scantling nailed flat on each other. 

The contractor's office on 33d Street backed up against the 32d Streetshaft site, and the basement was used as a storeroom for supplies forboth shafts. 

After the decision to do part of the work between Sixth and SeventhAvenues in open cut, an 8-in. Air main was laid in 33d Street to theWest Shafts, and air was supplied from the Intermediate Shaft for workon both streets in that neighborhood. 

_West-Shaft Plant. _--West of Sixth Avenue, between 32d and 33d Streetsand adjacent to the open-cut sections, the Railroad Company obtainedfrom the Hudson and Manhattan Railroad Company the use of a large areafrom which the buildings had recently been removed, and gave the use ofit to the contractor. This was of great value in prosecuting the westend of the work. The two West Shafts were located in the streets andwere supplied with short timber trestles similar to those at theIntermediate Shafts. One telpher was taken from each of the IntermediateShafts to operate at each of the West Shafts. In addition, a number ofstiff-leg derricks were set up along the open-cut section, and wereoperated by Lidgerwood or Lambert air hoisting engines, or by electricmotors, as circumstances dictated. A 15-ton Bay City locomotive cranewas also used along part of the open-cut work on 32d Street. 

Several concrete plants were installed at points along the open-cutsection, and were moved from place to place, the same generalarrangement being adopted as at the plants already described. No. 3 andNo. 4 Ransome mixers were used, and were generally set up at about thelevel of the top of the arch. The sand and stone storage bins were madeof scantlings spiked together, and were necessarily rather shallow onaccount of the proximity of the tunnels to the street surface. 

_Thirty-fifth Street Pier. _--For the receipt and disposal of materialsat the 35th Street pier, four stiff-leg derricks, operated by electrichoisting engines, were installed. Two were used in lifting the muckbuckets from the wagons and dumping their contents on the scows forfinal disposal (Fig. 4, Plate LVIII); and the other two were fitted withclam-shell buckets for unloading sand and broken stone from barges anddepositing the materials in large hoppers, from which they were drawninto wagons for transportation to the various concrete plants. A largepart of the cement (all of which was supplied by the Railroad Company)was also unloaded at the 35th Street pier and hauled directly to thework, the surplus being stored temporarily in the Company's cementwarehouses on 32d, 33d and 35th Streets, near First Avenue, from whichit was drawn as required. On the dock was located the main powdermagazine, a small concrete structure. Considerable use was also made ofneighboring piers for unloading electric conduits, lumber, steel, etc. 

[Illustration: FIG. 1. SPECIAL STEEL BUCKET PLAN OF BUCKET END VIEW SIDE VIEW OF BUCKET SECTION AT A-A]

_Tunnel Plant. _--The spoil buckets, designed by D. L. Hough and GeorgePerrine, Members, Am. Soc. C. E. , were a novel feature of the work. These buckets are shown in detail in Fig. 1 and various photographs. They were of 3 cu. Yd. Capacity and were split longitudinally, the twohalves being pinned at the apices of the ends. For lifting, they weresuspended from eyes at that point, and, when dumping, trip ropes werehooked into eyes at the bottom of each side; lifting the trip ropes orlowering the hoisting rope split the bucket, as shown in Fig. 4, PlateLVIII, and dumped the contents. They were transported in the tunnel onflat cars, and in the street on wagons, both cars and wagons beingprovided with cradles shaped to receive the bottom of the bucket. 

In the tunnels the loading was done with air-operated steam shovels, four (Model 20) Marion shovels being used at various points of the work. In Fig. 1, Plate LIX, one of these is shown loading the bucket. The carswere hauled by General Electric, standard, 10-ton, mine locomotives, thecurrent for which was taken at 220 volts from a pair of No. 00 coppertrolley wires suspended from the roof of the tunnel. The collector was asmall four-wheeled buggy riding on the wires and connected to thelocomotive by several hundred feet of cable wound on a reel for usebeyond the end of the trolley wire. Two 8-1/2-ton, Davenport, steamlocomotives were also used in 32d Street, toward the end of the work, after the headings had been holed through and the tunnels would quicklyclear themselves of gas and smoke. The steam shovels were supplementedby two Browning, 15-ton, locomotive cranes, which handled the spoil inplaces where timbering interfered with the operation of the shovels. Alltracks were of 3-ft. Gauge throughout and laid with 40-lb. Rails. 

Practically all the heavy drilling was done with Ingersoll drills (ModelE 52), the trimming being largely done with jap and baby drills. A largenumber of pumps were used at various points on the work, and practicallyall were of Cameron make, the largest ones at the shaft being 10 by 5 by13-in. The grout machines were of the vertical-cylinder, air-stirringtype. 

SHAFT SINKING. 

The sinking of the Intermediate Shafts was the first work undertaken bythe contractor. 

The 33d Street Shaft was 34. 5 ft. Long, 21 ft. Wide, and 83 ft. Deep. The rock surface averaged 5 ft. Below the ground surface. Sinking wasstarted on July 10th, 1905, and was completed on October 3d, 1905, therock being hard and dry. The average daily rate was 0. 73 ft. And anaverage of 17. 1 cu. Yd. Were excavated per day, with two shifts of 8 hr. Each. The first shift started at 6 A. M. And the second at 2. 30 P. M. , ending at 11 P. M. These hours were adopted in order to avoid unduedisturbance during the night. 

[Illustration: PLATE LIX, FIG. 1. --AIR-OPERATED STEAM SHOVEL USED INTUNNEL. ]

[Illustration: PLATE LIX, FIG. 2. --TIMBERING IN TOP HEADINGS ABOVEI-BEAMS. ]

[Illustration: PLATE LIX, FIG. 3. --FIRST SECTION OF CONCRETE LINING ATFIFTH AVENUE. ]

[Illustration: PLATE LIX, FIG. 4. --TIMBERING AND RUBBLE MASONRY OVERI-BEAMS. ]

Before blasting the first lift of rock, channel cuts 5 or 6 ft. Deepwere made along the sides of the shaft, in order to avoid damage to thewalls of neighboring buildings. Timbering was required for a depth ofonly 10 ft. Below the surface of the ground. 

A drift, 30. 6 ft. Long, 17 ft. Wide, and 27 ft. High, connected thesouth end of the shaft with the tunnels. The drift was excavated inthree stages, a top heading and a bench in two lifts. While blasting thecut in the top heading, there was enough concussion to break glass inthe neighboring buildings. The use of a radialax machine reduced theconcussion somewhat, but it was very quickly abandoned on account of thelength of time required for the drilling. 

The construction of the 32d Street Shaft was quite similar to the one on33d Street. It was 31. 5 ft. Long, 20. 5 ft. Wide, and 71 ft. Deep. Thedepth of earth excavation averaged 19. 5 ft. The rock in this shaft wasseamy and not quite as hard or dry as that in 33d Street, and timberingwas required for practically the full depth to the crown of the drift. Sinking was started on May 15th, 1905, and was completed on October26th, 1905. The daily average rate was 0. 30 ft. In earth and 0. 52 ft. Inrock. The drift was excavated in much the same manner as the one in 33dStreet, but the rock being softer the radialax machine was not used. 

TUNNEL EXCAVATION. 

During the early part of the work, the contractor devoted his entireattention to the work of excavation. Nearly all the excavation east ofFifth Avenue was done before any of the lining was placed. At a numberof points west of Fifth Avenue and at a few points to the east thenature of the rock was such that the two operations had to be donesimultaneously. 

_Single-Tunnel Method. _--For an average distance of 350 ft. West fromthe First Avenue Shafts there were four single tunnels. The rock wassound and comparatively dry. A top heading of the full size of thetunnel and about 8 ft. High was first driven. It was drilled by fourdrills mounted on two columns, and was blasted in the ordinary way. Thebench was about 13 ft. High. Tripod drills, standing on the bench, drilled the usual holes, but, owing to the lack of head-room, steelslong enough to reach the bottom of the bench could not be used. Tripoddrills were set as low as possible at the foot of the bench and drilledlifting holes. These holes were inclined downward from 10° to 15° to thehorizontal, and were spaced to converge at the location of the drainageditches. The heading was usually driven from 10 to 20 ft. In advance ofthe bench. At this distance a large part of the muck from the headingwas shot backward over the bench. In the single tunnels the muck wasloaded by hand. 

_Twin-Tunnel Methods. _--From the end of the single-track tunnel westwardto Fifth Avenue on 33d Street, and to Madison Avenue on 32d Street, withsome exceptions, each pair of tunnels was excavated for the entire widthat one operation. Three different methods of work were extensively used. They were the double-heading method, the center-heading method, and thefull-sized-heading method, and these differed only in the manner ofdrilling and blasting. The bench was usually within 10 or 15 ft. Of theface of the heading, and was drilled and fired in the same way as in thesingle tunnels. After the installation of the permanent plant, most ofthe muck was handled by steam shovels. 

In the double-heading method, shown on Plate LVII, the top headings foreach tunnel of the pair were driven separately, leaving a short rockcore-wall between them. The headings were drilled from columns in themanner described for the single tunnels. The temporary rock dividingwall between the headings was drilled by a tripod drill on the bench ofone of the headings, and was fired with the bench. 

In the center-heading method, also shown on Plate LVII, only one headingwas driven. It was rectangular in shape, about 8 ft. High and 14 ft. Wide. It was located on the center line between the tunnels. In general, the face was from 6 to 12 ft. , or the length of one or two rounds, inadvance of the remainder of the face at the top. The center heading wasdrilled by four drills mounted on two columns. By turning these drillsto the side, they were used for holes at right angles to the line of thetunnels, by which the remainder of the face of the heading was blasted. By turning the drills downward, the bench holes under the center headingwere also drilled. The center heading explored the rock in advance ofthe full-width heading, and gave a good idea as to the care needed infiring. 

For the full-width-heading method, Fig. 2, ten drills were mounted onfive columns set abreast across the face. Holes were drilled to form acut near the center line between the tunnels. The remainder of the holeswere located so that they would draw into the cut. The bench wasfrequently drilled from the same set-up of columns by turning thedrills downward. In sound rock this method proved to be the most rapidof any. 

Practically all trimming was left until immediately before theconcreting. It was then taken up as a separate operation, but proved tobe costly and tedious, and a hindrance to the placing of the lining. 

_Materials Encountered. _--All the rock encountered was the familiarHudson schist, but it varied widely in its mineral constituents and inits physical characteristics. In many places where the rock surface waspenetrated, a fine sand was found that was probably quicksand. Thematerial above the rock in the open-cut sections was mostly sand. 

[Illustration: FIG. 2. METHOD OF EXCAVATING WITH FULL-WIDTH HEADINGCROSS-TOWN TUNNELS, MANHATTAN SIDE ELEVATION FRONT ELEVATION PLAN SHOWING POSITION OF COLUMNS FOR DRILLING FACE]

The concurrence of the watercourse, shown on General Viele's map ofManhattan Island (Plate IX[D]), with the points where difficulties inthe construction of the tunnels were encountered has been noted in aprevious paper. 

In all cases where the course of this ancient stream was crossed (exceptat its final intersection of 33d Street), the rock was found to be verysoft and disintegrated, a large quantity of water was encountered, andheavy timbering was required. The construction at these localities willbe taken up later. In addition, disintegrated rock, but of a lesstroublesome character, was invariably met under the depressions in therock surface developed by the borings from the streets and test holesfrom the tunnels. Many of these places required timbering, and notimbering was elsewhere necessary except at the portals. Thesecoincident conditions were especially marked in 32d Street, which for along distance closely adjoins the course of the former creek. 

_Disposal of Spoil. _--The materials excavated from the tunnels weredumped at the 35th Street pier on barges furnished by the RailroadCompany under another contract, and were towed to points near theBayonne peninsula where the spoil was used principally in theconstruction of the Greenville Freight Yards and the line across theHackensack Meadows to the tunnels. Details of this work will be given ina subsequent paper. After December, 1907, when the excavation was about85% completed, the contractor furnished the barges and effected thecomplete disposal of the spoil. 

_Difficulties of Excavation. _--As stated in a previous paper, theexcavation of the Twin Tunnel in 33d Street was continued westward tothe west line of Fifth Avenue on the original grade. At that point thecontractor started three drifts in the three-track section. The relationof the drifts to each other and to the cross-section are shown by Fig. 3. The center heading was driven a little in advance of those on thesides. At a distance of 65 ft. West of Fifth Avenue the rock surface wasbroken through in the top of the heading, and a very fine sand wasencountered. For some distance east of this point the rock was badlydisintegrated, and the heading required timbering. Through the softmaterial, tight lagging was placed on the sides and roof of the heading, and the face was protected by breast boards. There was a moderate flowof water through the cracks, and, in spite of every effort, some of thefine sand was constantly carried into the heading. 

In one or two instances considerable ground was lost at the face. On theevening of December 14th, 1906, as a heavy coal wagon was passing along33d Street above the heading, the rear wheels dropped through theasphalt pavement. An examination disclosed a cavity under the pavementabout 14 ft. Long, 12 ft. Wide and 14 ft. Deep. Evidently, the fine sandhad gradually settled into the voids caused by the loss of material atthe face, and the settlement broke the brick sewer over the heading. Thesewer was temporarily repaired, and the hole in the street was filledbefore morning. A tight bulkhead was built across the heading, and workwas abandoned at that point. The north drift was advanced to a point108 ft. West of Fifth Avenue where sand was also encountered and aconsiderable run occurred. After that time all work on the three-tracksection was discontinued. 

The Company then took up the consideration of changes in plan. Todetermine the difficulties of driving a Twin Tunnel at a lowerelevation, an exploration drift, 8 ft. High and 12 ft. Wide, was drivenon the center line of the street as a top heading on the proposed newgrade. Test holes were drilled above this heading and to the sides. Theresults indicated that there was sufficient rock cover of fair qualityto enable the Twin Tunnel to be driven without great risk. The new plan(continuing the Twin Tunnel westward at a lower grade) was adopted inMarch, 1907, and work was immediately resumed at Fifth Avenue. 

The relation between the cross-sections under the old and new plans atthat point is shown by Fig. 3. Before the new section was excavated itwas necessary to support the timber work in the old headings. The planadopted is also shown by Fig. 3. The rock was excavated under the centerheading, as shown in cross-section, for a length of about 3 ft. A girdercomposed of two 18-in. I-beams was then put in position over each lineand supported on the sides by posts. The ends at the center linesbetween the tunnels were supported on short posts bearing on the rockbench. The support of the timbering in the headings was then transferredto the girders by additional posts. Blocking was also inserted betweenthe tops of the beams and the rock walls between the headings. Fig. 2, Plate LIX, gives a good idea of the timber work in the top headingsabove the I-beams. When the roof had been made secure, the removal ofthe bench was begun. As the work advanced it was necessary to replacethe short posts at the center of the tunnel by others of full height, and there was considerable settlement in the I-beams during thisoperation. When the bench had been removed to a point 61 ft. West ofFifth Avenue, settlement was detected in the street surface above. Benchexcavation was suspended and a section of the permanent lining, 35 ft. Long, was placed. The space between the lining and the beams and betweenthe beams and the roof was filled with rubble masonry. Grout pipes werebuilt into the masonry and later all voids were filled with grout. Fig. 3, Plate LIX, shows the first section of the concrete lining completedand part of the rubble in place; and Fig. 4, Plate LIX, shows details ofthe work above the tunnels. A second section of bench was next removedand more lining was placed. Work was continued in this way until all theroof at the old three-track headings had been secured. In this portionof the work the posts were embedded in the concrete. 

Between Fifth and Sixth Avenues there were two more sections of bad rockwhere it was necessary to support the roof with steel beams. At theselatter points there were no complications with the excavation for theThree-Track Tunnel, and the work was much simpler. To avoid leaving thecenter posts in the permanent work, two rows of temporary posts wereplaced, as shown by Fig. 1, Plate LX, the center wall and skewback werebuilt, and the posts were removed, as shown by Fig. 2, Plate LX, beforeplacing the remainder of the lining. 

In 32d Street the normal progress of the excavation was frequentlyinterrupted by encountering soft and unsound rock. In the excavationbetween the East River and the Intermediate Shafts it was possible toovercome these conditions by temporarily narrowing the excavation on oneside and supporting the roof on 16 by 16-in. Transverse timbers caughtin niches in the rock at the sides, leaving sufficient room for thesteam shovel to work through. In order to save time, the height of theexcavation was not increased before placing these timbers, so that, previous to the concreting, they all required to be raised to clear themasonry lining and were then supported on posts on the center linebetween the tunnels. This permitted the remainder of the excavation tobe made, and such additional timbering as was required was placed. Atmost of these sections a brick arch and water-proofing were used, onaccount of the presence of water. In certain places the center lineposts were buried in the core-wall, and, in order to permit the placingof the water-proofing, were then cut off one by one flush with its topas the load was transferred to the completed masonry. In other cases theload was transferred to posts clear of the masonry and the center lineposts were entirely removed. Under such conditions the normal concretemethods, to be described later, could not be used, and special formswere substituted. 

[Illustration: FIG. 3. CONSTRUCTION OF TWIN TUNNELS, THROUGH EXCAVATIONSTARTED FOR THREE-TRACK TUNNEL IN 33D STREET NEAR 5TH AVENUE]

In this section of the work the most serious difficulties wereencountered near Fourth Avenue a short distance east of the IntermediateShaft, and beneath the site of the old pond shown on General Viele'smap. The rock cover was known from the boring to be very thin, and thepresence of the subway overhead caused some anxiety. The excavation wasat first taken out to practically full width and timbered, but the rockbecame so treacherous that the heading was narrowed to a widthsufficient for one tunnel only. With this span the rock in the roof heldwithout timbering. As the masonry lining approached, sufficient trimmingwas done to permit the placing of the core-wall and one arch. Above thecompleted core-wall and brick arch the voids were filled solid withrubble masonry to give an unyielding support to the roof. The excavationof the remaining width of tunnel was then undertaken. Near the west sideof Fourth Avenue, the excavation broke out of rock at the top, and finesand and gravel with a large quantity of water were encountered. Thework of excavation was arduous, and proceeded very slowly, on account ofthe care with which it was executed. Only a small amount of sand enteredthe tunnel, but the lining was placed as soon as the excavation wascompleted. Rubble masonry packing and grout ejected through pipes builtinto the arch were used to fill the voids above the roof. As a furtherprecaution against the settlement of the subway, 2-in. Pipes were washeddown from the street above the point where soft ground was exposed inthe roof of the tunnel, and through them grout was forced into theground at various depths. Careful levels show that no settlement of thesubway has taken place. 

West of the Intermediate Shaft the tunnel was excavated for full widthuntil bad rock was encountered about 60 ft. West of Madison Avenue. (SeeGeneral Viele's map, Plate IX. ) Timbering was used for a short distance, and then the heading and bench were narrowed to 18 ft. , and steam-shovelexcavation was abandoned. As the heading advanced the rock grew steadilysofter, the difficult conditions in this locality culminating when aslushy disintegrated feldspar was met, requiring poling and breasting. Thereafter the rock improved markedly, but near the east side of FifthAvenue its thickness above the roof was found to be only 1-1/2 ft. , and the advance was stopped, pending a decision as to a change of plan. 

[Illustration: PLATE LX, FIG. 1. --DOUBLE ROW OF POSTS UNDER I-BEAMS, SUPPORTING ROOF IN BAD ROCK SECTION. ]

[Illustration: PLATE LX, FIG. 2. --CENTER WALL AND SKEWBACK UNDER I-BEAMS, AFTER REMOVAL OF DOUBLE ROW OF POSTS. ]

[Illustration: PLATE LX, FIG. 3. --TIMBERING IN FULL-WIDTH HEADING OFTHREE-TRACK TUNNEL. ]

[Illustration: PLATE LX, FIG. 4. --UNDERPINNING WALLS IN OPEN-CUTSECTION. ]

After some delay, an exploration drift, similar to the one alreadydescribed, was driven through to Sixth Avenue, and a change in plan wasmade, substantially the same as for the 33d Street tunnels. Enlargementto full size was at once started, but, for 400 ft. The rock was verysoft and poor, and required extremely careful handling. The explorationdrift was widened out to the full Twin-Tunnel width, and I-beams wereplaced and supported, in much the same manner as in 33d Street. The rockwas so soft that it was frequently necessary to drive poling boardsahead as the face was mined out with picks and shovels. The load wasvery heavy, and the work the most difficult encountered in the tunnels. 

After this stage of the enlargement was reached, the excavation of thebench and the placing of the lining proceeded alternately, with theI-beams temporarily supported on long posts while the concrete core-wallwas being built. Considerable settlement took place while shifting theposts, and eventually showed on the street surface and in the adjacentsidewalk vaults, but no damage was done to the structural portions ofthe buildings. 

While the above work had been going on westward from Fifth Avenue, theexcavation of the Twin Tunnel eastward from the end of the open-cutsection at Sixth Avenue had been proceeding rapidly, and, toward the endof the difficult Fifth Avenue work, it was being attacked from bothdirections. 

PROGRESS OF EXCAVATION. 

Owing to the numerous sections of poor rock, interspersed throughout thework with stretches of sound rock, the progress of the excavation wasvery irregular, especially in 32d Street. The rate of excavation in goodground is shown in Table 1. In the sections of bad ground, theoperations of excavation, timbering, and lining were often carried onalternately, and it is impracticable to include them in the table. 

TABLE 1. --PROGRESS AND METHODS OF EXCAVATION IN GOOD GROUND. 

THIRTY-THIRD STREET. 

============================================================ 1 | 2 | 3 |-----------------------------+--------+--------------------+ | | | | | | Type of excavation. |Tunnels. | Worked from: | | | | | | |-----------------------------+--------+--------------------+Full-sized single tunnel | B | 1st Ave. Shaft. | | | |Full-sized single tunnel | A | 1st Ave. Shaft. | | | |Full-sized twin tunnel |A and B | 1st Ave. Shaft. | | | | | | | | | |Full-sized twin tunnel |A and B |Intermediate shaft. | | | (West of shaft. ) | | | | | | |Full-sized twin tunnel |A and B |Intermediate shaft. | | | (East of shaft. ) | | | | | | |Full-sized twin tunnel |A and B |Intermediate shaft. | | | (East of shaft. ) | | | | | | |Exploration drift |A and B |Intermediate shaft. | | | (West of shaft. ) | | | |Twin tunnel. Enlargement |A and B | West shaft. | of exploration drift | | (East of shaft. ) |=============================+========+=====================

====================================================================== 4 | 5 | 6 | 7 |----------------------------------+--------+------------+------------+ | | Length | Average | DATES. | Time | tunnel | advance |----------------------------------|elapsed, | excavated, | per day, | | | in | in | in | From | To | days. |linear feet. |linear feet. |----------------------------------+--------+------------+------------+Feb. 28, 1906. |May 12, 1906. | 74 | 346 | 4. 7 | | | | | |Feb. 28, 1906. |Apr. 30, 1906. | 62 | 255 | 4. 1 | | | | | |Aug. 23, 1906. |Jan. 5, 1907. | 136 | 789 | 5. 8 | | | | | | | | | | | | | | | |Apr. 4, 1906. |Oct. 31, 1906. | 210 | 730 | 3. 5 | | | | | | | | | | | | | | | |Apr. 4, 1906. |Oct. 31, 1906. | 210 | 783 | 3. 7 | | | | | | | | | | | | | | | |Nov. 1, 1906. |Dec. 26, 1906. | 56 | 311 | 5. 5 | | | | | | | | | | | | | | | |Mar. 1, 1907. |July 23, 1907. | 145 | 947 | 6. 5 | | | | | | | | | | |Sept. 6, 1907. |Dec. 4, 1907. | 89 | 603 | 6. 8 | | | | | |===============+==================+========+============+=============

===================================================== 8-----------------------------------------------------

 Methods and conditions. 

-----------------------------------------------------Top heading and bench. Muck loaded by hand. 

 " " " " " " " "

Top full-width heading and bench. Muck loaded by steam shovel. Working exclusively on this heading. 

Top center heading and bench. Muck loaded by steam shovel. Working alternately in headings east and west of the shaft. 

Top center heading and bench. Muck loaded by steam shovel. Working alternately in headings east and west of the shaft. 

Top full-width heading and bench. Muck loaded by steam shovel working exclusively on this heading. 

Exploration drift about 9 ft. By 12 ft. Mucking by hand. Fourteen timber bents were placed in March, and seven in April, 1907. 

Drift excavated to full width and bench. Muck loaded by steam shovel. =====================================================

THIRTY-SECOND STREET. 

============================================================ 1 | 2 | 3 |-----------------------------+--------+--------------------+ | | | | | | Type of excavation. |Tunnels. | Worked from: | | | | | | |-----------------------------+--------+--------------------+Full-sized single tunnel | C | 1st Ave. Shaft. | | | |Full-sized single tunnel | D | 1st Ave. Shaft. | | | |Full-sized twin tunnel |C and D | 1st Ave. Shaft. | | | | | | | | | |Narrowed twin tunnel | C |Intermediate shaft. | | | (East of shaft. ) | | | | | | |Narrowed twin tunnel | C |Intermediate shaft. | | | (East of shaft. ) | | | | | | | | | | | | |Full-sized twin tunnel |C and D |Intermediate shaft. | | | (West of shaft. ) | | | |Exploration drift |C and D |Intermediate shaft. | | | (West of shaft. ) | | | |Twin tunnel. Enlargement }|C and D |{ Eastward from | of exploration drift }| |{ open cut. | | | |Twin tunnel. Enlargement }|C and D |{ Eastward from | of exploration drift }| |{ open cut. | | | |=============================+========+=====================

==================================+========+============+============= 4 | 5 | 6 | 7 |----------------------------------+--------+------------+------------+ | | Length | Average | DATES. | Time | tunnel | advance |----------------------------------|elapsed, | excavated, | per day, | | | in | in | in | From | To | days. |linear feet. |linear feet. |----------------------------------+--------+------------+------------+ Jan. 25, 1906. |Apr. 30, 1906. | 95 | 367 | 3. 9 | | | | | | Jan. 27, 1906. |Apr. 30, 1906. | 93 | 354 | 3. 8 | | | | | |{May. 22, 1906. |July 24, 1906. [E]}| 173 | 810 | 4. 7 |{Aug. 11, 1906. |Nov. 29, 1906. }| | | | | | | | | | | | | | Mar. 19, 1906. |May 28, 1906. | 70 | 58 | 0. 8 | | | | | | | | | | | | | | | |{May 29, 1906. |July 3, 1906. [E]}| 208 | 1, 206 | 5. 8 |{July 18, 1906. |July 31, 1906. }| | | |{Aug. 12, 1906. |Nov. 23, 1906. }| | | |{Jan. 15, 1907. |Feb. 5, 1907. }| | | |{Feb. 17, 1907. |Mar. 21, 1907. }| | | | | | | | | Dec. 1, 1905. |May. 10, 1906. | 161 | 225 | 1. 4 | | | | | | | | | | | Feb. 1, 1907. |Sept. 13, 1907. | 225 | 1, 033 | 4. 6 | | | | | | | | | | |}Feb. 1, 1908. |Feb. 14, 1908. | 14 | 65 | 4. 6 |} | | | | | | | | | |}Feb. 15, 1908. |Apr. 14, 1908. | 59 | 524 | 8. 9 |} | | | | | | | | | |======================================================================

[Footnote E: Time and distance omitted while workingthrough timbered stretches. ]

======================================================= 8------------------------------------------------------- Methods and conditions. -------------------------------------------------------Top heading and bench. Muck loaded by hand. 

 " " " " " " " "

Double heading and bench. Muck loaded by steam shovel. Stretches aggregating 200 ft. Narrowed to about 25 ft. And later enlarged are included. 

Excavation about 30 ft. Wide. Top full-width heading and bench. Muck loaded by hand. Steam shovel not installed. 

Excavation about 30 to 35 ft. Wide. Top full-width by hand and part by steam shovel. 

Double heading and bench. Part of the muck handled by hand and part by steam shovel. 

Exploration drift about 10 ft. By 13 in. Muck loaded by hand. 14 ft. Timbered. 

At portal of twin tunnels. Drift excavated to full width and bench. Muck loaded by hand. 12 ft. Timbered. 

Drift excavated to full width and bench. Muck loaded by steam shovel. Full-width tunnel timbered for 26 ft. Independently of the main excavation. =======================================================

THREE-TRACK TUNNEL EXCAVATION. 

When it became evident that the work through the Fifth Avenue sectionwould be extremely slow, shafts were sunk in each street between Sixthand Seventh Avenues. The shafts, as shown on Plate XIV, were located inthe streets, but in such a way as to block only half of the roadway. Atthe same time it was decided to construct in open cut about 200 ft. Ofthe Three-Track Tunnel at the west end of the contract in 32d Street, where the rock surface was below the top of the tunnel. It was hopedthat the remainder of the work could be built without opening thestreet, but further investigation showed that this was impracticable, and eventually all the Three-Track Tunnel in 32d Street, except 120 ft. East of the shaft, was built in open cut. 

_Thirty-second Street Work in Tunnel. _--Following the sinking of theshaft, a drift was driven across the street at the crown of the tunnel, and a top heading on the south side was excavated in both directions. Frequent cross-drifts to the north side showed that the rock was nowherevery sound and that, except for a short distance east of the shaft, itwas distinctly unfavorable for the wide Three-Track excavation. In thisstretch the north ends of these cross-cuts were connected by a secondheading, and wall-plates and sets of three-segment arch timbering wereset up to support the roof of the drifts. The cross-cuttings weregradually widened and timbered until the entire excavation had been madedown to the level of the wall-plates, as shown in Fig. 3, Plate LX. Thebench was then excavated in two lifts, leaving the wall-plates supportedon narrow longitudinal berms, which were removed in short sections topermit the placing of posts under the wall-plates. 

_Thirty-second Street Open-Cut Work. _--Before actual open-cut excavationwas started, all buildings facing it were underpinned to rock. For thispurpose, a trench was dug along the face of the buildings and of thesame depth as their cellars. Holes were cut in the front foundationwalls through which long needle-beams (Fig. 4, Plate LX) were insertedand jacked up on blocking placed on the cellar floor and in the trench, until the weight of the building had been taken off its foundations. Aclose-sheeted trench was then sunk to rock under the front buildingwalls, and a light rubble masonry retaining wall was built in it tosupport the building permanently. Frequently, the excavation for theunderpinning wall, which was taken out in sections from 30 to 40 ft. Long, and in places was carried to a depth of 40 ft. , was verytroublesome on account of the large quantity of water encountered andthe fineness of the sand, which exhibited a tendency to flow whensaturated. 

The Elevated Railroad columns in Sixth Avenue, near the north and southlines of 32d Street, were underpinned in a manner similar to thebuilding foundations, while those on the center line of the street weresupported by girders riveted to them close under the track level. Thegirders in turn were supported on posts footed on the new underpinningof the adjacent columns. On the completion of the tunnels, concretepiers were built up from the roof of the tunnel to form a permanentfoundation for the center-line columns. The area to be excavated underSixth Avenue was enclosed by a rubble masonry retaining wall constructedin a trench. 

Open-cut excavation was started by planking over the street on stringersresting on transverse 12 by 12-in. Caps. The caps were graduallyundermined and supported on temporary posts which were then replaced byshort posts resting on 12 by 12-in. Sills about 7 ft. Below the cap. Theoperation was then repeated and the sill was supported on another set ofshort posts resting on a second sill. When the excavation had beencarried down in this manner to the level of the top of the tunnel, diagonal 3 by 10-in. Timbers were cut in between the posts and sills toform a species of double A-frame, the legs of which rested in niches cutin the rock and on posts carried up the face of the underpinning wall, and the whole was stiffened with vertical tie-rods. This construction isshown by Fig. 3, Plate LXII. The brick sewer was replaced temporarily byone of riveted steel pipe. This pipe and the water and gas pipes andelectric conduits were suspended from the timbers as the pipes wereuncovered. 

Excavation in rock was made by sinking a pit to sub-grade for the fullwidth of the tunnel and advancing the face of the pit in several lifts, the muck being blown over the slope and loaded into buckets at its foot. 

The work was attacked at several places simultaneously, and the spoilwas hoisted by derricks located at convenient points along the side ofthe cut. 

_Thirty-third Street Work in Tunnel and Open Cut. _--The West 33d StreetShaft was similar to the one in 32d Street, and was sunk duringFebruary, March, and April, 1907, through 10 ft. Of earth, 21 ft. Ofsoft rock, and 29 ft. Of fairly hard rock. It was necessary to timberheavily the upper 30 ft. Of the shaft. The timber later showed evidencesof severe strain, and had to be reinforced. 

[Illustration: Plate LXI. --EXCAVATION AND TIMBERING IN HEAVY GROUNDOF THREE-TRACK TUNNEL OF 33D ST. ]

As soon as the shaft excavation was deep enough, a drift was drivenpart way across the tunnels, and top headings were started both east andwest to explore the rock. The heading to the west was divided into twodrifts, as shown on Plate LXI. These two drifts were continued to thewest end of the contract, and were then enlarged to a full-sized headingand timbered, as shown on Plate LXI and Fig. 3, Plate LX. The rock nearthe shaft contained many wet rusty seams, and settlement was detected inthe segmental tunnel timbering soon after the widening of the headingwas completed. Short props were placed under the timbers, and the streetsurface was opened with a view of stripping the earth down to the rockand thus lightening the load on the timbering. Street traffic wasmaintained on a timber structure with posts eventually carried down tothe rock surface, and the walls of the buildings on the north side ofthe street were underpinned to rock. The settlement of the tunneltimbering was checked for a time, and the bench was excavated as shownon Plate LXI. In this work the cut in the center was first made, and theshort props were replaced by struts, as shown; after this the berms wereremoved and the side posts were placed. While building the brick arches, holes were left in the masonry around the struts. After the masonry hadhardened, piers were built on the arches to support the segmentaltimbers. The struts were then removed and the openings filled withmasonry. The voids above the arch were packed with rock and afterwardthoroughly grouted. 

The timbers near the shaft continued to settle, and, although they hadbeen placed from 9 to 12 in. Above the level of the top of the masonry, by October 1st, they encroached 9 in. Within the line of masonry. It wasthen decided to remove the rock for a distance of 48 ft. West of theshaft, and build this portion of the tunnel in open cut. The postssupporting the deck forming the street surface were replaced by anA-frame structure similar to that developed for the 32d Street open cut, without interruption of the street traffic. 

After making the open cut to the westward of the shaft, there was a slipin the rock north of and adjoining the shaft. Fortunately, the timbersdid not give way entirely, and no damage was done. The open cut wasextended eastward for a distance of 46 ft. , making the total length oftunnel built in open cut on this street 94 ft. 

East of the shaft, for a distance of about 125 ft. , the rock was brokenand could not be excavated to full size without timbering the roof, butbetween this section of poor rock and those already mentioned inconnection with the work at Fifth Avenue, there was a stretch of 600 ft. Of good rock where all the spoil was handled with a steam shovel. 

TWIN-TUNNEL LINING. 

The masonry lining for the tunnels was not started until the late fallof 1906, after excavation had been in progress for a year and a half. Atthat time concreting was started in the single tunnels westward from theFirst Avenue Shafts, and by spring was in full swing in the TwinTunnels. 

The plans contemplated the use of a complete concrete lining exceptwhere large quantities of water were encountered; in which case thearches, beginning at a point 15° above the springing line, were to bebuilt of vitrified paving brick. By reference to Plate XII it will beseen that the water-proofing, which in the concrete-roof tunnelsextended the full height of the sides to the 15° line, was carried inthe brick-roof tunnels completely around the extrados of the arch. Thecross-sections also show the location of the electric conduits whichwere buried in the mass of the side and core-walls and which limited theheight to which the concrete could be carried in one operation. 

The same general scheme of operations was used wherever possiblethroughout the Twin-Tunnel work, but was subject to minor modificationsas circumstances dictated. Concrete was first deposited in the bottom, to the grade of the flow line of the drains; after it had set, collapsible box forms, of 2-in. Plank with 3-in. Plank tops, were laidon it to form the ditch and the shoulders for the flagstone covers. Thetrack, which had previously been blocked up on the rock between theditches, was raised and supported on the ditch boxes above the finishedfloor level. At the same time, light forms were braced from the ditchboxes to the grade of the base of the low-tension and telephone-ductbank. After depositing the concrete to this level, the telephone ductswere laid. 

The forms for the water-proofing or sand-wall up to the 15° line and forthe main side-walls and core-walls were built in 30-ft. Panels and weresupported on carriages, which, traveling on a broad-gauge track abovethe ditches, moved along the tunnel, section by section, as the workadvanced. The panels were hung loosely from joists carrying a platformon the top chord of the carriage trusses, and were adjustedtransversely by bracing and wedging them out from the carriage. Thesmall forms for the refuge niches, ladders, etc. , were collapsible, andwere spiked to the main panel forms just previous to the deposition ofthe concrete. The concrete was deposited from the platform on top of thecarriage, to which the cars were elevated in various ways. Plate LXIshows the details of the carriages, and is self-explanatory. 

The concrete for the sand-walls and the core-wall, to the level of thesidewalk, was deposited at the same time; two carriages in each tunnel, placed opposite each other, forming a 60-ft. Length, were used at eachsetting. The floor section of the 4-in. Tile drains had been laid withthe floor concrete, and, as the sand-wall concrete was deposited, thedrains were brought up simultaneously, broken stone being depositedbetween the tile and the rock to form a blind drain and afford access tothe open joints of the tile for the water entering the tunnel throughseams in the rock. The drains were spaced at intervals not exceeding 25ft. , depending on the wetness of the rock, and were placed at similarintervals in the core-wall under the lowest projecting points of therock on the center line between the tunnels. A small ditch lined withloose 6-in. Vitrified half pipe was provided in the top of the sand-wallto collect the water from the extrados of the arch and lead it to thetop of the drains. Great difficulty was experienced in maintaining thesedrains clear, and, on completion of the work, a large amount of laborwas expended in removing obstructions from the floor sections, the onlyportion then accessible. 

After water-proofing the sand-walls and laying the low-tension ducts, asecond pair of carriages, with panels on one side only, for 60 ft. Ofside-wall and skewback to the 15° line, were set and braced against thecore-wall. These forms are shown in connection with the carriage onPlate LXI. They were concreted to the base of the high-tension ductbank, and, after the concrete had hardened and the bank of ducts hadbeen laid, the concreting was completed in a second operation. 

In places where the roof was supported temporarily by posts and heavytimbering, such as at Fifth Avenue, the form carriages could not beused, and special methods were devised to suit the local conditions. Usually, the panels were stripped from the carriages and moved fromsection to section by hand, and, when in position, were braced to thetimbering. 

The arch centers were built up of two 5 by 3 by 3/8-in. Steel angles, and, when set, were blocked up on the sidewalks opposite each other inthe two tunnels. A temporary platform was laid on the bottom chordangles of the ribs, on which the concrete was dumped, the same as on theform carriages. The lagging used was 3 by 3-in. Dressed pine or spruce16 ft. Long, and was placed as the concreting of the arch proceededabove the 15° line on the side-wall and above the sidewalk on thecore-wall. After the arch had reached such a height that the concretecould not be passed over the lagging directly from the main platform, itwas cast on a small platform on the upper horizontal bracing of thecenters, shown in Fig. 3, Plate LIX, and was thence shoveled into thework. In the upper part of the arch the face of the concrete was kept ona radial plane, and, when only 3 ft. Remained to be placed, it was keyedin from one end, the key lagging being set in about 5-ft. Lengths. Thearches were concreted usually in 60-ft. Lengths. 

Where brick arches were used, the core-wall skewback was concretedbehind special forms set up on the sidewalks, or the arch ribs andlagging were used for forms, and the brick arch was not started untilafter the concrete had set. In laying the brick in the arch, the fivecourses of the ring were carried up as high as the void between theextrados and the rock would permit and still leave a working space inwhich to place the water-proofing. This was usually not more than 3 ft. , except on the core-wall side. The felt and pitch water-proofing was thenlaid for that height, joined to the previous water-proofing on theside-walls, and was followed by the brick armor course over thewater-proofing and by the rock packing, after which another lift ofbrick was laid and the operations were repeated. The large void (Fig. 1, Plate LXII) above the core-wall gave convenient access for working ontop of the adjacent sides of the roof, and the keying of the arches andthe water-proofing and rock packing above the core-wall were usuallycarried on from that point, the work progressing from one end. 

The concrete for all work above the floor was dumped on the platform ofthe carriages, to which it was transported in the early part of the workin cars running on a high-level track laid on long ties, resting on thefinished sidewalks. This arrangement, although requiring a large amountof timber for the track, permitted the muck to be carried out on thelow-level track without interference. Later, when the advance of theheading had ceased and the heavy mucking was over, all concrete wastransported on the floor level, and the cars were lifted to the carriageplatforms by elevators and were hauled by hoisting engines up a movableincline. The latter method is shown by Fig. 3, Plate LIX. 

_Water-Proofing. _--The water-proofing referred to above was in all casesfelt and pitch laid with six thicknesses of felt and seven of pitch. Thesub-contractor for the work was the Sicilian Asphalt Paving Company. Alljoints were lapped at least 1 ft. , and, where work was suspended for atime and a bevel lap could not be made, the edges of the felt were leftunpitched for 1 ft. And the newer work was interlaced with the old. Thismethod was not always successful, however, on account of the softeningof the unpitched felt on long-continued exposure to the water. The feltused was mainly "Tunaloid, " together with some "Hydrex. " It weighedabout 12 lb. Per 100 sq. Ft. When saturated and coated on one side only, and contained about 25% of wool. The coal-tar pitch used had a meltingpoint of 100° Fahr. 

After the completion of the tunnel, the concrete arch showed someleakage and in places unsightly lime deposits. It was determined toattempt to stop these leaks by the application of a water-proof cementcoating on the intrados of the arch. Extended experimental applicationof two varieties of materials used for this purpose--"Hydrolithic"cement and the U. S. Water-proofing Company's compound--have been madewith apparent success up to the present time, and the results after thelapse of a considerable period are awaited with interest. 

_Duct Laying. _--The position of the electric conduits, buried in theheart of the concrete walls, interfered greatly with the economical andspeedy placing of the lining, and their laying proved to be one of themost troublesome features of the work. The power conduits weresingle-way, with the bank for high-tension cables separated in theside-walls from the low-tension bank, as shown on Plate XII. Theconduits for telephone and telegraph service were four-way, and werelocated in the core-wall. All ducts had 3/4-in. Walls and a minimumclear opening of 3-3/8 in. Square, with corners rounded. They were laidwith joints broken in all directions, and in about 1/4-in. Beds of1:2-1/2 mortar. Flat steel bond-irons, 2 by 1/8 in. , with split and bentends, were placed in the joints at intervals of 3 ft. And projected intothe concrete 3 in. On each side, tying together the concrete on oppositesides of the ducts. The joints were wrapped with a 6-in. Strip of10-oz. Duck saturated with neat-cement grout, and, in addition, thepower conduits were completely covered with a 1/2-in. Coat of mortar toprevent the intrusion of cement and sand from the fluid concrete. Thefour-way conduits were plastered only over the wraps. Splicing chamberswere provided at intervals of 400 ft. 

[Illustration: PLATE LXII, FIG. 1. --WATER-PROOFING OVER BRICKARCHES. ]

[Illustration: PLATE LXII, FIG. 2. --TRESTLE USED IN CONCRETING INTHREE-TRACK TUNNEL. ]

[Illustration: PLATE LXII, FIG. 3. --METHOD OF STREET SUPPORT OVEROPEN-CUT EXCAVATION. ]

[Illustration: PLATE LXII, FIG. 4. --JUNCTION OF TWIN AND THREE-TRACKTUNNELS. ]

THREE-TRACK TUNNEL LINING. 

In the Three-Track Tunnels, a heavy brick arch was used for thoseportions constructed in tunnel, while, in the open-cut sections, theroof was of concrete. Both were completely water-proofed on the roof andsides, and in the tunnel sections the space above the brick roof wasfilled with rock packing. On account of the unstable nature of the rockencountered throughout, the voids in the packing were afterward filledwith grout. 

By reference to the cross-sections, Plate XII, it will be seen that thehaunches of the arch were tied together by steel I-beams anchored in theconcrete, with the object of making the structure self-supporting in theevent of the removal of the adjacent rock for deep cellar excavations. This construction materially influenced the contractor's method ofplacing the masonry lining. 

After depositing the floor concrete, by the same method that was used inthe Twin Tunnels, a timber trestle (Fig. 2, Plate LXII) was erected tothe height of the underside of the I-beam ties, the posts being footedin holes, about 3 in. Deep, left in the concrete floor to preventslipping. In the open-cut sections the sand-wall forms were of undressedplank tacked to the studding and braced from the trestle; in the tunnelsection they were spiked to the face of the posts supporting thetimbering. 

The side-wall forms were made up in panels about 3 by 10 ft. , and wereclamped to studs by U-shaped irons passing around the stud and bolted tothe cleats on the back of the panels, the studs being braced from thetrestle. The side-wall concrete was deposited in three sections. Thefirst was brought up just above the sidewalk and formed the bench forthe high-tension ducts; the second carried the wall up to the springingline. Before placing the third section the I-beam ties were set inposition (Fig. 3, Plate LXII) on top of the trestle, and the reinforcingrods in the haunch of the arch were hung from them. The concrete wascarried up to a skewback for the arch, as shown in the brick-roofcross-section (Plate XII) and embedded the ends of the ties. 

The centers for the arches stood on the I-beam ties, and the tops of thehangers, for the permanent support of the ties near their center, wereinserted through the lagging. The brick arch, water-proofing, and rockpacking were laid up in lifts, in the same manner as in the Twin Tunnel, with grout pipes built in at intervals of about 8 ft. The concrete archwas placed in sections, from 25 to 50 ft. In length, with a rather wetmixture and a back form on the steep slope of the extrados. 

The concrete for the sand-walls and lower part of side-walls was handledon tracks and platforms laid on cantilever beams at mid-height of thetrestle, as shown by Fig. 3, Plate LXII. For the walls above thespringing line, the tracks were laid on top of the I-beam ties, and someof the arch concrete, also, was delivered from the mixer at that leveland hauled up an incline to the level of the top of the arch. By far thegreater part, however, was turned out from mixers set on the completedarch, and was transported on tracks hung in part from the streettimbering. 

_Completion. _--Except in the heavily-timbered portions, such as at FifthAvenue, where the load had to be transferred from posts to the completedmasonry section by section, the lining of the tunnels presented nospecial difficulty. The large number of small forms to be set, and themutual interference of the concreting and duct-laying operations provedto be the most troublesome features of the work. 

The restoration of the streets, public utilities, etc. , at the open-cutsections was a slow and tedious operation, but the tunnels themselveswere completed in March, 1909, 3 years and 10 months after the inceptionof the work. The finished tunnels are shown by the photograph, Fig. 4, Plate LXII, taken at the junction of the twin and three-track types. 

FOOTNOTES:

[Footnote A: Presented at the meeting of December 1st, 1909. ]

[Footnote B: Of the paper by Mr. Noble. ]

[Footnote C: Of the paper by Mr. Noble. ]

[Footnote D: Of the paper by Mr. Noble. ]