Chapter 6

When the original lagging had become too much worn for further use, it was resurfaced with strips of 7/8 by 2½-in., clear, tongued and grooved, hard pine, placed vertically, which did fairly well and lasted to the end (about 1,000 ft.), although it was not altogether satisfactory, and the last eight or ten sections built had to be rubbed down with a wooden float in order to obtain a suitable finish.

In designing the forms for all exposed surfaces in the tunnels, it was the desire of the contractors to obtain directly from them a surface which would be satisfactory to the engineers without further finishing than the patching of minor defects. In this they were generally quite successful, and excellent results were obtained, as shown in the view of the finished tunnel, Fig. 2, Plate XXVII. The surface of the bench-walls was obtained solely by spading the face with a flat spade as the work progressed. No after treatment was resorted to, except for the few sections where the forms became worn. The top of the bench-wall was finished with a float about 2 or 3 hours after the concrete was placed.

When the work was well organized, a bench-wall was built at each end each day, one day in the North Tunnel, and the following day in the South. During the time sand-walls were being built, a sand-wall and bench-wall were built on alternate days in each tunnel, care being taken that when a bench-wall was being built in one tunnel, the sand-wall was being built in the other, this being necessary in order to equalize the work of the night gang and the conduit layers as well as the transportation.

The conduit layers on the day shift, two or three men and a foreman, required about 2 hours in the forenoon and 1 hour in the afternoon to lay their portion of the conduits, and usually finished this work by 3 P.M. At other times during the shift they were utilized at those points where rock packing was heaviest, and when the packing was brought in in the large cars, as shown in Fig. 1, Plate XXVI, these men helped unload it so that the track could be cleared as soon as possible. When water-proofing was to be done, the number of men in this gang was increased, so as to enable them to do that work also.

A gang of four rough carpenters and a foreman was employed on the day shift; they moved and set the bench-wall forms or sand-wall forms, as the case might be, and moved the traveling gantry into position. This was done in the afternoon, and required about 3 hours. They also took out, cleaned, repaired, and set all ditch forms, all passenger forms, circuit-breaker forms, and did all other repair work. The ladder forms, the refuge-niche forms, and overhead conductor pocket forms were attended to by one man, who set, removed, cleaned, and repaired them. The carpenters on the night shift set the arch centers and gantries, also the manhole forms when needed. The conduit layers on the night shift laid up half the 4-way conduits (3-high) and one-third of the single ducts (4-high). This one gang laid the conduits in two sections of bench-wall each night, that is, one section at Weehawken and the other at the western end.

In concreting the bench-walls, the concrete was first placed on the side containing the single conduit until it reached the top of the four tiers laid, then the concrete gang was turned over to the side with the 4-way conduits while four more tiers of single conduits were laid, the work thus progressing, the conduits being laid on one side while concrete was placed on the other. On the side of the 4-way conduits the concrete was built in two layers while that on the side of the single ducts was built in three; the interval between the different layers was not sufficiently long to prevent a complete bond being obtained, and there were only one or two instances where there was any mark on the face to indicate a break.

After the work had been in progress some time, it was found to be quite feasible to build all the 4-way conduits at night and half the single conduits, that is, 6 ducts high, as the mandrels proved amply sufficient to hold them in place; in fact, had it been necessary, the writer has no doubt that all the ducts might have been laid and held in place with very little extra precaution, by the use of the expanding mandrels, as described under the head of conduit laying. A =V=-shaped joint about ½ in. deep was made between each section of bench-wall so that the expansion cracks would follow this joint rather than show irregularly on the face. These joints divided the face into the even 25-ft. panels, and were very effectual in concealing what few cracks there were.

After the construction of the sand-walls was discontinued, the space behind the bench-walls, between the neat line and the rock, was filled with rock packing, which was generally built, part way up at least, as a dry wall ahead of the construction of the bench-wall, or it was put in place simultaneously with the concrete, care being taken to keep it as free as possible for the drainage of any water there might be. Toward the latter part of the work, owing to the difficulty of getting sufficient rock packing during the day, a rough back form for the bench-wall was built at the neat line, in places where the section was at all large, and the space was filled with rock afterward, generally at night or on Sundays.

In the sections where water-proofing was required, where no sand-wall was built, the rock was taken out for 2 ft. outside the neat line, if the excavation was not already that far out (at the expense of the contractors, who preferred to do this rather than build the sand-walls for the short sections required), so that there would be sufficient room for placing the water-proofing on the back of the bench-walls, as shown by Fig. 18, _E_. The water-proofing of these sections was left until just before the arch was to be built, and after being placed it was protected by a single row of brick laid on edge before the rock packing was filled in.

_Arches._--The centering used for the arches is shown very clearly in Fig. 4, Plate XXV, which is a view of the back end of the first section built at Weehawken. In this part of the tunnel, the lower part of the arch, about 5 ft. above the bench-wall, was built first, as previously referred to, but the centers, as will be seen, were built so that they could be used for the whole of the arch. The forward bulkhead, and the shoveling platform on a section being built, are shown in Fig. 3, Plate XXVI.

The front bulkheads used were made in nine sections, bolted to a 2½ by 2½-in. angle bent to the radius of the arch, as shown in Fig. 3, Plate XXVI, and fitting on the end of the lagging; when set they were braced partly against the rock of the roof and partly against the gantry. After the ribs and part of the lagging had been set by the night gang for a fresh section of arch, the braces holding the bulkheads were knocked out, the concrete placed during the day having set sufficiently by this time; the whole of the bulkhead was then easily moved ahead, sliding along the lagging to the forward end, and made ready for the next day's work. The middle section at the top was taken out temporarily, to facilitate working at the sides, until it was needed.

The traveling gantry used in handling the concrete for the arch is shown in Fig. 1, Plate XXVI, which also shows the form for the circuit-breaker chamber, and a car of rock packing on the track beneath.

The arches were built in 10-ft. sections, the ribs being spaced 5 ft. apart, the end ribs of each section supporting the end of the lagging on two adjoining sections. Five sets of lagging and ten ribs were used at each place where the arch was being built, thus giving each section practically 4 days' set before removing the centers. Probably in the greater part of the work the centers could have been removed in from 40 to 48 hours after the concrete had been placed, but 3 days was considered the least time which would certainly be safe at all times, and the contractors thought that the very slight additional expense involved in leaving the centers up 4 days was more than warranted by the additional feeling of security.

The lagging was made from 3 by 6-in. clear, hard pine, 10 ft. long, dressed to about 2½ in. in thickness, about 5½ in. in width, and the sides to radial lines. As it was placed, every third or fourth piece was lightly nailed to the ribs; when the latter were released and taken down, the nails pulled out, and the lagging was left in place until one piece was pried out, allowing the others to fall. A light =A=-frame, about 8 ft. long, spanning the bench-walls, was placed below, in order to break the fall and allow the lagging to slide to the top of the bench-walls rather than fall to the track beneath.

Cross-passages between the two tunnels were built every 300 ft., their form being shown on Plate VIII of the paper by Mr. Jacobs. There were two circuit-breaker chambers, one at Station 286 and the other at Station 310. Steel doors are provided so that all the openings between the two tunnels can be closed. At Station 294+24, the core-wall broke through for a length of about 40 ft., and instead of filling this in, a storage chamber 34 ft. long and 11 ft. wide, inside, was built there, the form for which is shown in Fig. 2, Plate XXVI. This photograph, as well as Fig. 1, Plate XXVI, a form for a circuit-breaker chamber, shows the method of setting the steel doors in the forms, so that they were built into the concrete instead of being fastened in with expansion bolts afterward, thus showing a perfect fit and a much neater job.

During construction the arches in each tunnel were kept even with each other, so that when the cross-passages were reached, they, and the sections of arch which they joined, could be completed at one operation.

By the methods used on this work, one section of arch was easily built in a shift, so that the monolithic construction of each section was easily secured, and concrete, as wet as it was possible to handle with shovels, could be used for all except the last 5 ft. or so at the top, thus getting a structure which was as nearly impervious as possible under the circumstances.

The gangs placing the arches were paid over-time when they were required to work after 6 o'clock to finish their section, which was generally only necessary when the quantity of rock packing to be placed was very large. If they finished their section before 6 o'clock, however, they were allowed to quit when this was done, and were given a full day's pay. The difference in time, when there was any, was usually due to the greater or less quantity of rock packing, as the excavation varied from the standard section line.

In building the arches, the night gang set the two ribs (one at the center and one at the forward end of the section to be built), placed the lagging on the sides, 4 or 5 ft. high, built the shoveling platform on the horizontal cross-braces of the ribs, and placed the traveling gantry in position for use. The forward end of the gantry (that is, the end farthest from the arch being built), as shown in Fig. 1, Plate XXVI, was loaded with rock packing to be used as required. As the concrete was brought into the tunnel it was hoisted and dumped on the end of the gantry next the arch, and shoveled from there to the platform on the ribs and from there into place. The rock packing brought in during the day was dumped on the front or back end of the gantry, as was most convenient, and handled into the work in the intervals between batches of concrete. The concrete and rock packing, with the back-lagging and water-proofing, where these were used, were placed simultaneously, or nearly so, and brought up the sides together until the key was reached; the latter was then worked from the back toward the front. The key was usually made about 5 ft. wide, the lagging for this width was made 5 ft. long and put up in two sections. It was found to be more convenient to have the key of this width than narrower.

The method used in making the closures where two sections of the arch came together is shown by Fig. 17.

_Water-proofing._--As already pointed out, the original design for the lining of these tunnels provided for a brick arch. It was intended to cover this arch with water-proofing, this latter extending over the whole of the roof and down the sides as far as the bottom of the conduit lines. The water-proofing was to be placed against the sand-walls on the sides, up to the top of the side walls, Figs. 10 and 14. Over the arch, after being placed, it was to be protected by an armor course of brick, laid flat, the space between the brick and the excavation, which was required to be not less than 4 in. (and, as a matter of fact, was actually a great deal more), being filled with rock packing. Besides filling the space, this latter was designed to allow any water from the roof of the tunnel to find its way easily to the top of the sand-wall, from there being carried through the 4-in. cast-iron pipes, shown on Plate VIII[4] to the side ditches in the floor of the tunnel.

[Footnote 4: Of the paper by Mr. Jacobs.]

All the water-proofing placed in these tunnels was of felt and pitch, six-ply felt and seven layers of pitch. The felt was required to be Hydrex, or of equal quality, and the pitch, "Straight run coal-tar pitch which will soften at 60° Fahr., of a grade in which the distillate oils will have a specific gravity of 1.05."

In addition to tests as to the above qualities, the pitch was analyzed to determine the amount of free carbon it contained, and was not accepted if this fell below 20 per cent.

It was considered quite important that there should be absolutely free drainage on the outer side of the lining, so that there would be no chance for any water to acquire a head. More than three-quarters of the length of these tunnels is below the level of mean high water, and while it was hardly expected that there would be any direct connection between the water in the Hudson River and the groundwater of the section penetrated, it was thought wise to provide ample drainage.

Before the lining was started, however, the excavation had progressed sufficiently to show that the tunnels, while very wet in places, and varying from that to quite damp, would be, on the whole, much dryer than had been anticipated. It was then decided to substitute concrete for the brick in the arch and omit the water-proofing over the top, except at places where water came into the tunnels in sufficiently large quantities to form practically a continuous stream. Three general types of construction for the arch were decided on, as shown in Fig. 18. The first, as shown at _A_, was to be used where the tunnel was quite dry. In this type, the sand-wall was omitted entirely, and the concrete and rock packing were built up together, the rock packing impinging to a certain extent on the concrete, and the concrete squeezing somewhat into the rock packing, as shown by Fig. 4, Plate XXV. The section shown at _B_ was used where the tunnels were damp, or where there were slight droppers not forming a continuous stream. The back lagging, of 1-in. boards, which was left in place, provided a practically smooth outer surface on the concrete arch, and allowing the concrete and rock packing to be built almost simultaneously. It was considered that the free drainage through the rock packing, the surface of the boards, and the smooth outer surface of the concrete in the arch would allow the comparatively small quantity of water in these parts of the tunnel to find its way to the sides, and thence to the ditches at the bottom, rather than to percolate through the concrete, and this proved to be very generally the case, as is shown by the dry condition of the tunnel as built. The back lagging was used over the arch, both where the sand-wall was built and where it was omitted, as well as being placed over the water-proofing of the arch as an armor course where water-proofing was required. Where the sand-walls were built and water-proofed, and where the water-proofing was not carried over the arch, the water-proofing was turned in at the top, as shown at _C_, Fig. 18.

The third method provided for water-proofing the whole of the arch, and was the same as _B_ except for the addition of the water-proofing inside the back lagging. In placing this water-proofing, the felt was cut in strips about 11 ft. long (about 1 ft. longer than the length of a section of arch), and six thicknesses were cemented together with hot pitch. These mats were then laid shingle-fashion, as shown at _D_, Fig. 18, up the sides of the arch until a space about 5 ft. wide remained at the crown; shorter mats were then brought out over this, laying them perpendicular to the axis of the tunnel. Care was taken in making all laps, irrespective of the direction in which the arch was built, so that they would lay with the grade, that is, so that the water would tend to flow over the edges of the laps rather than against them.

Most of the wet sections of the tunnel were at the ends, where sand-walls had been built for the purpose of providing a smooth surface against which the water-proofing was to be placed; there were several wet places at isolated points in the tunnels, however, and, in order to avoid building sand-walls at these points, the method shown at _E_, Fig. 18, was adopted. This involved a slightly larger excavation, 2 ft. outside of the neat line, up to the height of the top of the bench, where there was not already that much room. The bench-wall was built with a back form on the neat line, the water-proofing was placed as shown, protected by an armor course of brick, and then continued over the arch when this latter was built. The excavation and refilling with rock packing were done at the contractor's expense, which he was willing to assume rather than build these short sections of sand-wall.

The method of water-proofing that part of the timbered section which was very wet, is shown at _F_, Fig. 18, and in Fig. 4, Plate XXVI, and Fig. 1, Plate XXVII. A lagging of 1-in. boards was nailed up the sides and to the soffit of the segmental timbering, all the spaces outside of this lagging being carefully filled with rock packing. Before starting any concrete work, a single thickness of water-proofing felt was nailed to the inner side of the lagging, which not only served to protect the finished surfaces of the concrete from the water which fell copiously from the roof, but also provided a comparatively dry surface to which the regular six-ply water-proofing could be cemented with pitch and held in position, while the concrete was placed against it.

In placing the water-proofing in this section on the sides, the strips of felt were placed vertically, nailed at the top to the wall-plate, to support their weight, and lapped and cemented with pitch to the sides as on the sand-walls, except that there was no trouble from the overhang. After the bench-wall had been built, the felt was cut just below the nails and about 2 ft. above the top of the bench, so that the mats which were placed over the arch could be inserted behind it. The roof was covered with three-ply mats and lapped over a little more than half, as shown diagrammatically on the drawing.

When the upper part of the arch was reached, where the cementing strength of the pitch was not sufficient to hold the felt in place, the mats were braced temporarily from the centering, as shown by Fig. 1, Plate XXVII, until the concrete could be packed against it.

Where the water-proofing was placed against the sand-wall, the method of securing the sheets at the top is shown in the small sketch on Fig. 14 and by Figs. 3 and 4, Plate XXIV. Fig. 3, Plate XXV, shows the laps of the sheets and the method of hanging. At the start an attempt was made to stick the water-proofing to the sand-wall, but this could not be done on account of its dampness and the overhang at the top.

The sand-wall water-proofing was kept about 35 ft. ahead of the finished bench-wall, as shown by Fig. 3, Plate XXV. As the bench-wall form was moved ahead and set, the mat was braced back against the sand-wall from the forms at a point just above the top of the finished bench, care being taken to avoid wrinkles, as, if these were once formed, it was practically impossible to straighten them out.

The completion of the bench-wall left the upper part of this water-proofing stretched taut across the curved top of the sand-wall, forming a chord of the arc. As the arch was built up, the top was gradually slackened so as to allow the concrete to press the mat back into place until the top of the sand-wall was reached, when the end was turned in, as shown at _C_, Fig. 18, or the water-proofing was continued over the arch, if that was necessary.