Part 3

The plans and designs for these tunnels are shown on Plate XXXII. In this short length of about 977 ft. there are no less than nine different kinds of cross-section. The reason for these changes is the fact that the two lines of track are here not straight and parallel to the center line between the tunnels, but are curved, although symmetrical about this center line. The various changes of section are to enable the tunnels to be built in straight lengths, thus avoiding the disadvantages attending the use of curved forms, and at the same time minimizing the quantity of excavation, an item which accounts for from 60 to 70% of the total cost of tunnels of this type. Of course, there are corresponding and obvious disadvantages in the adoption of many short lengths of different cross-sections, and these disadvantages were well brought out in the course of the work; on the whole, however, they may be said to have justified their adoption. These New York Land Tunnels were divided into three contracts, viz.: From Station 190 + 15 (the Portal to the open work of the Terminal Station at the east side of Tenth Avenue, New York City) to Station 197 + 60, called "Section Gy-East." The next contract, called "Section Gy-West Supplementary," extended from Station 197 + 60 to Station 199 + 20, which is the east side of Eleventh Avenue. The third contract was called "Section Gy-West," and extended from Station 199 + 20 to Station 231 + 78 (the dividing line between the States of New York and New Jersey). Thus, for nearly all its length, this contract consists of shield-driven tunnel. The portion between Stations 199 + 20 and 199 + 91.5, however, was of the Land Tunnel type, and therefore will be included here. A fourth contract extended from Station 231 + 78 to the Weehawken Shaft at Station 263 + 50, and of this all but 230 ft. was of the shield-driven type, only the portion next to the Weehawken Shaft being of the Land Tunnel type.

The four contracts were let to one contractor (The O'Rourke Engineering Construction Company), and the work was carried on simultaneously in all four, so that the division into contracts had no bearing on the methods of work adopted, and these will now be described as a whole and with no further reference to the different sections.

Excavation.

Work was started on the New York side on April 18th, 1904, the Weehawken shaft being at that date still under construction. As will have been noted in the description of the shafts, the contractor found a shaft already prepared for his use, and cross-drifts at grade and at right angles to the future tunnels, and extending across their entire width. The first essential was to get access to the shield chambers, which were to lie about 330 ft. to the west of the shaft, so that the construction of these enlargements in which the shields for the subaqueous tunnels were to be built might be finished as soon as possible and thus allow the earliest possible start to be made with the shield-driven tunnels.

With this in view, two bottom headings, on the center line of each of the two tracks, were driven westward from the western cross-heading at the foot of the shaft. When about 138 ft. had been made in this way, the two headings were brought together and a break-up was made to the crown level of the tunnel, as the depth of rock cover was doubtful. From this break-up a top heading was driven westward to Station 200 + 30. While widening the heading out at Station 200 + 20 the rock was penetrated on the south side. The exposed wet sand and gravel started to run, and, as a consequence, a change in design was made, the shield chambers (and consequently the start of the shield-driven tunnels) being moved eastward from their original location 133 ft. to their present location. A certain amount of time was necessarily spent in making these changes of design, which involved a rearrangement of the whole layout from the Terminal Station to the start of the River Tunnels. On July 5th, 1904, however, the new design was formally approved. No sooner had this been decided than a strike arose on the work, and this was not settled until August 1st, 1904, but from that time the work progressed without delay. No further reference will be made to the work in the shield chambers, as that will come under the heading of "River Tunnels," being of the segmental, cast-iron lined type.

A top heading was now driven over the original bottom heading west of the shaft, and at the same time the original cross-drifts from the shaft were amalgamated with and broken down by a heading driven at the crown level of the "Intercepting Arch" which here cuts across the ordinary run of tunnel at right angles and affords access to the tunnels from the shafts.

The excavation of the upper portion of the intercepting arch at its southern end gave some trouble, and caused some anxiety, as the rock cover was penetrated and the wet sand and gravel were exposed. This made it necessary to timber all this section heavily, and the tracks of the New York Central Railroad directly above were successfully supported. The general way in which this timbering was carried out will be described under the head of "Timbering."

Meanwhile, the excavation of the tunnels west of the intercepting arch was continued until the North and South Tunnels were taken out to their full outlines, leaving a core of rock between them. This core was gradually removed, and timbering supporting the rock above the middle wall was put in as excavation went on. The ground, which was entirely of micaceous schist, typical of this part of Manhattan, seamed with veins of granite, was rather heavy at the west end, or adjacent to the shield chambers, and required complete segmental timbering across the whole span. One heavy fall of rock in the corewall between the North and South Tunnels took place on November 2d, but fortunately did not extend beyond the limits of the permanent work. On November 7th, 1904, the excavation east of the intercepting arch was begun by driving a bottom heading in the South Tunnel. This was continued to Station 197 + 14 and then was taken up to the crown level and worked as a top heading with the view of keeping track, by making exploratory borings upward from the roof at frequent intervals, of the rock surface, which was here irregular and not known with any degree of certainty. The work was not pressed with any vigor, because all efforts were then being bent toward excavating from the River Tunnels as much rock as possible. In Section Gy-East the conditions were exceptionally variable, as the rock was subject to sudden changes from a soft crumbling mica schist to broad bands of hard granite, and, in addition, the rock surface was very irregular, and, for a good length of this section, was below the crown of the tunnel, a condition which led to the adoption of the cut-and-cover method for part of the work.

The irregularity in conditions called for varying methods of procedure, but in general the methods were as shown on Plate XXXIII, and described as follows:

_In Solid Rock._--Where there was plenty of good rock cover, a top middle heading was driven, which was afterward widened out to the full cross-section of the twin tunnel arches. If necessary, a few lengths of segmental timbering were put in before taking down the bench, which was generally kept some 40 or 50 ft. behind the breast of the heading. After the bench was down, the middle conduit trench was excavated and the trimming done.

_In Soft Rock._--Where there was not enough rock cover, or where there was actual soft ground in the roof, wall-plate headings at the springing line level were driven ahead of the remainder of the work. The wall-plates were laid in these, the roof was taken out in short lengths, and segmental timbering spanning from wall-plate to wall-plate was put in. The roof being thus held, the bench excavation proceeded without trouble. Where the rock was penetrated and soft ground showed in the roof, poling boards were driven ahead over the crown-bars, as shown in Fig. 4.

_Cut-and-Cover Work._--After some 225 ft. had been driven from the intercepting arch, it was found that the crown of the tunnel was continually in soft ground. To ascertain the extent of this condition the contractor decided to make soundings as far as Tenth Avenue, which was done by sinking trial pits and making wash-borings in the street. These soundings showed that there would be soft ground in the crown from Station 194 + 75 to Station 194 + 25 (at one point to a depth of 12 ft. below the crown), and on each side of this section the cover was insufficient from Station 193 + 58 to Station 195 + 30. This condition being known, it was decided to adopt cut-and-cover work for this length, the principal reasons being that repairs to sewers, streets, and drains would be no more, and probably less, expensive than with the tunnel method; the underpinning of a heavy brick brewery building adjoining the works on the north side would be facilitated, and the opening in the street, through which muck and materials could be handled, would relieve the congested shaft, through which the large volume of muck from the River Tunnels was then being conveyed. On the other hand, the cut-and-cover method was adversely affected by the presence of a heavy timber trestle built down the south side of the street and over which passed all the excavation from the Terminal Station, amounting to a very heavy traffic. As this trestle had to be supported, it complicated the situation considerably. Very little active progress was made between June, 1905, and April, 1906, as the contractor's energies during that time were much taken up with the progress of the shield-driven tunnels. In April, 1906, preparations were made to start a 50-ft. length of open cut, rangers being fixed and sheathing driven; and the sewer which ran down the middle of this street was diverted to the outside of the open-cut length.

April and May were occupied in driving the sheathing down to rock, supporting the trestle, underpinning the adjoining brewery, and excavating the soft material above the rock. On June 2d, 1906, rock was reached, and, by July 31st, the excavation was down to subgrade over nearly the whole 50 ft. in the first length. In the meantime another length was opened up, and eventually a third.

The surface of the rock now seemed to be rising, and the heavy buildings had been passed, so that tunneling was reverted to for the rest of the work, though many difficulties were caused by soft rock in the roof from time to time.

When the excavation for the open-cut work of the Terminal Station had advanced to the line of Tenth Avenue, the contractor started a heading from this point and drove westward under Tenth Avenue until the headings driven eastward from the cut-and-cover portion, were met.

This was done to expedite the work under Tenth Avenue, where the ground was not very good, where there were several important gas and water mains in the street, and where, moreover, the tunnels were of exceptionally large span (24 ft. 6 in.), making a total width of some 60 ft. for the excavation. The excavation for the New York Tunnels was practically finished in January, 1908.

_Drilling and Blasting._--The foregoing short description will serve to show in a general way the scheme adopted in making the excavation. A few details on drilling and blasting methods may not be out of place.

Percussive drills run by air pressure were used. They were Ingersoll-Sergeant, Nos. 3½, A-86, C-24, and F-24. The air came from the high-pressure compressor previously described. This compressor, without assistance, could supply air for nine drills, but, when fed by compressed air from the lower pressure, its capacity was increased three or four times.

The air was compressed to 100 lb. per sq. in. in the power-house, and was delivered at about 80 lb. per sq. in. at the drills. A 3-in. air line was used. The drill steel was 1-1/8-to 1-3/8-in. octagonal. The holes were about 3¼ in. in diameter at starting and 2-5/8 in. at the full depth of 10 ft. The powder used on the New York side was 40% Forcite, the near presence of heavy buildings and lack of much rock cover necessitating light charges and many holes spaced close together.

To compensate the contractor for the inevitable excavation done outside the neat lines of the masonry lining, the excavation was paid for to the "Standard Section Line" which was 12 in. outside the neat lines on top and sides and 6 in. outside at the bottom of the cross-section. The actual amount of excavation done was about 11% greater than that paid for. The distance excavated beyond the neat line, because of the very heavy timbering necessary, was about 2.1 ft. instead of the 1 ft. allowed, and at the bottom about 0.85 ft. instead of the 0.50 ft. paid for.

For a period of 5 months detailed records were kept of the drilling and blasting. About 12,900 cu. yd. of excavation are included. A sketch and table showing the method of driving the heading, the number and location of the holes drilled, and the amount of powder used, is given in Fig. 5. From this and similar figures the information in Table 5 is derived.

TABLE 5.

+========================+=======+=======+=======+======+=======+======| | | FEET OF HOLE | POUNDS OF POWDER | | |DRILLED PER CUBIC YARD | USED PER CUBIC YARD | | | OF EXCAVATION. | OF EXCAVATION. | | +-------+-------+-------+------+-------+------| |Portion of excavation. |15-ft. |19-ft. |24-ft. | | | | | | 4-in. | 6-in. | 6-in. |15-ft.|19-ft. |24-ft.| | |span-- |span-- |span-- |4-in. | 6-in. | 6-in.| | | twin | twin | twin | | | | | |tunnel.|tunnel.|tunnel.| | | | |------------------------+-------+-------+-------+------+-------+------+ |Wall-plate heading[C] | 13.0 | 10.97 | 10.97 |3.77 | 2.85 | 2.85 | | | | | | | | | |Total heading[C] | 7.87 | 8.17 | 7.81 |2.31 | 2.02 | 1.78 | | | | | | | | | |Bench and raker bench[C]| 5.97 | 6.15 | 7.56 |0.94 | 0.93 | 1.13 | | | | | | | | | |Trench[C] | 9.82 | 15.96 | 18.10 |1.84 | 2.49 | 2.73 | |------------------------+-------+-------+-------+------+-------+------+ |Average for section[C] | 6.69 | 7.43 | 8.95 |1.28 | 1.30 | 1.45 | |------------------------+-------+-------+-------+------+-------+------| |Actual amount[D] | 6.82 | 7.27 | 8.95 |1.22 | 1.24 | 1.27 | +========================+=======+=======+=======+======+=======+======+

[C] Figures taken from typical cross-sections.

[D] This gives the actual amount of drilling done and powder used per cubic yard for the whole period of 5 months of observation, but as this length included 280 ft. of heading and only 220 ft. of bench, the average figures (for powder especially) are too low.

Table 6 gives the rate and cost of drilling, and the cost of powder. It will be seen that the average rate of drilling was 2.71 ft. per hour per drill or 27.1 ft. per drill per shift.

Table 7 shows the result of observation as to the time taken in various subdivisions of the drilling operations. These observations were not carried on for a long enough period to give correct results, but the percentages of time spent on each division of the operation are believed to be about right. The headings of this table are self-explanatory. The necessary delays include all time spent in changing bits, making air-line connections, etc. The unnecessary delays are stoppages caused by lack of supplies or insufficient air pressure.

By Table 6 it will be noticed that the cost of labor for drilling and sharpening steels was about $0.29 per lin. ft. of hole drilled. The total cost, including repairs, supply of air, etc., came to about $0.38, as will be seen from Table 8.

_Timbering._--On the New York side nearly the whole length of the excavation needed timbering, to a greater or less extent, and for the most part required timbering of quite a heavy type.

TABLE 6.--ROCK TUNNEL EXCAVATION UNDER 32D STREET, EAST OF CUT-AND-COVER SECTION. DRILLING AND BLASTING.--DETAILED COST OF LABOR IN DRILLING, ALSO QUANTITY AND COST OF POWDER USED.

+=====================================================================+ | DRILLING AND BLASTING. | |-----+-----+------+------+------+-----+-----+------+-----+-----+-----| |Type.|Date.|Total feet drilled. | No. drill shifts| Feet drilled | | | | | | | of (10-hour.) |per man per hour.| | +-----+------+------+------+-----+-----+------+-----+-----+-----+ | |1907 |Head- |Bench |Total |Head-|Bench|Total |Head-|Bench|Total| | | | ing | | | ing | | | ing | | | |-----+-----+------+------+------+-----+-----+------+-----+-----+-----+ |_Ke._|May | 2,971| 5,578| 8,549| 98 | 204 | 302 |3.031|2.734|2.831| | |June | 2,093| 6,194| 8,287| 85 | 223 | 308 |2.462|2.777|2.691| | |July | | 7,627| 7,627| | 268 | 268 | |2.845|2.845| | |Aug. | | 2,552| 2,552| | 95 | 95 | |2.688|2.688| | |Sept.| | 2,133| 2,133| | 79 | 79 | |2.700|2.700| | +-----+------+------+------+-----+-----+------+-----+-----+-----+ | |Total| 5,064|24,084|29,148| 183 | 869 |1,052 |2.767|2.77 |2.77 | |-----+-----+------+------+------+-----+-----+------+-----+-----+-----+ |_Ki._|May | 6,976| | 6,976| 216 | | 216 |3.229| |3.229| | |June | 4,089| | 4,089| 135 | | 135 |3.029| |3.029| | |July | | 3,733| 3,733| | 140 | 140 | |2.666|2.666| | |Aug. | | 6,715| 6,715| | 249 | 249 | |2.769|2.769| | |Estim| |14,742|14,742| | 46 | 546 | |2.700|2.700| | +-----+------+------+------+-----+-----+------+-----+-----+-----+ | |Total|11,065|25,190|36,255| 351 | 935|1,286 |3.152|2.694|2.819| |-----+-----+------+------+------+-----+-----+------+-----+-----+-----+ |_Ko._|May | | 1,617| 1,617| | 55| 55 | |2.921|2.921| | |June | | 2,948| 2,948| | 107| 107 | |2.755|2.755| | |July | | 3,734| 3,734| | 131| 131 | |2.850|2.850| | |Aug. | | 8,260| 8,260| | 290| 290 | |2.848|2.848| | |Estim| | 4,787| 4,787| | 285| 285 | |1.180|1.680| | +-----+------+------+------+-----+-----+------+-----+-----+-----+ | |Total| |21,346|21,346| | 868| 868 | |2.460|2.460| |-----+-----+------+------+------+-----+-----+------+-----+-----+-----+ |Grand|Total|16,129|70,620|86,749| 534 |2,672|3,206 |3.020|2.710|2.710| +=====+=====+======+======+======+=====+=====+======+=====+=====+=====+

+==================================================+=====================+ | DRILLING AND BLASTING | POWDER USED. | |-----+----------+------+--------------------------+--------+-------+----+ | | | | Cost of labor only. | | | | | | | | Drilling and sharpening. | | | | | | | +------+------+-------+----+ | | | | | | | | | Per | | | Cost | | | | | | | | cubic | | | per | | | | | | | | yard. | | | cubic | | | | | | | | | | |yard at| | | | | | | | | | | 11 | | | | | | | | | | | cents | | | | | | | | | | | per | | | | | | | | | | |pound. | | | +----------+------+------+------+-------+----+--------+-------+----+ |Type.| Quantity | |Total.| Per |Actual.|Paid| Total |Actual.|Paid| | | of | | |linear| |for |Quantity| |for.| | |excavation| | |feet. | | | | | | | | in cubic | | | | | | | | | | | yards. | | | | | | | | | | +----------+------+------+------+-------+----+--------+-------+----+ | | Actual. | Paid | $ | $ | $ | |Pounds. | $ | $ | | | [E] | for | | | | | | | | | | | [F] | | | | | | | | |-----+----------+------+------+------+-------+----+--------+-------+----+ |_Ke._| 1,736 | 1,664| 2,331| 0.27 | 1.34 |1.40| 1,595 | 0.10 |0.10| | | 809 | 698| 2,440| 0.29 | 3.01 |3.49| 1,960 | 0.27 |0.31| | | 1,022 | 960| 2,031| 0.26 | 1.98 |2.11| 966 | 0.10 |0.11| | | 743 | 716| 640| 0.25 | 0.86 |0.89| 430 | 0.06 |0.07| | | 238 | 238| 533| 0.25 | 2.24 |2.24| 280 | 0.13 |0.13| | |----------+------+------+------+-------+----+--------+-------+----+ | | 4,548 | 4,276| 7,975| 0.27 | 1.75 |1.87| 5,231 | 0.13 |0.13| |-----+----------+------+------+------+-------+----+--------+-------+----+ |_Ki._| 614 | 527| 1,604| 0.23 | 2.61 |3.04| 1,230 | 0.22 |0.26| | | 357 | 259| 1,234| 0.30 | 3.45 |4.76| 1,036 | 0.32 |0.44| | | 530 | 404| 1,084| 0.29 | 2.04 |2.68| 550 | 0.11 |0.15| | | 925 | 890| 1,901| 0.28 | 2.05 |2.13| 905 | 0.10 |0.11| | | 3,254 | 2,908| 4,570| 0.31 | 1.40 |1.57| 2,470 | 0.08 |0.09| | |----------+------+------+------+-------+----+--------+-------+----+ | | 5,680 | 4,988|10,393| 0.29 | 1.83 |2.08| 6,191 | 0.12 |0.14| |-----+----------+------+------+------+-------+----+--------+-------+----+ |_Ko._| 250 | 188| 471| 0.29 | 1.88 |2.50| 376 | 0.17 |0.22| | | 496 | 347| 883| 0.29 | 1.78 |2.54| 357 | 0.08 |0.11| | | 626 | 606| 1,003| 0.27 | 1.60 |1.65| 609 | 0.11 |0.11| | | 718 | 709| 2,161| 0.26 | 3.00 |3.04| 918 | 0.14 |0.14| | | 605 | 535| 2,397| 0.50 | 3.96 |4.48| 762 | 0.14 |0.16| | |----------+------+------+------+-------+----+--------+-------+----+ | | 2,695 | 2,385| 6,915| 0.32 | 2.57 |2.90| 3,022 | 0.12 |0.14| | |---------+-------+------+------+-------+----+--------+-------+----+ | |12,923 |11,649|25,283| 0.29 | 1.96 |2.17|14,444 | 0.12 |0.14| +=====+==========+======+======+======+=======+====+========+=======+====+

The work done during the 5 months when these analyzed cost figures were kept includes 280 ft. of bench and 220 ft. of heading. This excess of bench over heading causes the general average amounts per cubic yard to be too low.

[E] Actual amount of excavation.

[F] Amount of excavation paid for.