Chapter IV: Part 4

Next, for Grades: The considerations for grade were very similar to those for line. If the vertical diameter of the tunnel had been true at each 25-ft. interval surveyed, it would have been correct to plot the elevations of the crown (or invert) as a longitudinal section of the tunnel, and to have set up over those points others 6 in. above (as the metal lining could have been 6 in. lower than the standard section, which is equivalent to the track being an equal amount higher), and below these crown or invert elevations others 3 in. lower (as the metal lining could be 3 in. higher).

Then, by joining the points 6 in. above in one line and those 3 in. below in another, there would have been obtained lines of limitation between which the track grades must lie. However, as the tunnel diameter was not uniformly correct, a modification of this method had to be made, as in the case of the line determination, the principle, however, remaining the same.

The elevations were taken on the inner edges of the circumferential flanges of the metal lining, not only in the bottom, but also in the top, of the tunnel, at each 25-ft. interval; then, for the upper limit of the track at each such interval the following was plotted:

Elevation of inner edge of flange at top, minus 16.58 ft.

This 16.58 ft. (or 16 ft. 7 in.) was obtained thus: The standard height from the top of the rail to the inner edge of the iron flange is 17 ft. 1 in., but, as the track may be 6 in. above the standard or normal, the minimum height permissible is 16 ft. 7 in. For the lower limit of track at each 25-ft. interval the following was plotted:

Elevation of inner edge of flange at bottom, plus 3.83 ft.

This 3.83 ft. (or 3 ft. 10 in.) was obtained thus: The standard height from the top of the rail to the inner edge of the iron flange is 4 ft. 1 in. (5 ft. to outside of iron, less 11 in. for depth of flange), but, as the track may be 3 in. below the standard, the minimum height permissible is 4 ft, 1 in. less 3 in., or 3 ft. 10 in.

By plotting the elevations thus obtained, two lines were obtained which were not parallel but were closer together or further apart according as the actual vertical diameter was less or greater than the standard, and the track grade had to lie within these two lines in order to comply with the requirements indicated above. The results of these operations for the North Tunnel are shown on Plate XXXVI.

The greatest deviations between the lines and grades in the subaqueous tunnels as determined by these means and those as originally laid out in the contract drawings are on the Weehawken side, and were caused by the unexpected behavior of the tunnel when the shields were driven "blind" into the silt, causing a rise which could not be overcome, and the thrusting aside of one tunnel by the passage of the neighboring one. Had this unfortunate incident not occurred, it is clear that it would have been possible to adhere very closely indeed to the contract lines and grades, although the deviation is small, considering all things.

The internal outline of the concrete cross-section is uniform throughout, and is built on the lines and grades thus described.

_Steel Rod Reinforcement of Concrete._--The original intention had been to line the metal lining of the tube tunnels with plain concrete, but, as the discussion on the foundation question continued, it was felt advisable, while still it was intended to put in the foundations, to guard against any stresses which were likely to come on the structure, by using a system of steel rods embedded circumferentially within the concrete. Designs were made on this basis, and even the necessary material prepared, before the decision to omit the piles altogether was reached. However, in order to provide a safeguard for the structure where it is partly or wholly beyond the solid rock, it was decided to use reinforcement, even with the piles omitted.

For this purpose the tunnel was considered as a girder, and longitudinal reinforcement was provided at the top and bottom. The top reinforcement extends from a point 25 ft. behind the point where the crown of the tunnel passes out of rock on the New York side to where the crown passes into rock on the New Jersey side. The bottom reinforcement extends from where the invert of the tunnel passes out of rock on the New York side to where it passes into rock on the New Jersey side.

The reinforcement both at top and bottom consists of twenty 1-in. square twisted rods, ten placed symmetrically on either side of the vertical axis, 9 in. apart from center to center and set 4 in. (to their centers) back from the face of the concrete.

As a further precaution, circumferentially-placed rods were used on the landward side of the river lines, mainly to assist in preventing the distortion of shape which might occur here, either under present conditions, such as under the Fowler Warehouse at Weehawken, or under any possible different future conditions, such as might be brought about by building some new structure in the vicinity of the tunnels.

For purposes of classification of the circumferential reinforcement, the tunnel was divided into two types, "_B_" and "_C_"; (Type "_A_" covering the portion which, being wholly in solid rock, was not reinforced at all).

Type "_B_" covers the part of the tunnels on both sides of the river lying between the point where the top of the tunnel passes out of rock and the point where the invert passes out of rock on the Manhattan side, or out of gravel on the Weehawken side. The reinforcement consists of twenty 1-in. square longitudinal rods in the crown of the tunnel, as described for the general longitudinal reinforcement, together with 1-in. square circumferential rods at 10-in. centers, and extending over the arch to 2 ft. 3 in. below the horizontal axis.

Type "_C_" extends from the latter limit of Type "_B_" to the river line on each side, and consists of longitudinal reinforcement in both top and bottom, as described before, together with circumferential reinforcement entirely around the tunnel, and formed of 1-in. square twisted rods at 15-in. centers.

Type "_D_" consists of longitudinal reinforcement only, and extends from river line to river line, thus occupying 72.5% of the length in which concrete is used. The reinforcement consists of twenty 1-in. twisted rods at 9-in. centers in the crown, and twenty 1-in. rods at 9-in. centers in the invert. In addition to the three standard types, "_B_," "_C_," and "_D_," there were two sub-types which were used in Type "_D_," and in conjunction with it wherever the thickness of the center of the concrete arch became less than 1 ft. 6 in., measuring to the outside of the metal lining. This thickness was one of the limits used in laying out the lines and grades, and in general the arch was not less than this. There were one or two short lengths, however, where it was less, for, if the arch thickness requirement had been adhered to, it would have resulted in a break of line or grade for the sake of perhaps only a few feet of thin arch, and it was here that the sub-types came into play.

Sub-type 1 was used where the arch was less than 1 ft. 6 in. thick at the top. The extra reinforcement here consisted of 1-in. square twisted rods, 16 ft. long, laid circumferentially in the crown at 10-in. centers.

Sub-type 2 was used where the arch was less than 1 ft. 6 in. thick at the side. The extra reinforcement here consisted of 1-in. square twisted rods, 16 ft. long, laid circumferentially, at the side on which the concrete was thin, at 10-in. centers. Very little of either of these two sub-types was used. The entire scheme is shown graphically and clearly on Plate XXXVII.

_Cross-Passage Lining._--There are two main types of cross-passages: Lined with steel plates, and unlined.

There is only one example of lining with steel plates, namely, the most western one at Weehawken. This is built in rock which carried so much water that, in order to keep the tunnels and the passage dry, it was decided to build a concrete-lined passage, without attempting to stop the flow of water, and within this to place a riveted steel lining, not in contact with the concrete, but with a space between the two. This space was drained and the water led back to the shield chamber and thence to the Weehawken Shaft sump. The interior of the steel lining is covered with concrete.

In the passages not lined with steel plates the square concrete lining is rendered on the inside with a water-proof plaster. Each of the passages is provided with a steel door.

_Provisions in Concrete Lining for Surveys and Observations._--The long protracted discussion as to the provision for foundations in these tunnels led to many surveys, tests, and observations, which were carried out during the constructive period, and, as it was desired to continue as many of these observations as possible up to and after the time when traffic started, certain provisions were made in the concrete lining whereby these requirements might be fulfilled. The chief points on which information was desired were as follows:

The change in elevation of the tunnel,
The change in lateral position of the tunnel,
The change in shape of the tunnel,
The tidal oscillation of the tunnel.

A detailed account of these observations will be found in another paper on this work, but it may be said now that it was very desirable to be able to get this information independently of the traffic as far as possible, and therefore provision was made for carrying on the observations from the side benches.

For studying the changes in level of the tunnel, a permanent bench-mark is established in each tunnel where it is in the solid rock and therefore not subject to changes of elevation; throughout the tunnel, brass studs are set in the bench at intervals of about 300 ft. A series of levels is run every month from the stable bench-mark on each of these brass plugs, thus obtaining an indication of the change of elevation that the tunnels have undergone during the month.

These results are checked on permanent bench-marks in the subaqueous portion of the tunnels. These consist of rods, encased in pipes of larger diameter, which extend down through the tunnel invert into the bed-rock below the tunnel. Leakage is kept out by a stuffing-box in the invert. By measuring between a point on these rods where they pass through the invert and the tunnel itself a direct reading of the change of elevation of the tunnel is obtained. These measurements are taken at weekly intervals, and, as the tunnels are subject to tidal influences, being lower at high tide than at low tide, are always taken under the same conditions as to height of water in the river. These permanent bench-marks are at Stations 209 + 05 and 256 + 02 (about 100 ft. on the shoreward side of the river line in each case) in the South Tunnel, at Stations 220 + 00 and 243 + 86, also in the South Tunnel, and at Station 231 + 78 in the North Tunnel. In order to study the lateral change of position, a base line was established on the side bench at each end of each tunnel in the portion built through the solid rock.

At intervals of about 300 ft. throughout each tunnel, alignment pockets are formed in the concrete arch, also above the bench, on the south bench of the North Tunnel and the north bench of the South Tunnel. In each pocket is placed a graduated and verniered brass bar, so that, when the base line is projected on these bars, the lateral movement of the tunnel can be read directly. As it was desirable to have as much cross-connection as possible between the tunnels at the points where the instruments were to be set up, five of the main survey stations were set opposite each of the five cross-passages. Then, for the purpose of increasing the cross-connection still further, pipes 6 in. in diameter were put through from one tunnel to the other at axis level at Stations 220 + 60, 231 + 78, 234 + 64, 241 + 99, and 251 + 13, and a survey station was put in opposite each one.

Points were established at Station 220 + 00, which is the point of intersection for the curve on the original center line of the tunnel, and also at Station 220 + 23, where the intersection of the track center line comes in the North Tunnel. As it was desirable to have the survey stations not much more than 300 ft. apart, so as to obtain clear sights, other stations were established so that the distances between survey stations were at about that interval.

For studying changes of shape in the tunnel, brass "diameter markers" were inserted at each survey station in the concrete lining at the extremities of the vertical and horizontal axes. These were pieces of brass bar, 3/8 in. in diameter and 6 in. long, set in the concrete and projecting 5/8 in. into the tunnel, so that a tape could be easily held against the marker and read.

For obtaining the tidal oscillation of elevation of the tunnel, recording gauges are attached to the invert of the tunnel at each of the five permanent bench-marks referred to above in such a way that the recording pencil of the gauge is actuated by the rod of the permanent bench-mark. A roll of graduated paper is driven by clock-work below the recording pencil which thus marks automatically the relative movement between the moving tunnel and the stable rods. These have shown that in the subaqueous part of the tunnel there is a regular tidal fluctuation of elevation, the tunnel moving down as the tide rises, and rising again when the tide falls. For an average tide of about 5 ft. the tunnel oscillation would be about 1/8 in. Before the concrete lining was placed, there was a tidal change in the shape of the tunnel, which flattened about 1/64 in. at high tide. After the concrete lining was placed, this distortion seemed to cease.

The general design and plan of the work have been described, and before giving any account of the contractor's methods in carrying it out, Table 22, showing the chief quantities of work in the river tunnels, is presented.

Methods of Construction.

The following is an account of the methods used by the contractor in carrying out the plans which have already been described. First, it may be well to point out the sequence of events as they developed in this work. These events may be divided into six periods.

_1._--Excavation and Iron Lining: June, 1903, to
November, 1906;

_2._--Caulking and grummeting the iron lining:
November, 1906, to June, 1907;

_3._--Surveys, tests and observations: April, 1907, to
April, 1908;

_4._--Building cross-passages and capping pile bores:
April, 1908, to November, 1908;

_5._--Placing the concrete lining: November, 1908, to
June, 1909;

_6._--Cleaning up and various small works: June, 1909,
to November, 1909.

The tunnels were under an average air pressure of 25 lb. per sq. in. above normal for all except Periods 5 and 6, during which times there was no air pressure in the tunnels.

All the work will be described in this paper except that under Period 3 which will be found in another paper.

_Period 1.--Excavation and Iron Lining, June, 1903, to November, 1906._--Table 23 gives the chief dates in connection with this period.

_Manhattan Shield Chambers._--The Manhattan shield chamber construction will be first described. The Weehawken shield chambers have been described under the Land Tunnel Section, as they are of the regular masonry-lined Land Tunnels type, whereas the Manhattan chambers are of segmental iron lining with a concrete inner lining.

During the progress of excavation, the location of the New York shield chambers was moved back 133 ft., as previously described in the "Land Tunnel" Section, and when the location had been finally decided, there was a middle top heading driven all through the length now occupied by the shield chamber. Narrow cross-drifts were taken out at right angles to the top heading, and from the ends of these the wall-plate headings were taken out. Heavy timbering was used, as the rock cover was only about 6 ft., and the whole span to be covered was 60 ft. The process adopted was to excavate and timber the north side first, place the iron lining, and then excavate the south side, using the iron of the north side as the supports for the north ends of the segmental timbering of the south. The only incident of note was that at 2:00 A.M., on October 20th, 1904, the rock at the west end of the south wall-plate heading was pierced. Water soon flooded the workings, and considerable disturbance was caused in the New York Central Railroad yard above. The cavity on the surface was soon filled in, but to stop the flow of mud and water was quite a troublesome job.

TABLE 22.--QUANTITIES OF WORK IN SUBAQUEOUS TUNNELS.

============================+=========================================
| TYPE.
|----------+--------------+--------------+
DESCRIPTION, QUANTITY, |MANHATTAN | CAST IRON, | CAST IRON, |
LENGTH, ETC. |shield | ordinary | ordinary |
|chambers. | pocketless. | pocket. |
----------------------------+----------+--------------+--------------+
Length, in feet. | 59.00| 4,374.99 | 2,146.3 |
----------------------------+----------+--------------+--------------+
Excavation, in cubic yards. | | | |
Total. | 1,884 | 67,344 | 33,038 |
Per linear foot. | 31.9 | 15.4 | 15.4 |
Cast-iron tunnel lining, | | | |
in pounds. | | | |
Total. |847,042 |39,643,120 |19,715,405 |
Per linear foot. | 14,357 | 9,061 | 9,186 |
Cast-steel tunnel lining, | | | |
in pounds. | | | |
Total. | | 1,544,962 | 757,938 |
Per linear foot. | | 353.1 | 353.1 |
Steel bolts and washers, | | | |
in pounds. | | | |
Total. | 23,627 | 1,475,991 | 724,095 |
Per linear foot. | 400.46| 337.37 | 397.00 |
Rust joints, in linear feet.| | | |
Total. | 3,376 | 170,755 | 83,935 |
Per linear foot. | 57.2 | 39.0 | 39.1 |
Concrete, in cubic yards. | | | |
Total. | 766 | 20,030 | 9,827 |
Per linear foot. | 12.98| 4.58 | 4.58 |
Steel beams, plates, etc., | | | |
in pounds. | | | |
Total. | 12,346 | 83,774 | 41,098 |
Per linear foot. | 2,092.5 | 19.1 | 19.1 |
Steel bolts, hooks, etc., | | | |
in pounds. | | | |
Total. | 1,328 | 36,980 | 18,142 |
Per linear foot. | 22.5 | 84.5 | 84.5 |
Expanded metal, in pounds. | | | |
Total. | 594 | 2,215 | 1,086 |
Per linear foot. | 10.07| 0.506| 0.506|
Vitrified conduits, in | | | |
duct feet. | | | |
Total. | 2,560 | 235,903 | 115,728 |
Per linear foot. | 43.49| 53.92 | 53.92 |
============================+==========+==============+==============+

============================+==========================================
|
|--------------+-------------+-------------
DESCRIPTION, QUANTITY, | CAST IRON, | CAST STEEL, |
LENGTH, ETC. | heavy | ordinary | Total.
| pocketless. | pocketless. |
----------------------------+--------------+-------------+-------------
Length, in feet. | 5,522.05 | 152.66 |12,255.00 ft.
----------------------------+--------------+-------------+-------------
Excavation, in cubic yards. | | |
Total. | 85,001 | 2,349 | 189,616
Per linear foot. | 15.4 | 15.4 | cu. yd.
Cast-iron tunnel lining, | | |
in pounds. | | |
Total. |61,559,845 | | 121,765,412
Per linear foot. | 11,148 | | lb.
Cast-steel tunnel lining, | | |
in pounds. | | |
Total. | 2,730,905 |1,549,711 | 6,583,516
Per linear foot. | 494.5 | 10,151.4 | lb.
Steel bolts and washers, | | |
in pounds. | | |
Total. | 2,935,455 | 51,266 | 5,210,434
Per linear foot. | 581.59 | 335.82 | lb.
Rust joints, in linear feet.| | |
Total. | 218,656 | 5,996 | 482,718
Per linear foot. | 39.6 | 39.3 | ft.
Concrete, in cubic yards. | | |
Total. | 25,282 | 713 | 56,618
Per linear foot. | 4.58 | 4.58 | cu. yd.
Steel beams, plates, etc., | | |
in pounds. | | |
Total. | 105,738 | 7,432 | 250,388
Per linear foot. | 19.1 | 48.7 | lb.
Steel bolts, hooks, etc., | | |
in pounds. | | |
Total. | 46,675 | 1,471 | 104,596
Per linear foot. | 84.5 | 96.4 | lb.
Expanded metal, in pounds. | | |
Total. | 2,795 | 62 | 6,752
Per linear foot. | 0.506| 0.406| lb.
Vitrified conduits, in | | |
duct feet. | | |
Total. | 297,752 | 7,757 | 659,700
Per linear foot. | 53.92 | 50.81 | duct ft.
============================+==============+=============+============

TABLE 23.--EXCAVATION AND IRON LINING.

====================================+================+================| | North | North | | Manhattan. | Weehawken. | ------------------------------------+----------------+----------------| Shaft and preliminary headings. | June 10, '03. | June 11, '03. | Begun. | | | Shaft and preliminary headings. |December 11, '03|September 1, '04| Finished. | | | Excavation of shield chamber. Begun.| May 24, '04. |January 16, '05.| Excavation of shield chamber. |January 21, '05.| March 25, '05. | Finished. | | | Cast-iron lining of shield chambers.|February 4, '05.| None. | Begun. | | | Cast-iron lining of shield chambers.| March 13, '05. | None. | Finished. | | | Excavation of tunnels begun before |October 17, '04.|January 13, '05.| installation of shield. | | | Commenced building falsework for | March 6, '05. | March 23, '05. | shield. | | | Shield parts received at shaft. | March 11, '05. | March 20, '05. | Erection of shield begun. | March 13, '05. | March 27, '05. | Erection of shield (structural | March 27, '05. | April 12, '05. | steel). Finished. | | | Erection of shield (hydraulic | May 11, '05. | May 25, '05. | fittings). Finished. | | | First ring of permanent cast-iron | May 12, '05. | May 29, '05. | lining put in. | | | First air lock bulkhead wall. Begun.| May 29, '05. | June 15, '05. | First air lock bulkhead wall. | June 7, '05. | June 23, '05. | Finished. | | | Air pressure first put in tunnel. | June 25, '05. | June 29, '05. | Rock disappeared from invert of |December 1, '05.|October 31, '05.| tunnel. | | | First pair of bore segments built in|December 9, '05.|January 12, '06.| tunnel. | | | Rip-rap of river bulkhead wall met. |February 8, '06.| None. | First pile met (in river bulkhead |February 18, '06|January 3, '06. | wall at Manhattan, and Fowler | | | warehouse foundation at Weehawken). | | | Last pile met. | March 2, '06. |February 5, '06.| First ring erected on river side of | March 3, '06. |February 6, '06.| shore line. | | | Removing hood of shield. Begun. | March 27, '06. |February 6, '06.| Removing hood of shield. Finished. | April 1, '06. |February 8, '06.| Second air-lock bulkhead wall. | May 12, '06. | March 19, '06. | Begun. | | | Second air-lock bulkhead wall. | May 21, '06. | March 24, '06. | Finished. | | | ------------------------------------+----------------+----------------| Tunnel holed through with meeting | September 12, 1906. | tunnel. | | Last ring of permanent cast-iron | October 9, 1906. | lining built in. | | ====================================+================+================|

====================================+================+================| | South | South | | Manhattan. | Weehawken. | ------------------------------------+----------------+----------------| Shaft and preliminary headings. |June 10, '03. |June 11, '03. | Begun. | | | Shaft and preliminary headings. |December 11, |September 1, 04| Finished. |'03. | | Excavation of shield chamber. Begun.|May 24, '04. |January 16, '05.| Excavation of shield chamber. |May 13, '05. |April 19, '05. | Finished. | | | Cast-iron lining of shield chambers.|May 15, '05. |None. | Begun. | | | Cast-iron lining of shield chambers.|June 14, '05. |None. | Finished. | | | Excavation of tunnels begun before |January 5, '05. |January 25, '05.| installation of shield. | | | Commenced building falsework for |June 19, '05. |April 17, '05. | shield. | | | Shield parts received at shaft. |June 22, '05. |April 24, '05. | Erection of shield begun. |June 22, '05. |April 24, '05. | Erection of shield (structural |June 8, '05. |May 6, '05. | steel). Finished. | | | Erection of shield (hydraulic |August 27, '05. |June 13, '05. | fittings). Finished. | | | First ring of permanent cast-iron |August 27, '05. |June 14, '05. | lining put in. | | | First air lock bulkhead wall. Begun.|September 18, |June 21, '05. | |'05 | | First air lock bulkhead wall. |September 23, |July 3, '05. | Finished. |'05 | | Air pressure first put in tunnel. |October 6, '05. |July 8, '05. | Rock disappeared from invert of |February 8, '06.|September 21, 05| tunnel. | | | First pair of bore segments built in|February 16, |December 12, '05| tunnel. |'06. | | Rip-rap of river bulkhead wall met. |April 11, '06. |None. | First pile met (in river bulkhead |April 18, '06. |December 4, '06.| wall at Manhattan, and Fowler | | | warehouse foundation at Weehawken). | | | Last pile met. |May 1, '06. |January 9 '06. | First ring erected on river side of |May 9, '06. |January 19, '06.| shore line. | | | Removing hood of shield. Begun. |May 9, '06. |January 19, '06.| Removing hood of shield. Finished. |May 12, '06. |January 24, '06.| Second air-lock bulkhead wall. |July 13, '06. |March 11, '06. | Begun. | | | Second air-lock bulkhead wall. |July 21, '06. |March 18, '06. | Finished. | | | ------------------------------------+----------------+----------------| Tunnel holed through with meeting | October 9, 1906. | tunnel. | | Last ring of permanent cast-iron | November 18, 1906. | lining built in. | | ====================================+================+================+

The excavation was begun on May 24th, 1904, and finished on May 15th, 1905. The segments were placed by an erector consisting of a timber boom supported by cross-timbers running on car wheels on longitudinal timbers at each side of the tunnel. Motion was transmitted to the boom by two sets of tackle, and the heavy (5,000-lb.) segments were easily handled. The erection of the lining was started on February 4th, 1905, and finished on June 14th, 1905.

While the shield chambers were being excavated, bottom headings were run along the lines of the river tunnels and continued until the lack of rock cover prevented their being driven further. These were afterward enlarged to the full section as far as possible. The typical working force in the shield chambers was as follows:

_Ten-hour Shifts._

_Drilling and Blasting._

1 Foreman @ $3.50
6 Drillers " 3.00
6 Drillers' helpers " 2.00
1 Blacksmith " 3.50
1 Blacksmith's helper " 2.25
1 Powderman " 2.00
1 Waterboy " 2.00
1 Nipper " 2.00
1 Machinist " 3.00
1 Machinist's helper " 1.80

_Mucking._

1 or 2 Foremen @ $3.00
16 Muckers " 2.00

_Erection of Shields._--The tunneling shields have been described in some detail in the section of this paper dealing with the contractor's plant. They consist essentially of two parts, the structural steelwork and the hydraulic fittings. The former was made by the Riter Conley Manufacturing Company, of Pittsburg, Pa., and put up by the Terry and Tench Company, of New York City; the hydraulic fittings were made and put in by the Watson-Stillman Company, of New York City.

On the New York side, the shields were built inside the iron lining of the shield chambers, hence no falsework was needed, as the necessary hoisting tackle could be slung from the iron lining; at Weehawken, however, the erection was done in the bare rock excavation, so that timber falsework had to be used. The assembly and riveting took about 2 weeks for each shield; the riveting was done with pneumatic riveters, using compressed air direct from the tunnel supply.

After the structural steel had been finished, the shields, which had hitherto been set on the floor of the chambers in order to give room for working over the top, were jacked up to grade; this involved lifting a weight of 113 tons. While the hydraulic fittings were being put in, the shields were moved forward on a cradle, built of concrete with steel rails embedded, on which the shield was driven for the length in which the tunnel was in solid rock.

The installation of the hydraulic fittings took from 4 to 6 weeks per shield. The total weight of each finished shield was about 193 tons. The completed shield, as it appeared in the tunnel, is shown by Fig. 1, Plate XXXVIII. The typical force working on shield erection was as follows:

_Ten-hour Shifts._

_Shield Erection._ (_Terry and Tench._)

1 Superintendent @ $13.00 per day
4 Foremen " 5.50 " "
1 Timekeeper " 2.50 " "
2 Engineers " 4.50 " "
34 Iron workers " 4.50 " "
7 Laborers " 2.25 " "

_Hydraulic Work._ (_Watson-Stillman Company._)

4 Mechanics @ $4.00 per day
_General Labor._ (_O'Rourke Engineering Construction Company._)

1 Inspector @ $4.00 per day
1 Foreman " 4.00 " "
8 Laborers " 2.00 " "
1 Engineer " 2.50 " "

After the shield was finished and in position, the first two rings of the lining were erected in the tail of the shield. These first rings were then firmly braced to the rock and the chamber lining; then the shield was shoved ahead by its own jacks, another ring was built, and so on.

The description of the actual methods of work in the shield-driven tunnels can now be given; this will be divided generally into the different kinds of conditions met at the working face, for example, Full Face of Rock, Mixed Face, Full Face of Sand and Gravel, Under River Bulkhead, and Full Face of Silt.

The last heading is the one under which by far the longest length of tunnel was driven, and, as not much has hitherto appeared descriptive of the handling of a shield, through this material, considerable space will be devoted to it.

_Full Face of Rock._--As was described when dealing with the shield chambers, as much as possible of the rock excavation was done before the shields were installed. On the New York side, about 146 ft. of tunnel was completely excavated, with 71 ft. of bottom headings beyond that, and at Weehawken, 58 and 40 ft. of tunnel and heading beyond, respectively. This was chiefly done to avoid handling the rock through the narrow shield doors. Test holes were driven ahead at short intervals to make sure that the rock cover was not being lost, but, nevertheless, at Weehawken, on February 14th, 1905, a blast broke through the rock and let the mud flow in, filling the tunnel for half its height for a distance of 300 ft. from its face.

Throughout the rock section the shield traveled on a cradle of concrete in which were embedded either two or three steel rails. In the portion in which the whole of the excavation had been taken out, it was only necessary to trim off projecting corners of rock. In the portion in which only a bottom heading had been driven, the excavation was completed just in front of the shield, the drilling below axis level being done from the heading itself, and above that from the front sliding platforms of the shield. The holes were placed near together and drilled short, and very light charges of powder were used, so as to lessen the chance of knocking the shield about too much. In this work the small shield doors hampered the work greatly, and it might have been well to have provided a larger bottom opening which could have been subdivided or partly closed when soft ground was met; on the other hand, the quantity thus handled was small, owing to the fact that the greater part of the rock was excavated before the shields were installed.

The space outside the lining was grouted with a 1:1 mixture of Portland cement and sand. Large voids were hand-packed with stone before grouting. The details of grouting will be described later.

A typical working gang is given herewith. Two such gangs were worked per shield per 24 hours, 10 hours per shift. All this work was done under normal air pressure.

_General:_

½ Tunnel superintendent @ $200.00 per month
1 Assistant tunnel superintendent " 5.00 per day
1 General foreman " 5.00 " "
½ Electrician " 3.50 " "
½ Electrician's helper " 3.00 " "
½ Pipefitter " 3.00 " "
½ Pipefitter's helper " 2.75 " "

_Drilling:_

1 Foreman " 5.00 " "
3 Drillers " 4.00 " "
3 Drillers' helpers " 3.00 " "
1 Nipper " 2.50 " "
½ Waterboy " 2.50 " "
½ Powderboy " 2.75 " "

_Mucking:_

1 Foreman " 3.50 " "
8 Muckers " 2.75 " "

_Erecting Iron and Driving Shield:_

1 Erector runner " 4.00 " "
3 Iron workers " 3.00 " "

The duties of such a gang were as follows: The tunnel superintendent looked after both shifts of one shield. The assistant or "walking boss" had charge of all work in the tunnel on one shift. The general foreman had charge of the labor at the face. The electricians looked after repairs, extensions of the cables, and lamp renewals. The pipefitters worked in both tunnels repairing leaks in pipes between the power-house and the working faces, extending the pipe lines, and attending to shield repairs, and in the latter work the erector runner helped.

The drillers stuck to their own jobs, which were not subject to interruption as long as the bottom headings lasted. One waterboy and one powderboy served two tunnels. The muckers helped the iron men put up the rings of lining, as well as doing their own work. The iron men tightened bolts, whenever not actually building up iron. The list does not include the transportation gang, which will be described under its own heading.

The rate of progress attained was 4.2 ft. per day per shield where most of the excavation had been done before, and 2.1 ft. where none had been done before.

When the shields had got far enough away from the shield chamber, and before rock cover was lost, the first air-lock bulkhead walls were put in.

_Air-Lock Bulkhead Walls._--The specifications required these walls and all their fittings to be strong enough to stand a pressure of 50 lb. per sq. in. Accordingly, all the walls were of concrete, 10 ft. in thickness, except the first two, which were 8 ft. in thickness, and grouted up tight.

There were three locks in each bulkhead wall capable of holding men, namely, the top or emergency lock which is set high in order to afford a safe means of getting away in case of a flood; this lock was used continuously for producing the lines and levels into the tunnels. It was very small and cramped for this purpose, and a larger one would have been better, both for lines and emergencies. This lock was directly connected with the overhead platform (also called for in the specifications) which ran the whole length of the tunnels. Side by side, on the level of the lower or working platform of the tunnel, were the man lock and the muck lock. In addition a number of pipes were built in to give access to the cables and for passing pipes, rails, etc., in and out.

After each tunnel was about 1,200 ft. ahead of the first walls, a second wall was built just like the first, and no others were put in, so that altogether there were eight walls. This second wall not only gave an added safeguard to the tunnel but enabled the air pressure at the working face to be divided between the two walls, and this compression or decompression in stages, separated by a spell of walking exercise, was found to be very good for the health of those working in the air.

_Mixed Face._--When the rock cover became so thin that it was risky to go on without the air pressure, the air pressure was turned on, starting with from 12 to 18 lb., which was enough to stop the water from the gravel on top of the rock. At first, when the surface of the rock was penetrated, the soft face was held up by horizontal boards braced from the shield until the shield was shoved. The braces were then taken out and, as soon as the shield had been shoved, were replaced by others. As the amount of soft ground in the face increased, the system of timbering was gradually changed to one of 2-in. poling boards resting on top of the shield and supported at the face by vertical breast boards, in turn held by 6 by 6-in. walings braced both through the upper doors to the iron lining and from the sliding platforms of the shield. The latter were in their forward position before the shield was shoved, the pressure being turned off and the exhaust valves opened just before the shove began. As the shield went ahead, the platform jacks gradually exhausted and thus held enough pressure on the face to keep it up. Fig. 17 is a sketch of this method. In driving through mixed ground a typical working gang was about as follows:

_General:_

1/3 Tunnel superintendent @ $300.00 per month
1 Assistant tunnel superintendent " 5.00 per day
1 General foreman " 5.00 " "
½ Electrician " 3.50 " "
½ Electrician's helper " 3.00 " "
½ Pipefitter " 3.25 " "
½ Pipefitter's helper " 3.00 " "

_Drilling:_

1 Foreman " 5.00 " "
2 Drillers " 3.25 " "
2 Drillers' helpers " 3.00 " "

_Timbering:_

2 Timbermen @ $2.50 per day
2 Timbermen's helpers " 2.00 " "

_Mucking:_

1 Foreman " 3.50 " "
6 Muckers " 2.75 " "

_Erecting Iron and Driving Shield:_

1 Erector runner " 3.25 " "
3 Iron workers " 3.00 " "

The average rate of progress was 2.6 ft. per day.

In this case there were three such gangs, each on an 8-hour shift.

_Full Face of Sand and Gravel._--This condition of affairs was only met at Weehawken. Two systems of timbering were used. In the first system, Fig. 17, the ground was excavated 2 ft. 6 in. ahead of the cutting edge, the roof being held by longitudinal poling boards, resting on the outside of the skin at their back end and on vertical breast boards at the forward end. When the upper part of the face was dry, it was held by vertical breast boards braced from the sliding platform and through the shield doors to cross-timbers in the tunnel; the lower part, which was always wet, was held by horizontal breast boards braced through the lower shield pockets to cross-timbers in the tunnel. This system worked all right as long as the ground in the top was sandy enough and had sufficient cohesion to allow the polings to be put in, but, when the upper part was in gravel, thus making it impossible to put in the longitudinal polings or the vertical breasting, the second system came in. Here the excavation was only carried 1 ft. 3 in. (half a shove) ahead of the cutting edge, and the longitudinal polings were replaced by transverse boards supported by pipes which were placed in the holes provided in the shield to accommodate some telescopic poling struts which had been designed but not made. These pipes acted as cantilevers, and were in two parts, a 2½-in. pipe wedged tight into the holes and smaller pipes sliding inside them. After a small section of the ground had been excavated, a board was placed against it, one of the pipes was drawn out under it, and wedges were driven between it and the board. These polings were kept below the level of the hood, so that when the shield was shoved they would come inside of it; in addition, they were braced with vertical posts from the sliding platforms. The upper part of the face was held by longitudinal breast boards braced from the sliding platform by vertical "soldier" pieces. The lower part of the face was supported by vertical sheet-piling braced to the tunnel through the lower doors. Sometimes two rows of piling were used, but generally one, as shown in Fig. 17. Notwithstanding the fact that the breasting was only 1 ft. 3 in. ahead of the hood, the shield was moved its full stroke of 2 ft. 6 in., the ground around the cutting edge of the hood being scraped away by men working bars in the place from which the temporary breast boards at the circumference had been removed. The back pressure on the sliding platform jacks, when the exhaust valves were only partly open, offered a good deal of resistance, and held the face as long as the movement of the shield was continuous.

On one occasion, when for some reason the shield was stopped with the shove only partly done, and the exhaust valves had not been shut off, the platforms continued to slide and allowed the face to collapse; the shield platforms and doorways, however, caught the falling sand and gravel and the flow choked itself.

As soon as the rock surface was penetrated and the sand and gravel were met, which happened almost at the same time in the two Weehawken Tunnels, the escape of air increased enormously, and it at once became clear that it was impossible to keep enough air in the two tunnels by the methods then in use, even when working the three compressors, each capable of compressing 4,400 cu. ft. of free air per min. at top speed. When the shields just entered the sand and gravel, the face had been held by light breasting, without any special effort to prevent the escape of air, but when it was found impossible to supply enough air, a large amount of straw and clay was used in front of the boards.

This cut down the escape, but, as much air was escaping through the joints of the iron lining, these were plastered with Portland cement. Even then, the loss was too great, therefore one tunnel was shut down entirely and all the air was sent to the other. This allowed a pressure of 10 lb. to be kept up in the working tunnel, and this, though less than the head, was enough to allow progress to be made. In order to use one tunnel as a drain for the other, the two faces were always kept within 150 ft. of each other by working them alternately. The timbered face was never grouted, though this would have reduced the loss of air, as at the same time it would have decreased the progress very much, and any one who saw the racing engines in the power-house, and realized that a breakdown of one of them would mean the loss of the faces, was ready to admit that the quicker this particular period was cut short, the better.

Above the sand and gravel lay the silt, and, when it showed in the roof, the escape of air was immediately reduced and the two faces could be worked simultaneously. Almost at the same time the piles supporting the large warehouse, known as the Fowler Building, were met. Although the face now took much less timber, the same system of breast boards as had been used in the gravel was kept up, but in skeleton form. They were set 2 ft. 6 in. ahead of the shield, however, instead of 1 ft. 3 in., and the transverse roof poling boards were replaced by longitudinals resting on the shield. The more piles in the face the less timbering was done. The piles were cut into handy lengths with axes and chisels.

All timbering was light compared with the weight of the ground, but, as the shove took place as soon as the set was made, it served its purpose. When a face was closed down the whole system was greatly reinforced by braces from the shield, the face of which was closed by the doors.

In driving through such a face the typical 8-hour shift gang was about as follows:

_General:_

1/3 Tunnel superintendent @ $300.00 per month.
1 Assistant tunnel superintendent " 5.00 per day.
1 General foreman " 5.00 " "
½ Pipefitter " 3.25 " "
½ Pipefitter's helper " 2.75 " "
½ Electrician " 3.00 " "
½ Electrician's helper " 2.75 " "

_Timbering:_

3 Timbermen " 2.50 " "
3 Timbermen's helpers " 2.00 " "

_Mucking:_

1 Foreman " 3.50 " "
6 Muckers " 2.75 " "

_Erecting Iron and Driving Shield:_

1 Erector runner " 3.25 " "
1 Foreman " 4.00 " "
4 Iron workers " 3.00 " "

The drillers were not kept on after the rock disappeared; a foreman was added who divided his time between iron erection and mucking.

The average rate of progress in sand and gravel without piles was 5.1 ft. per day per shield. When piles and silt were met in the upper part of the face, the speed increased to 7.0 ft. per day.

_Passing Under River Bulkhead._--At Weehawken no trouble was found in passing under the river wall, as the bulkhead consisted of only cribwork supported on silt, and, though the piles obstructed the motion of the shield, they were easily cut out, and the cribwork itself was well above the top of the shield.

On the New York side, however, conditions were not nearly as good. The heavy masonry bulkhead was supported on piles and rip-rap, as shown in Fig. 18. The line of the top of the shield was about 6 ft. above the bottom of the rip-rap, the spaces between the stones of which were quite open and allowed a free flow of water directly from the river. As soon, therefore, as the cutting edge of the shield entered the rip-rap there was a blow, the air escaping freely to the ground surface behind the bulkhead and to the river in front of it. Clay puddle, or mud made from the excavated silt, was used in large quantities to plug up the interstices between the stone in the working face, the air pressure being slightly greater than that needed to keep out the water holding it in place. The excavation of the rip-rap was a tedious affair, for it had to be removed one stone at a time and the spaces between the newly exposed stones plugged with mud immediately. One man stood ready with the mud while another loosened the stones with a bar. When the shield had advanced its own length in the rip-rap, another point for the escape of the air was exposed at the rear end of the shield. This loss was closed at the leading end of the last ring with mud and cement sacks.

As long as the shield was stationary it was possible, by using these methods and exercising great care and watchfulness, to prevent excessive loss of air; but, while the shield was being shoved ahead, the difficulties were much increased, for the movement of the shield displaced the bags and mud as fast as they were placed, and it was only by shoving slowly and having a large number of men looking out for leaks and stopping them up the instant they developed that excessive loss of air could be prevented. In erecting the iron lining, as each segment was brought into position, it was necessary to clean off the leading surface of the previous ring and the adjacent portion of the tail of the shield; this was always accompanied by a slight "blow," and for some time the air pressure in the tunnel dropped from 25 to 20 lb., that is, from greater than the balancing pressure to less, every time a segment was placed, and on two occasions the "blow" became so great that the tunnel pressure was reduced considerably further, and in consequence the water from the river rushed in and was not stopped until it had risen about 4 ft. in the tunnel invert. On such occasions the surface of the river was greatly disturbed, rising more than 20 ft. in the air in a sort of geyser. A large quantity of grout (about 2,500 bbl. of cement and a similar quantity of sand in the North Tunnel and 1,000 bbl. in the South Tunnel) was used at this point; it was forced through the tunnel lining immediately behind the shield, greatly reducing the loss of air and helping to bind the rip-rap together.

When the shield had traveled 25 ft. through the rip-rap, the piles which support the bulkhead were met. One hundred of these which were spaced at 3-ft. centers in each direction, were cut out of the path of each shield in a distance of 35 ft. The presence of the piles caused considerable extra labor, as each pile had to be cut into several pieces with axes to enable it to be removed through the shield doors, otherwise they presented no difficulties. It was not necessary to timber the face, as the piles supported it most effectively.

When the river line had been passed, the "blow" still continued, and as there was no heavy ground above the tunnel the light silt was carried away into the water by the escaping air. At one time the cover over the crown of the tunnel was reduced to such an extent that for a distance of 30 ft. there was less than 10 ft. of very soft silt, and in some places none at all. Therefore, the shield was stopped and the air pressure reduced until it was less than the balancing pressure; the blow then ceased, and about 28,000 cement bags filled with mud were dumped into the hole (the location made it impossible to dump them _en masse_ from a scow). They were then weighted down with rip-rap. This sealed the blow, and the work was continued without any further disturbance from this source. Just before the blow reached its maximum it was found that two of the piles which had been encountered were directly in the path of one of the proposed screw-piles. It was therefore decided to pull these, and this was done with two 40-ton hydraulic jacks supported by the upper sliding platforms and acting on a horizontal timber which was connected to the piles by tie-rods and chains. The working force here was similar to that employed in the sand and gravel section previously described.

_In Full Face of Silt._--A full face of silt was first met under the New York Central Railroad freight yard on the New York side. Up to this point the ground passed through had been either solid rock or a mixed face of rock and gravel. In both of these the full excavation had to be taken out before the shield could be shoved, and the soft ground had needed timbering. When the rock, gravel, and hardpan gave place to a full face of silt, the timber was removed, all the shield doors were opened, and the shield was shoved into the ground without any excavation being done by hand ahead of the diaphragm. As the shield advanced, the silt was forced through the open doors into the tunnel. After the work had gone on in this way for some time, taking in about 90% of the full volume of the tunnel excavation per foot forward, the air pressure was raised from 20 to 22 lb. The result was that the silt in the face got harder and flowed less readily through the shield, and the amount taken in fell to about 65% of the full volume. This manner of shoving at once caused a disturbance on the surface and the railroad tracks above the tunnel were raised, so that the pressure was lowered to 16 lb., then the muck got softer and the full volume of excavation was taken in; after a while the pressure was again raised to 20 lb.

The forcing of the shield through the silt resulted in a rising of the bed of the river, the amount that the bed was raised depending on the quantity of material brought into the shield.

If the whole volume of excavation was being brought in, the surface of the bed was not affected; when about 50% was being taken in, the surface was raised about 3 ft.; if the shield was being driven blind, the bed was raised about 7 ft.

The number of open doors was regulated so as to take in the minimum quantity of muck consistent with causing no surface disturbance. On the average, in the North Manhattan Tunnel, all the doors were open, but in the South Tunnel there were generally only five or six out of the total nine.