Transactions of the American Society of Civil Engineers, vol. LXVIII, Sept. 1910

Chapter II: Part 2

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_Excavation in Part Rock and Part Earth._--This was probably the most difficult work encountered, particularly when the rock was covered with boulders and coarse sharp sand which permitted a free escape of air. It was necessary, before removing the rock immediately under the soft ground, to excavate the earth in advance of the shield to a point beyond where the rock was to be disturbed, and to support, in some way, the roof, sides, and face of the opening thus made. The hoods were designed mainly for the purpose of supporting the roof and the sides. With the fixed hood it was necessary either to excavate for the distance of the desired shove in front of it or else to force the hood into the undisturbed material. To avoid this difficulty, the sliding hoods were tried as an experiment.

In using the sliding hood, which will be described in detail in Mr. Japp's paper, the segments commencing at the top were forced forward by the screw rod, one at a time, as far as possible into the undisturbed material. Just enough material was then removed from underneath and in front of the section to free it, and it was again forced forward. These operations were repeated until the section had been extended far enough for a shove. As soon as two or three sections had been pushed forward in this way, the face near the advance end of the sliding hood was protected by a breast board set on edge and braced from the face. Gradually, all the segments were worked forward, and, at the same time, the whole soft ground face was sheeted with timber. At times polings were placed over the extended segments in order to make room for a second shove, as shown on Plate LXVIII. When the shield was advanced the nuts on the screw rods were loosened and the sections of the hoods were telescoped on to the shield. The idea was ingenious, but proved impracticable, because of the unequal relative movements of the top and bottom of the shield in shoving, bringing transverse strains on the hood sections.

With the fixed hood, poling boards were used to support the roof and sides, and the face was supported in the manner described for the sliding hoods. The polings were usually maple or oak planks, 2 in. thick, about 8 in. wide, and 6-1/2 ft. long. In advancing the face, the top board of the old breast was first removed, then the material was carefully worked out for the length of the poling. The latter was then placed, with the rear end resting over the hood and the forward end forced as far as possible into the undisturbed material. When two or three polings had been placed, a breast board was set. After several polings were in position, their forward ends were supported by some form a cantilever attached to the hood. Plate LXIX shows one kind of supports. In this way all the soft material was excavated down to the rock surface, and the roof, sides, and face were sheeted with timber. In shoving, the polings in the roof and sides were lost. It was found that the breast could usually be advanced 5 ft. with safety. The fixed hood made it possible to set the face about 7 or 8 ft. in front of the cutting edge without increasing the length of the polings. This distance was ample for two shoves, and was generally adopted, although a great many faces were set for one shove only.

Fixed hoods were substituted for those of the sliding type, originally placed on Shields _B_ and _D_ at Manhattan, at about the time the latter encountered the rock at the reef.

In placing the polings and breasting, all voids behind them were filled as far as possible with marsh hay or bags of sawdust or clay. To prevent loss of air in open material, the joints between the boards were plastered with clay especially prepared for the purpose in a pug mill. The sliding extensions to the floors of the working compartments were often used, in the early part of the work, to support the timber face or loose rock, as shown in Fig. 1, Plate LXVIII. At such times the front of the extensions was held tightly against the planking by the pressure of the floor jacks. While shoving, the pressure on the floor jacks was gradually released, allowing the floors to slide back into the shield and still afford support to the face. The extensions also afforded convenient working platforms. They were subject to severe bending strains while the shield was being shoved, however, and the cast-iron rams were frequently broken or jammed. The extensions did not last beyond the edge of the ledge at Manhattan, nor more than about half through the rock work at Long Island City. The fixed extensions originally placed on Shields _A_ and _C_ at Manhattan were not substantial enough, and lasted only a few days.

Wherever the rock face was sufficiently sound and high, a bottom heading was driven some 20 or 30 ft. in advance of the shield. The heading was driven and the cradle placed independently of the face of the soft ground above, and in the manner described for all-rock sections. The remainder of the rock face was removed by firing top and side rounds into the bottom heading after the soft ground had been excavated. Great care had to be taken in firing in order not to disturb the timber work or break the rock away from under the breast boards. If either occurred, a serious run was likely to follow. The bottom-heading method is shown by Figs. 1, 2, and 3, Plate LXVIII, and the breasting and poling by Fig. 2, Plate LXX.

In the early part of the work, where a bottom heading was impracticable, the soft ground was first excavated as described above, and the rock was drilled by machines mounted on tripods, and fired as a bench. By this plan no drilling could be done until the soft ground was removed. This is called the rock-bench method.

Later the rock-cut method was devised. Drills were set up on columns in the bottom compartments of the shield, and the face was drilled while work was in progress on the soft ground above. The drilling was done either for a horizontal or vertical cut and side and top rounds. The drillers were protected while at work by platforms of timber built out from the floors of the compartments above. This plan, while probably not quite as economical of explosives, saved nearly all the delay due to drilling the bench.

_All-Earth Section._--As described by Messrs. Hay and Fitzmaurice, in a paper on the Blackwall Tunnel,[C] the contractor had used, with marked success, shutters in the face of the shield for excavating in loose open material. He naturally adopted the method for the East River work. When the shields in Tunnels _B_ and _D_, at Manhattan, the first to be driven through soft ground, reached a point under the actual bulkhead line, work was partly suspended and shutters were put in place in the face of the top and center compartments. The shutters in the center compartments in Shield _D_ are shown in Fig. 3, Plate LXX, while the method of work with the shutters is shown by Figs. 4, 5, 6, and 7, Plate LXVIII. Fig. 4 on that plate shows the shield ready for a shove. As the pressure was applied to the shield jacks, men loosened the nuts on the screws holding the ends of the shutters, and allowed the latter to slide back into the working compartments. At the end of the shove, the shutters were in the position shown in Fig. 5, Plate LXVIII. In preparing for a new shove, the slides in the shutters were opened, and the material in front was raked into the shield. At the same time, the shutters were gradually worked forward. The two upper shutters in a compartment were generally advanced from 12 to 15 in., after which the muck could be shoveled out over the bottom shutters, as shown on Fig. 6, Plate LXVIII, and Fig. 3, Plate LXX. No shutters were placed in the bottom compartments, and as the air pressure was not generally high enough to keep the face dry at the bottom, these compartments were pretty well filled with the soft, wet quicksand. Just before shoving, this material was excavated to a point where it ran in faster than it could be taken out. Much of the excavation in the bottom compartment was done by the blow-pipe. During the shove the material from the bottom compartment often ran back through the open door in the transverse bulkhead, as shown by Fig. 5, Plate LXVIII.

In the Blackwall Tunnel the material was reported to have been loose enough to keep in close contact with the shutters at all times. In the East River Tunnels this was not the case. The sand at the top was dry and would often stand with a vertical face for some hours. In advancing the shutters, it was difficult to bring them into close contact with the face at the end of the operation. The soft material at the bottom was constantly running into the lower compartment and undermining the stiff dry material at the top. The latter gradually broke away, and, at times, the actual face was some feet in advance of the shutters. Under those circumstances, the air escaped freely through the unprotected sand face. The joints of the shutters were plastered with clay, but this did not keep the air from passing out through the lower compartments. This condition facilitated the formation of blows, which were of constant occurrence where shutters were used in the sand. In Tunnels _B_ and _D_, at Manhattan, the shutters were used in the above manner clear across to the reef. In Tunnel _C_, which was considerably behind Tunnels _B_ and _D_, the shutters, although placed, were never used against the face, and the excavation was carried on by poling the top and breasting the face. The change resulted in much better progress and fewer blows. The excavation through the soft material in Tunnel _C_ had just been completed when Tunnel _A_ was started, and the gangs of workmen were exchanged.

The work in soft ground in Tunnel _A_ thus gained the benefit of the experience in Tunnel _C_. Shutters were placed only in the top compartments in this tunnel, and, as in Tunnel _C_, were never used in contact with the face. The method of work is shown by Figs. 1, 2, and 3, Plate LXXI. The result was still more rapid progress in Tunnel _A_, and although the loss of air was fully as great in this tunnel as in the other three, there was only one blow which caused any considerable loss of pressure. In Tunnels _A_ and _C_ the diaphragms in the rear of the center compartments of the lower tiers of working chambers were removed before the shields entered the soft ground. The change was not of as much advantage in soft ground as in rock, but it facilitated the removal of the soft wet sand in the bottom. In Tunnel _A_, after encountering gravel, a belt conveyor was suspended from the traveling stage with one end projecting through the opening into the working compartment. The use of the conveyor made it possible to continue mucking at the face while the bottom plates of the iron lining were being put in place, and resulted in a material increase in the rate of progress.

The shutters were not placed on the Long Island shields at all. Just before the shields passed into all soft ground, a fixed hood was attached to each.

The method of working in soft ground from Long Island City is illustrated by Plate LXXII. The full lines at the face of the shield show the position of the earth before a shove of the shield, and the dotted lines show the same after the shove. The face was mined out to the front of the hood and breasted down to a little below the floor of the top pockets of the shield. In the middle pocket the earth was allowed to take its natural slope back on the floor. Toward the rear of the bottom pockets it was held by stop-planks. The air pressure was always about equal to the hydrostatic head at the middle of the shield, so that the face in the upper and middle pockets was dry. In the lower pockets it was wet, and flowed under the pressure of shoving the shield. By this method 4,195 lin. ft. of tunnel was excavated by the four Long Island shields in 120 days, from November 1st, 1907, to March 1st, 1908. This was an average of 8.74 ft. per day per shield.

The rate of progress, the nature of the materials, and the methods adopted are shown in Table 2.

_Preparations for Junction of Shields._--As previously mentioned, the Manhattan shields were stopped at the edge of the reef. Before making the final shove of those shields, special polings were placed with unusual care. The excavation was bell-shaped to receive the Long Island shields. The arrangement of the polings is shown by Figs. 4 and 5, Plate LXXI. After the shields were shoved into final position, as shown at the right in Fig. 5, the rear end of the polings rested over the cutting edge and allowed room for the removal of the hood. After the latter had been accomplished, the temporary bulkheads of concrete and clay bags were built as a precaution against blows when the shields were close together. An 8-in. pipe was then driven forward through the bulkhead for distances varying from 30 to 100 ft., in order to check the alignment and grade between the two workings before the shields were actually shoved together. The errors in the surveys were negligible, but here, as elsewhere, the shields were not exactly in the desired position, and it took careful handling to bring the cutting edges together. The Long Island shields were driven to meet those from Manhattan.

TABLE 2.--RATE OF PROGRESS, NATURE OF MATERIALS, AND METHODS ADOPTED IN CONSTRUCTION OF EAST RIVER TUNNELS.

LINE A, LONG ISLAND.

LINE B, LONG ISLAND.

LINE C, LONG ISLAND.

LINE D, LONG ISLAND.

LINE A, MANHATTAN.

LINE B, MANHATTAN.
--------------+-----------------+-------------------+-------------------------+
| | Station: | Date: |
| |---------+---------+------------+------------+
| | | | | |
Material. | Method. | From | To | From | To |
--------------+-----------------+---------+---------+------------+------------+
{|Top heading |108+35 |107+87 |July 6, '05|July 27, '05|
{|Top lift of bench|108+35 |107+87 |Aug. 3, '05|Aug. 14, '05|
Rock {|Bottom lift of | | | | |
{| bench |108+35 |108+15 |Aug. 26, '05|Aug. 30, '05|
{|Bottom lift of | | | | |
{| bench |108+15 |107+87 |Sept 11, '05|Sept 26, '05|
| | | | | |
Rock |Bottom heading |107+87 |107+00 |Oct. 23, '05|Jan. 17, '06|
| | | | | |
Mixed |Bottom heading |107+00 |106+64 |Jan. 17, '06|Feb. 12, '06|
| | | | | |
Mixed |Rock bench |106+64 |106+31 |Feb. 12, '06|Mar. 1, '06|
| | | | | |
Earth |Poling and |106+31 |105+58 |Mar. 1, '06|Apr. 3, '06|
| breasting | | | | |
| | | | | |
|Shutters in | | | | |
Earth | contact with |105+58 | 99+19 |Apr. 9, '06|Nov. 1, '06|
| face | | | | |
| | | | | |
Mixed |Rock bench | 99+19 | 98+44 |Nov. 1. '06|Dec. 29, '06|
| | | | | |
Mixed |Bottom heading | 98+44 | 97+76 |Dec, 29, '06|Feb. 12, '07|
| | | | | |
Mixed |Rock cut | 97+66 | 93+84 |Feb. 12, '07|Aug. 6, '07|
| | | | | |
Rock |Full face | 93+84 | 93+21 |Aug. 6, '07|Sept 2, '07|
| | | | | |
Rock |Center Heading | 93+21 | 92+30 |Sept 2, '07|Oct. 12, '07|
| | | | | |
Rock |Bottom heading | 92+30 | 90+99 |Oct. 12, '07|Dec. 6, '07|
| | | | | |
Mixed |Rock cut | 90+99 | 90+49.6 |Dec. 6, '07|Jan. 3, '08|
--------------+-----------------+---------+---------+------------+------------+

LINE C, MANHATTAN.

--------------+-----------------+-------------------+-------------------------+
| | Station: | Date: |
| |---------+---------+------------+------------+
| | | | | |
Material. | Method. | From | To | From | To |
--------------+-----------------+---------+---------+------------+------------+
{|Top heading |107+79.03|107+69 |Dec. 20, '04|Dec. 27, '04|
{|Top heading |107+69 |107+23 |Jan. 1, '05|Jan. 15, '05|
Rock {|Excavating bench |107+79 |107+23 |Jan. 21, '05|Feb. 28, '05|
{|Bottom heading |107+23 |106+72 |Mar. 1, '05|Mar. 11, '05|
{|Bottom heading |107+23 |107+15 |Oct. 12, '05|Oct. 27, '05|
{| | | | | |
| | | | | |
Rock |Bottom heading |107+15 |106+62 |Nov. 6, '05|Dec. 2, '05|
| | | | | |
| | | | | |
Mixed |Bottom heading |106+62 |106+55 |Dec. 2, '05|Dec. 23, '05|
| | | | | |
| | | | | |
Mixed |Bottom heading |106+55 |106+17 |Feb. 12, '06|Mar. 22, '06|
| | | | | |
Mixed |Rock cut |106+17 |105+85 |Apr. 2, '06|Apr. 20, '06|
| | | | | |
Mixed |Rock cut |105+85 |105+55 |July 27, '06|Aug. 26, '06|
| | | | | |
| | | | | |
Earth |Breasting and |105+55 | 99+40 |Aug. 26, '06|Jan. 2, '07|
| poling | | | | |
| | | | | |
Mixed |Rock cut | 99+40 | 98+70 |Jan. 2, '07|Feb. 6, '07|
| | | | | |
Rock |Full face | 98+70 | 98+60 |Feb. 6, '07|Feb. 12, '07|
| | | | | |
Mixed |Bottom heading | 98+60 | 98+39 |Feb. 12, '07|Mar. 6, '07|
| | | | | |
Rock |Bottom heading | 98+39 | 98+17 |Mar. 6, '07|Mar. 15, '07|
| | | | | |
Mixed |Rock cut | 98+17 | 95+68 |Mar. 15, '07|July 30, '07|
| | | | | |
Rock |Middle heading | 95+68 | 94+61 |July 30, '07|Aug. 21, '07|
| | | | | |
Mixed |Rock cut | 94+61 | 93+56 |Aug. 21, '07|Oct. 3, '07|
| | | | | |
Rock |Middle heading | 93+56 | 92+73 |Oct. 3, '07|Nov. 11, '07|
| | | | | |
Mixed |Rock cut | 92+73 | 90+55 |Nov. 11, '07|Feb. 13, '08|
| | | | | |
Mixed |Rock cut | 90+55 | 90+44.4 |Feb. 25, '08|Mar. 3, '08|
--------------+-----------------+---------+---------+------------+------------+

--------------+------+--------+--------+--------------------------------------+
| | |Rate of | |
|Number| |progress| |
| of | Linear |in feet | |
Material. | days.| feet. |per day.| Remarks |
--------------+------+--------+--------+--------------------------------------+
{| 7} | | {|Stopped to brace portal. No work done |
{| 14} | | {|from March 12th to October 11th, 1905,|
Rock {| 38} | 54 | 0.77 {|except a little trimming in September.|
{| 10} | | {|All work up to this date done in |
{| 15} | | {|normal air. Heading advanced to 106+70|
{| } | | {|and bulkheaded. |
| | | | |
Rock | 26 | 53 | 2.04 | |
| | | | |
| | | {|Heading advanced to 106 + 40. Shut |
Mixed | 21 | 7 | 0.33 {|down in order that Line D might have a|
| | | {|lead. |
| | | | |
Mixed | 38 | 38 | 1.00 {|Shut down on account of air shortage. |
| | | | |
Mixed | 18 | 32 | 1.78 |Shut down on account of air shortage. |
| | | | |
Mixed | 30 | 30 | 1.00 |Shut down April 20th to July 27th, |
| | | |1906. |
| | | | |
Earth |127 | 615 | 4.84 | |
| | | | |
| | | | |
Mixed | 35 | 70 | 2.00 | |
| | | | |
Rock | 6 | 10 | 1.66 | |
| | | | |
Mixed | 22 | 21 | 0.95 | |
| | | | |
Rock | 9 | 22 | 2.44 | |
| | | | |
Mixed |110 | 249 | 2.26 |Heading advanced to 97+82. |
| | | | |
Rock | 49 | 107 | 2.18 | " " " 94+35. |
| | | | |
Mixed | 43 | 106 | 2.46 | |
| | | | |
Rock | 39 | 83 | 2.13 | |
| | | | |
Mixed | 94 | 218 | 2.32 |Shut down until Line D shields met. |
| | | | |
Mixed | 6 | 11 | 1.83 | |
--------------+------+--------+--------+--------------------------------------+

LINE D, MANHATTAN.
--------------+-----------------+---------+---------+------------+------------+
{|Top heading |107+70.49|107+16 |Dec. 9, '04|Jan. 31, '05|}
{|Removing bench |107+70.49|107+35 |Jan. 1, '05|Jan. 27, '05|}
Rock {|Bottom heading |107+35 |106+80 |Jan. 30, '05|Feb. 10, '05|}
{|Trimming |107+70 |106+80 |Mar. 29, '05|Apr. 12, '05|}
{|Trimming |107+70 |106+80 |Aug. 31, '05|Sept 19, '05|}
| | | | | |
Rock |Bottom heading |106+80 |106+67 |Oct. 5, '05|Nov. 8, '05|
| | | | | |
Mixed |Bottom heading |106+67 |106+39 |Nov. 8, '05|Dec. 23, '05|
| | | | | |
|Sliding hood and | | | | |
Mixed |breasting. Rock |106+29 |105+70 |Dec. 23, '05|Jan. 24, '06|
|bench | | | | |
| | | | | |
Earth |Poling and |105+70 |104+61 |Jan. 24, '06|Feb. 27, '06|
|breasting | | | | |
| | | | | |
| | | | | |
Earth |Poling, breasting|104+61 |103+90 |Mar. 2, '06|Mar. 31, '06|
| and shutters | | | | |
| | | | | |
| | | | | |
Earth |Shutters |103+90 | 99+41 |Apr. 20, '06|Sept 3, '06|
| | | | | |
| | | | | |
Mixed |Bottom bench | 99+41 | 99+17 |Sept 3, '06|Sept 23, '06|
| | | | | |
Mixed |Bottom heading | 99+17 | 98+50 |Oct. 2, '06|Nov. 24, '06|
| | | | | |
| | | | | |
Rock |Bottom heading | 98+50 | 97+72 |Nov. 24, '06|Jan. 16, '07|
| | | | | |
Mixed |Bottom heading | 97+72 | 97+27 |Jan. 16, '07|Feb. 10, '07|
| | | | | |
Mixed |Rock cut | 97+27 | 95+72 |Feb. 10, '07|Apr. 23, '07|
| | | | | |
Rock |Middle heading | 95+72 | 95+57 |Apr. 23, '07|May 11, '07|
| | | | | |
Rock |Middle heading | 95+57 | 94+65 |May 23, '07|June 17, '07|
| | | | | |
| | | | | |
Mixed |Middle heading | 94+65 | 94+41 |June 17, '07|June 25, '07|
| | | | | |
Mixed |Rock cut | 94+41 | 94+03 |June 25, '07|July 13, '07|
| | | | | |
Rock |Middle heading | 94+03 | 92+64 |July 13, '07|Sept 12, '07|
| | | | | |
Mixed |Middle heading | 92+64 | 92+54 |Sept 12, '07|Sept 20, '07|
| | | | | |
Rock |Middle heading | 92+54 | 92+50 |Sept 20, '07|Sept 21, '07|
| | | | | |
Mixed |Rock cut | 92+50 | 90+38.66|Sept 21, '07|Jan. 8, '08|
--------------+-----------------+---------+---------+------------+------------+

--------------+------+--------+--------+--------------------------------------+
| | |Rate of | |
|Number| |progress| |
| of | Linear |in feet | |
Material. | days.| feet. |per day.| Remarks |
--------------+------+--------+--------+--------------------------------------+
{| | | | |
{| | | | |
Rock {|123 | 90 | 0.73 |In normal air. |
{| | | | |
{| | | | |
| | | | |
Rock | 34 | 13 | 0.40 |Bottom heading timbered. |
| | | | |
Mixed | 45 | 38 | 0.84 | |
| | | | |
| | | | |
Mixed | 32 | 59 | 1.84 | |
| | | | |
| | | | |
Earth | 31 | 109 | 3.41 | |
| | | | |
| | | | |
| | | |Three days' delay to set shutters in |
Earth | 29 | 71 | 2.45 |top. Shut down 20 days to permit |
| | | |consolidation of the river bed and to |
| | | |repair broken plates. |
| | | | |
Earth |136 | 449 | 3.40 |Four days of 136, delay account of |
| | | |flood. |
| | | | |
Mixed | 20 | 24 | 1.20 | |
| | | | |
Mixed | 53 | 67 | 1.27 |Thirteen days' shut-down to put on |
| | | |hood. |
| | | | |
Rock | 53 | 78 | 1.47 | |
| | | | |
Mixed | 25 | 45 | 1.40 | |
| | | | |
Mixed | 72 | 155 | 2.15 | |
| | | | |
Rock | 18 | 15 | 0.83 | |
| | | | |
Rock | 25 | 92 | 3.68 |Twelve days' delay to repair cutting |
| | | |edge. |
| | | | |
Mixed | 8 | 24 | 3.00 | |
| | | | |
Mixed | 18 | 38 | 2.11 | |
| | | | |
Rock | 61 | 139 | 2.28 | |
| | | | |
Mixed | 8 | 10 | 1.25 | |
| | | | |
Rock | 1 | 4 | 4.00 | |
| | | | |
Mixed |109 | 211.34| 1.94 | |
--------------+------+--------+--------+--------------------------------------+

Openings were made between the headings as follows:

Tunnel _D_, February 20th, 1908; Tunnel _B_, March 3d, 1908; Tunnel _C_, March 5th, 1908; Tunnel _A_, March 18th, 1908.

It was necessary to cut away the projecting floors of the working compartments before the cutting edges could be shoved together.

_Contractor's Organization._--Tunnel operations were carried on continuously for thirteen days out of fourteen, regular work being shut down for repairs on alternate Sundays. When the required pressure was more than 32 lb., four gangs of laborers were employed, each gang working two shifts of 3 hours each, with an intermission of 3 hours between the shifts. When the pressure was less than 32 lb., three gangs were employed, each gang covering 8 hours, but with an intermission of about 1/2 hour in low pressure for lunch.

_Air Pressures Required._--During the greater portion of the work in soft ground, pressure was maintained which would about balance the hydrostatic head at the axis of the tunnel. This required a pressure varying from 30 to 34 lb. per sq. in. above that of the atmosphere. In Tunnels _B_ and _D_, at Manhattan, during the work in soft ground, pressures as high as 37 lb. were maintained for considerable periods of time; in the firm material near the reef 28 lb. was often sufficient. While removing the broken plates, the pressure was raised for a short time to 42 lb., and was maintained between 37-1/2 and 40 lb. for a little more than one month.

_Air Supply._--For regular operation the contractor furnished four compressors on each side of the river, each having a rated capacity of 5,000 cu. ft. of free air per minute delivered at 50 lb. above normal, when running at the rate of 100 rev. per min. An additional compressor of the same capacity was supplied on each side of the river, in compliance with the requirement for 25% excess capacity; the additional compressors had also high-pressure air cylinders which could be connected at will, and in which the pressure could be increased to 150 lb., and the air used to supply rock drills, grouting machines, etc. The entire combination on each side of the river, therefore, was rated at 25,000 cu. ft. of free air per minute, or a mean of 6,250 cu. ft. per heading. Its safe working capacity was not far from 20,000 cu. ft. per min.

The shields broke through rock surface in Tunnels _B_, _C_, and _D_, at Manhattan, in November and December, 1905. The consumption of air in the four tunnels soon exceeded 15,000 cu. ft. for 24 hours, and in Tunnel _D_, on several occasions, it exceeded 7,000 cu. ft. for a like period. Blows had become frequent, and it was evident that the air plant was inadequate for driving four tunnels at once in the open material east of the Manhattan rock. Work in Tunnel _A_, therefore, was not resumed, after the suspension on December 29th, for about ten months, and Tunnel _C_ was also closed down for more than four months of the time between December, 1905, and July, 1906. During this period the capacity of the plant was increased from the rated 25,000 cu. ft. of free air per minute, to 35,000. In Tunnel _D_ the material had gradually become firmer, with more clay and less escape of air, as the Blackwell's Island Reef was approached, and, at the end of the period, the rock surface was within 3 ft. of the top of the shield; in Tunnel _B_, the rock of the reef was still a little below the shield, but the overlying material contained a large proportion of clay and held air very well. Tunnel _C_ was still in open material, but, with two lines safe and with the increased air plant, it was deemed best to resume work in Tunnel _A_, which was done on October 23d, 1906. Thenceforward work was continuous in all headings until the meeting points with the Long Island shields were reached.

This period, January to October, 1906, inclusive, was the most strenuous of the entire work, particularly the first six months. With one and, at times, two tunnels closed down, the consumption of air in the headings from Manhattan was an average of more than 20,000 cu. ft. per min. for periods of from 30 to 60 days; it was often more than 25,000 cu. ft. for 24 hours, with a maximum of nearly 29,000 cu. ft., and doubtless this was exceeded considerably for shorter periods. On several occasions the quantity supplied to a single tunnel averaged more than 15,000 cu. ft. per min. for 24 hours. The greatest averages for 24 hours were obtained later in Tunnel _A_, after the resumption of work there, and exceeded 19,000 cu. ft., but the conditions in the headings of the other lines were then so favorable that the work was carried on continuously in all.

The deficiency in the original plant at Manhattan was so marked, and the need of driving all headings from Long Island simultaneously so clear, that it was decided to increase the rated capacity of the Long Island compressor plant to 45,400 cu. ft. of free air per minute, which was 10,400 cu. ft. greater than the capacity of the Manhattan plant after the latter had been augmented.

The earth encountered on emerging from rock, when driving westward from Long Island, was far more compact and less permeable to air than on the Manhattan side, but for a distance of from 400 to 600 ft. immediately east of the reef, it was a clean open sand, and, while the shields were passing through this, the quantity of air supplied to the four headings seldom fell below 20,000 cu. ft. per min.; it was usually more than 25,000 cu. ft., with a recorded maximum of 33,400 cu. ft. Although this was greater than ever used on the Manhattan side, it was more uniformly distributed among the several headings, and in none equalled the maximum observed on the Manhattan side, the largest having been 12,700 cu. ft. per min. for 24 hours; it must be remembered, however, that at one time only two tunnels were in progress in the bad material in the tunnels from Manhattan.

From the foregoing experience, it would seem that the plant finally furnished at Long Island, having a rated capacity of 45,400 cu. ft. of free air per minute, would have been a reasonable compliance with the original actual needs on the Manhattan side and _vice versa_; the plant finally developed on the Manhattan side, having a rated capacity of 35,000 cu. ft. of free air per minute, would have sufficed for the Long Island side.

The total quantity of free air compressed for the supply of the working chambers of the tunnels and the Long Island caissons was 34,109,000,000 cu. ft., and, in addition, 10,615,000,000 cu. ft. were compressed to between 80 and 125 lb. for power purposes, of which at least 80% was exhausted in the compressed-air working chambers. The total supply of free air to each heading while under pressure, therefore, averaged about 3,550 cu. ft. per min.

The quantity of air escaping during a sudden blow-out is apparently much smaller than might be supposed. Investigation of a number of cases, showing large pressure losses combined with a long stretch of tunnel supplying a relatively large reservoir of air, disclosed that a maximum loss of about 220,000 cu. ft. of free air occurred in 10 min. This averages only a little more than 19,000 cu. ft. per min., the maximum recorded supply to one tunnel for a period of 24 hours. Of this quantity, however, probably from 30 to 40% escaped in the first 45 seconds, while the remainder was a more or less steady loss up to the time when the supply could be increased sufficiently to maintain the lowered pressure. Very few blows showed losses approaching this in quantity, but the inherent inaccuracy of the observations make the foregoing figures only roughly approximate.

[Footnote C: _Minutes of Proceedings_, Inst. C. E., Vol. CXXX, p. 50.]

SPECIAL DIFFICULTIES.

The most serious difficulties of the work came near the start. In Tunnel _D_ blows and falls of sand from the face were frequent after soft ground was met in the top. About six weeks after entering the full sand face, and before the shutters had been installed, the shield showed a decided tendency to settle, carrying the tunnel lining down with it and resulting in a number of badly broken plates in the bottom of the rings. Notwithstanding the use of extremely high vertical leads,[D] the sand was so soft that the settlement of the shield continued for about fifteen rings, the maximum being nearly 9 in. below grade. The hydrostatic head at mid-height of the tunnel was 32-1/2 lb., and the raising of the air pressure to 37 lb., as was done at this time, was attended with grave danger of serious blows, on account of the recent disturbance of the natural cover by the pulling and re-driving of piles in the reconstruction of the Long Island ferry slips directly above. It dried the face materially, however, and the shield began to rise again, and had practically regained the grade when the anticipated blow-outs occurred, culminating with the entrance of rip-rap from the river bed into the shield and the flooding of the tunnel with 4 ft. of sand and water at the forward end. The escape of air was very great, and, as a pressure of more than 28 lb. could not be maintained, the face was bulkheaded and the tunnel was shut down for three weeks in order to permit the river bed to consolidate.

This was the most serious difficulty encountered on any part of the work, and, coming at the very start, was exceedingly discouraging. During the shut-down the broken plates were reinforced temporarily with steel ribs and reinforced concrete (Fig. 1, Plate LXXIII) which, on completion of the work, were replaced by cast-steel segments, as described elsewhere. Practically, no further movement of iron took place, and the loss of grade caused by the settlement of the shield, which was by far the largest that ever occurred in this work, was not sufficient to require a change in the designed grade or alignment of the track. Work was resumed with the shutters in use at the face as an aid to excavation. The features of extreme seriousness did not recur, but for two months the escape of air continued to be extremely large, an average of 15,000 cu. ft. per min. being required on many days during this period.

In Tunnel _B_, after passing out from under the bulkhead line, in April, 1906, the loss of air became very great, and blow-outs were of almost daily occurrence until the end of June. At the time of the blows the pressure in the tunnel would drop from 2 to 8 lb., and it generally took some hours to raise the pressure to what it was before the blow. During that time regular operations were interrupted. In the latter part of June a permit was obtained allowing the clay blanket to be increased in thickness up to a depth of water of 27 ft. at mean low tide. The additional blanket was deposited during the latter part of June and early in July, and almost entirely stopped the blows.

By the end of the month the natural clay, previously described, formed the greater portion of the face, and, from that time forward, played an important part in reducing the quantity of air required. During April and the early part of May the work was under the ferry racks of the Long Island Railroad. The blanket had to be placed by dumping the clay from wheel-barrows through holes in the decking.

In Tunnel _A_ a bottom heading had been driven 23 ft. in advance of the face at the time work was stopped at the end of 1905. During the ten months of inactivity the seams in the rock above opened. The rock surface was only from 2 to 4 ft. below the top of the cutting edge for a distance of about 60 ft. Over the rock there were large boulders embedded in sharp sand. It was an exceedingly difficult operation to remove the boulders and place the polings without starting a run. The open seams over the bottom heading also frequently caused trouble, as there were numerous slides of rock from the face which broke up the breasting and allowed the soft material from above to run into the shield. There were two runs of from 50 to 75 cu. yd. and many smaller ones.

[Footnote D: The lead of the shield is the angular divergence of its axis from the axis of the tunnel and, in this tunnel, was measured as the offset in 23 ft. It was called + when the shield was pointed upward from grade, and - when pointed downward.]

GUIDING THE SHIELDS.

Little difficulty was experienced at any time in driving the shield close to the desired line, but it was much harder to keep it on grade. In rock section, where the cradle could be set far enough in advance to become hard before the shield was shoved over it, there was no trouble whatever. Where the cradle could be placed only a very short time before it had to take the weight of the shield, the case was quite different. The shield had a tendency to settle at the cutting edge, and when once pointed downward it was extremely difficult to change its direction. It was generally accomplished by embedding railroad rails or heavy oak plank in the cradle on solid foundation. This often had to be repeated several times before it was successful. In soft ground it was much easier to change the direction of the shield, but, owing to the varying nature of the material, it was sometimes impossible to determine in advance how the shield should be pointed. It was found by experience at Manhattan that the iron lining remained in the best position in relation to grade when the underside of the bottom of the shield at the rear end was driven on grade of the bottom of the iron, but if the rate of progress was slow, it was better to drive the shield a little higher.

In the headings from Long Island, which, as a rule, were in soft ground, the cutting edges of the shields were kept from 4 to 8 in. higher, with respect to the grade line, than the rails. The shields would then usually move parallel to the grade line, though this was modified considerably by the way the mucking was done and by the stiffness of the ground at the bottom of the shield.

On the average, the shields were shoved by from ten to twelve of the bottom jacks, with a pressure of about 4,000 lb. per sq. in. The jacks had 9-in. plungers, which made the average total force required to shove the shield 2,800,000 lb. In the soft ground, where shutters were used, all of the twenty-seven jacks were frequently used, and on several occasions the pressure exceeded 6,000 lb. per sq. in. With a unit pressure of 6,000 lb. per sq. in., the total pressure on the shield with all twenty-seven jacks in operation was 5,154 tons.

INJURIES TO SHIELDS.

There were only two instances of damage to the essential structural features of the shields. The most serious was in Tunnel _D_ where the cutting edge at the bottom of the shield was forced up a slightly sloping ledge of rock. A bow was formed in the steel casting which was markedly increased with the next few shoves. Work was suspended, and a heavy cast-steel patch, filling out the bow, was attached to the bent segments, as shown in Fig. 2, Plate LXXIII. No further trouble was experienced with the deformed portion. The other instance was in Tunnel _B_, from Long Island, where a somewhat similar but less serious accident occurred and was treated in a like manner.

_Bulkheads._--At Manhattan, bulkheads had to be built near the shafts before the tunnels could be put under pressure. After 500 ft. of tunnel had been built on each line, the second bulkheads were constructed. The air pressure between the first and second bulkheads was then reduced to between 15 and 20 lb. When the shields had been advanced for 1,500 ft., the third set of bulkheads was built. Nearly all the broken plates which were removed were located between the first and third bulkheads at Manhattan. Before undertaking this operation, the doors of the locks in the No. 3 bulkheads were reversed to take pressure from the west. By this means it was possible to carry on the work of dismantling the shields under comparatively low pressure simultaneously with the removal of the broken plates.

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Transactions of the American Society of Civil Engineers, vol. LXVIII, Sept. 1910Chapter II: Part 2

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