Part 2
In all cases where the course of this ancient stream was crossed (except at its final intersection of 33d Street), the rock was found to be very soft and disintegrated, a large quantity of water was encountered, and heavy timbering was required. The construction at these localities will be taken up later. In addition, disintegrated rock, but of a less troublesome character, was invariably met under the depressions in the rock surface developed by the borings from the streets and test holes from the tunnels. Many of these places required timbering, and no timbering was elsewhere necessary except at the portals. These coincident conditions were especially marked in 32d Street, which for a long distance closely adjoins the course of the former creek.
_Disposal of Spoil._--The materials excavated from the tunnels were dumped at the 35th Street pier on barges furnished by the Railroad Company under another contract, and were towed to points near the Bayonne peninsula where the spoil was used principally in the construction of the Greenville Freight Yards and the line across the Hackensack Meadows to the tunnels. Details of this work will be given in a subsequent paper. After December, 1907, when the excavation was about 85% completed, the contractor furnished the barges and effected the complete disposal of the spoil.
_Difficulties of Excavation._--As stated in a previous paper, the excavation of the Twin Tunnel in 33d Street was continued westward to the west line of Fifth Avenue on the original grade. At that point the contractor started three drifts in the three-track section. The relation of the drifts to each other and to the cross-section are shown by Fig. 3. The center heading was driven a little in advance of those on the sides. At a distance of 65 ft. west of Fifth Avenue the rock surface was broken through in the top of the heading, and a very fine sand was encountered. For some distance east of this point the rock was badly disintegrated, and the heading required timbering. Through the soft material, tight lagging was placed on the sides and roof of the heading, and the face was protected by breast boards. There was a moderate flow of water through the cracks, and, in spite of every effort, some of the fine sand was constantly carried into the heading.
In one or two instances considerable ground was lost at the face. On the evening of December 14th, 1906, as a heavy coal wagon was passing along 33d Street above the heading, the rear wheels dropped through the asphalt pavement. An examination disclosed a cavity under the pavement about 14 ft. long, 12 ft. wide and 14 ft. deep. Evidently, the fine sand had gradually settled into the voids caused by the loss of material at the face, and the settlement broke the brick sewer over the heading. The sewer was temporarily repaired, and the hole in the street was filled before morning. A tight bulkhead was built across the heading, and work was abandoned at that point. The north drift was advanced to a point 108 ft. west of Fifth Avenue where sand was also encountered and a considerable run occurred. After that time all work on the three-track section was discontinued.
The Company then took up the consideration of changes in plan. To determine the difficulties of driving a Twin Tunnel at a lower elevation, an exploration drift, 8 ft. high and 12 ft. wide, was driven on the center line of the street as a top heading on the proposed new grade. Test holes were drilled above this heading and to the sides. The results indicated that there was sufficient rock cover of fair quality to enable the Twin Tunnel to be driven without great risk. The new plan (continuing the Twin Tunnel westward at a lower grade) was adopted in March, 1907, and work was immediately resumed at Fifth Avenue.
The relation between the cross-sections under the old and new plans at that point is shown by Fig. 3. Before the new section was excavated it was necessary to support the timber work in the old headings. The plan adopted is also shown by Fig. 3. The rock was excavated under the center heading, as shown in cross-section, for a length of about 3 ft. A girder composed of two 18-in. I-beams was then put in position over each line and supported on the sides by posts. The ends at the center lines between the tunnels were supported on short posts bearing on the rock bench. The support of the timbering in the headings was then transferred to the girders by additional posts. Blocking was also inserted between the tops of the beams and the rock walls between the headings. Fig. 2, Plate LIX, gives a good idea of the timber work in the top headings above the I-beams. When the roof had been made secure, the removal of the bench was begun. As the work advanced it was necessary to replace the short posts at the center of the tunnel by others of full height, and there was considerable settlement in the I-beams during this operation. When the bench had been removed to a point 61 ft. west of Fifth Avenue, settlement was detected in the street surface above. Bench excavation was suspended and a section of the permanent lining, 35 ft. long, was placed. The space between the lining and the beams and between the beams and the roof was filled with rubble masonry. Grout pipes were built into the masonry and later all voids were filled with grout. Fig. 3, Plate LIX, shows the first section of the concrete lining completed and part of the rubble in place; and Fig. 4, Plate LIX, shows details of the work above the tunnels. A second section of bench was next removed and more lining was placed. Work was continued in this way until all the roof at the old three-track headings had been secured. In this portion of the work the posts were embedded in the concrete.
Between Fifth and Sixth Avenues there were two more sections of bad rock where it was necessary to support the roof with steel beams. At these latter points there were no complications with the excavation for the Three-Track Tunnel, and the work was much simpler. To avoid leaving the center posts in the permanent work, two rows of temporary posts were placed, as shown by Fig. 1, Plate LX, the center wall and skewback were built, and the posts were removed, as shown by Fig. 2, Plate LX, before placing the remainder of the lining.
In 32d Street the normal progress of the excavation was frequently interrupted by encountering soft and unsound rock. In the excavation between the East River and the Intermediate Shafts it was possible to overcome these conditions by temporarily narrowing the excavation on one side and supporting the roof on 16 by 16-in. transverse timbers caught in niches in the rock at the sides, leaving sufficient room for the steam shovel to work through. In order to save time, the height of the excavation was not increased before placing these timbers, so that, previous to the concreting, they all required to be raised to clear the masonry lining and were then supported on posts on the center line between the tunnels. This permitted the remainder of the excavation to be made, and such additional timbering as was required was placed. At most of these sections a brick arch and water-proofing were used, on account of the presence of water. In certain places the center line posts were buried in the core-wall, and, in order to permit the placing of the water-proofing, were then cut off one by one flush with its top as the load was transferred to the completed masonry. In other cases the load was transferred to posts clear of the masonry and the center line posts were entirely removed. Under such conditions the normal concrete methods, to be described later, could not be used, and special forms were substituted.
In this section of the work the most serious difficulties were encountered near Fourth Avenue a short distance east of the Intermediate Shaft, and beneath the site of the old pond shown on General Viele's map. The rock cover was known from the boring to be very thin, and the presence of the subway overhead caused some anxiety. The excavation was at first taken out to practically full width and timbered, but the rock became so treacherous that the heading was narrowed to a width sufficient for one tunnel only. With this span the rock in the roof held without timbering. As the masonry lining approached, sufficient trimming was done to permit the placing of the core-wall and one arch. Above the completed core-wall and brick arch the voids were filled solid with rubble masonry to give an unyielding support to the roof. The excavation of the remaining width of tunnel was then undertaken. Near the west side of Fourth Avenue, the excavation broke out of rock at the top, and fine sand and gravel with a large quantity of water were encountered. The work of excavation was arduous, and proceeded very slowly, on account of the care with which it was executed. Only a small amount of sand entered the tunnel, but the lining was placed as soon as the excavation was completed. Rubble masonry packing and grout ejected through pipes built into the arch were used to fill the voids above the roof. As a further precaution against the settlement of the subway, 2-in. pipes were washed down from the street above the point where soft ground was exposed in the roof of the tunnel, and through them grout was forced into the ground at various depths. Careful levels show that no settlement of the subway has taken place.
West of the Intermediate Shaft the tunnel was excavated for full width until bad rock was encountered about 60 ft. west of Madison Avenue. (See General Viele's map, Plate IX.) Timbering was used for a short distance, and then the heading and bench were narrowed to 18 ft., and steam-shovel excavation was abandoned. As the heading advanced the rock grew steadily softer, the difficult conditions in this locality culminating when a slushy disintegrated feldspar was met, requiring poling and breasting. Thereafter the rock improved markedly, but near the east side of Fifth Avenue its thickness above the roof was found to be only 1-1/2 ft., and the advance was stopped, pending a decision as to a change of plan.
After some delay, an exploration drift, similar to the one already described, was driven through to Sixth Avenue, and a change in plan was made, substantially the same as for the 33d Street tunnels. Enlargement to full size was at once started, but, for 400 ft. the rock was very soft and poor, and required extremely careful handling. The exploration drift was widened out to the full Twin-Tunnel width, and I-beams were placed and supported, in much the same manner as in 33d Street. The rock was so soft that it was frequently necessary to drive poling boards ahead as the face was mined out with picks and shovels. The load was very heavy, and the work the most difficult encountered in the tunnels.
After this stage of the enlargement was reached, the excavation of the bench and the placing of the lining proceeded alternately, with the I-beams temporarily supported on long posts while the concrete core-wall was being built. Considerable settlement took place while shifting the posts, and eventually showed on the street surface and in the adjacent sidewalk vaults, but no damage was done to the structural portions of the buildings.
While the above work had been going on westward from Fifth Avenue, the excavation of the Twin Tunnel eastward from the end of the open-cut section at Sixth Avenue had been proceeding rapidly, and, toward the end of the difficult Fifth Avenue work, it was being attacked from both directions.
PROGRESS OF EXCAVATION.
Owing to the numerous sections of poor rock, interspersed throughout the work with stretches of sound rock, the progress of the excavation was very irregular, especially in 32d Street. The rate of excavation in good ground is shown in Table 1. In the sections of bad ground, the operations of excavation, timbering, and lining were often carried on alternately, and it is impracticable to include them in the table.
TABLE 1.--PROGRESS AND METHODS OF EXCAVATION IN GOOD GROUND.
THIRTY-THIRD STREET.
============================================================ 1 | 2 | 3 | -----------------------------+--------+--------------------+ | | | | | | Type of excavation. |Tunnels.| Worked from: | | | | | | | -----------------------------+--------+--------------------+ Full-sized single tunnel | B | 1st Ave. shaft. | | | | Full-sized single tunnel | A | 1st Ave. shaft. | | | | Full-sized twin tunnel |A and B | 1st Ave. shaft. | | | | | | | | | | Full-sized twin tunnel |A and B |Intermediate shaft. | | | (West of shaft.) | | | | | | | Full-sized twin tunnel |A and B |Intermediate shaft. | | | (East of shaft.) | | | | | | | Full-sized twin tunnel |A and B |Intermediate shaft. | | | (East of shaft.) | | | | | | | Exploration drift |A and B |Intermediate shaft. | | | (West of shaft.) | | | | Twin tunnel. Enlargement |A and B | West shaft. | of exploration drift | | (East of shaft.) | =============================+========+=====================
====================================================================== 4 | 5 | 6 | 7 | ----------------------------------+--------+------------+------------+ | | Length | Average | DATES. | Time | tunnel | advance | ----------------------------------|elapsed,| excavated, | per day, | | | in | in | in | From | To | days. |linear feet.|linear feet.| ----------------------------------+--------+------------+------------+ Feb. 28, 1906. |May 12, 1906. | 74 | 346 | 4.7 | | | | | | Feb. 28, 1906. |Apr. 30, 1906. | 62 | 255 | 4.1 | | | | | | Aug. 23, 1906. |Jan. 5, 1907. | 136 | 789 | 5.8 | | | | | | | | | | | | | | | | Apr. 4, 1906. |Oct. 31, 1906. | 210 | 730 | 3.5 | | | | | | | | | | | | | | | | Apr. 4, 1906. |Oct. 31, 1906. | 210 | 783 | 3.7 | | | | | | | | | | | | | | | | Nov. 1, 1906. |Dec. 26, 1906. | 56 | 311 | 5.5 | | | | | | | | | | | | | | | | Mar. 1, 1907. |July 23, 1907. | 145 | 947 | 6.5 | | | | | | | | | | | Sept. 6, 1907. |Dec. 4, 1907. | 89 | 603 | 6.8 | | | | | | ===============+==================+========+============+=============
===================================================== 8 -----------------------------------------------------
Methods and conditions.
Top heading and bench. Muck loaded by hand.
" " " " " " " "
Top full-width heading and bench. Muck loaded by steam shovel. Working exclusively on this heading.
Top center heading and bench. Muck loaded by steam shovel. Working alternately in headings east and west of the shaft.
Top center heading and bench. Muck loaded by steam shovel. Working alternately in headings east and west of the shaft.
Top full-width heading and bench. Muck loaded by steam shovel working exclusively on this heading.
Exploration drift about 9 ft. by 12 ft. Mucking by hand. Fourteen timber bents were placed in March, and seven in April, 1907.
Drift excavated to full width and bench. Muck loaded by steam shovel. =====================================================
THIRTY-SECOND STREET.
============================================================ 1 | 2 | 3 | -----------------------------+--------+--------------------+ | | | | | | Type of excavation. |Tunnels.| Worked from: | | | | | | | -----------------------------+--------+--------------------+ Full-sized single tunnel | C | 1st Ave. shaft. | | | | Full-sized single tunnel | D | 1st Ave. shaft. | | | | Full-sized twin tunnel |C and D | 1st Ave. shaft. | | | | | | | | | | Narrowed twin tunnel | C |Intermediate shaft. | | | (East of shaft.) | | | | | | | Narrowed twin tunnel | C |Intermediate shaft. | | | (East of shaft.) | | | | | | | | | | | | | Full-sized twin tunnel |C and D |Intermediate shaft. | | | (West of shaft.) | | | | Exploration drift |C and D |Intermediate shaft. | | | (West of shaft.) | | | | Twin tunnel. Enlargement }|C and D |{ Eastward from | of exploration drift }| |{ open cut. | | | | Twin tunnel. Enlargement }|C and D |{ Eastward from | of exploration drift }| |{ open cut. | | | | =============================+========+=====================
==================================+========+============+============= 4 | 5 | 6 | 7 | ----------------------------------+--------+------------+------------+ | | Length | Average | DATES. | Time | tunnel | advance | ----------------------------------|elapsed,| excavated, | per day, | | | in | in | in | From | To | days. |linear feet.|linear feet.| ----------------------------------+--------+------------+------------+ Jan. 25, 1906.|Apr. 30, 1906. | 95 | 367 | 3.9 | | | | | | Jan. 27, 1906.|Apr. 30, 1906. | 93 | 354 | 3.8 | | | | | | {May. 22, 1906.|July 24, 1906.[E]}| 173 | 810 | 4.7 | {Aug. 11, 1906.|Nov. 29, 1906. }| | | | | | | | | | | | | | Mar. 19, 1906.|May 28, 1906. | 70 | 58 | 0.8 | | | | | | | | | | | | | | | | {May 29, 1906.|July 3, 1906.[E]}| 208 | 1,206 | 5.8 | {July 18, 1906.|July 31, 1906. }| | | | {Aug. 12, 1906.|Nov. 23, 1906. }| | | | {Jan. 15, 1907.|Feb. 5, 1907. }| | | | {Feb. 17, 1907.|Mar. 21, 1907. }| | | | | | | | | Dec. 1, 1905.|May. 10, 1906. | 161 | 225 | 1.4 | | | | | | | | | | | Feb. 1, 1907.|Sept.13, 1907. | 225 | 1,033 | 4.6 | | | | | | | | | | | }Feb. 1, 1908.|Feb. 14, 1908. | 14 | 65 | 4.6 | } | | | | | | | | | | }Feb. 15, 1908.|Apr. 14, 1908. | 59 | 524 | 8.9 | } | | | | | | | | | | ======================================================================
[Footnote E: Time and distance omitted while working through timbered stretches.]
======================================================= 8 ------------------------------------------------------- Methods and conditions. ------------------------------------------------------- Top heading and bench. Muck loaded by hand.
" " " " " " " "