Brooklyn Approaches to Downtown Tubes (1917)
Public Service Record · Vol. IV, No. 11, November 1917.
Brooklyn Approaches to Downtown Tubes.
By Aksel H. Jorgensen, Section Engineer, Tunnel Division
The enlargement of transit facilities, as provided for under the Dual System Contracts, in a great many instances has required work of a most difficult and unusual character. The selection of any particular section as the most interesting would probably be impossible in view of the many problems peculiar to each, but there are few which exceed in interest, either generally or from an engineering standpoint, the construction on Section 3 of Route No.33.
The contract was delivered on October 9, 1914, to the Flinn-O'Rourke Company, Inc., on a unit price bid totaling $3,395,152. The contract time is 36 months. A station at Willoughby and Lawrence Streets, recently added, will increase the above amount by approximately $275,000.
General Outline of Work. The work, now nearly completed, involves the construction of two separate railway lines in Fulton, Montague and Willoughby Streets, Brooklyn.
Under Fulton Street between Clark and Willoughby Streets the work forms the connecting link between the Old Slip-Clark Street River Tunnels and the First Subway, operated by the Interborough Rapid Transit Company. From west of Clinton Street on through Montague, Fulton and Willoughby Streets to Flatbush Avenue Extension the work forms the connecting link between the Whitehall-Montague Street River Tunnels and the Fourth Avenue subway, operated by the New York Consolidated Railroad Company.
Types of Construction. An unusual number of widely varying types of construction are used, the plans providing for three sections of shield driven tunnels in pairs, a station under Montague Street between Clinton and Court Streets, to be constructed by tunneling between the tubes, and three sections of steel bent structure, one of which is double decked and forms the Borough Hall Station, with upper and lower platforms and diverging approaches. Connections by means of passageways are to be made between this station, the Court Street Station and the present Borough Hall Station.
Two shafts are provided for entrance to the Court Street Station, one, located west of Clinton Street, is designed for elevators and stairways with a large mezzanine at the top and the other, located west of Court Street, is designed for escalators and stairways with a double mezzanine. A third shaft, located at Court Street, will provide ventilation for the Montague Street tubes. At Fulton and Johnson Streets a shaft embracing both tunnels is being constructed to provide ventilation for the Fulton and Clark Street lines. These shafts range in depth from 61 to 77 feet; three are of structural steel design and the fourth, the Court Street shaft, is of reinforced concrete design.
New Station. A station not provided for in the contract, as stated above, has recently been added to the work at Willoughby and Lawrence Streets. This work will consist of reconstructing the partially completed Lawrence Street crossover and fan chamber, of tunneling between the tubes for the purpose of constructing an island platform, and of providing a new location for the crossover.
Many other features of the work are of particular interest, among which may be mentioned the underpinning of the two existing subways, many buildings, and the elevated structure along Fulton Street, and the maintenance of the heaviest surface car traffic in New York over a cut 1000 feet in length, varying in depth from 50 to 63 feet.
Five Railway Levels. Through Fulton Street between Myrtle Avenue and Willoughby Street, near Borough Hall, there will exist upon the completion of the work five distinct railway levels which will provide five separate routes between Borough Hall and lower Manhattan. The five levels of transit at this point compare with the corresponding number of transit levels at Grand Central, a fact which is not generally recognized, owing to the depth of the so-called New York Municipal tunnels which are not connected in any way with the upper structure.
A cross section taken through the south approach to the new Borough Hall Station is illustrated in Plate I. At this point the cast-iron tunnels of the Whitehall-Montague Street line begin curving into Willoughby Street, having passed diagonally under the Fulton Street structure. Above these tubes are the tracks of the Old Slip-Clark Street line. Through the Borough Hall Station these tracks are arranged one directly over the other but, as shown, after leaving the station they diverge in order to connect with the First Subway which is shown to the left of the upper track. The remaining two levels consist of the surface lines and the elevated railroad. A sixth route to Manhattan is possible by way of the Fourth Avenue subway over the Manhattan Bridge if a change of cars is made at DeKalb Avenue by transfer from the Whitehall-Montague Street line.
Plate I. Section showing 5 levels at Borough Hall. (Click image to enlarge.)
Underpinning Buildings. Because of the fact that the material underlying the surface throughout this section consisted of sand ranging from fine to coarse with occasional gravel beds and boulders, it was necessary to underpin or otherwise support several buildings, the highest of which rose to eleven stories.
The general scheme of underpinning was to provide either continuous walls under the building fronts or piers under the column footings. The walls or piers were carried to subgrade of the subway or tunnel excavation, or to 5 feet below a 1.5 to 1 slope line, depending on their distance from the excavation lines.
During the construction of new footings or retaining walls the building fronts were supported by needle beams spanning the pits. At the end of each needle screw-jacks were used to carry the load to spread footings and also to take up any settlement of the building. Where I beam grillages constituted the column footings the entire grillage was supported intact by means of hangers as shown on Plate II.
The excavation for the continuous wall supports was carried on by sinking a series of pits, each pit being concreted before work on the adjoining pit was begun. Horizontal open joint sheeting was used, mortised or cleated in the corners and also held by vertical timbers at the center of all four sides against which the cross bracing was wedged.
Ground Water Plane. In carrying out the underpinning work, some of which extended several feet below mean high water, no difficulty was experienced with ground water, owing to the fact that the ground water plane is considerably below mean high water in this section of Brooklyn. This fact was established before work was commenced by a series of observations covering a period of several years, as it was recognized that it was a matter of the greatest importance to avoid ground water, if possible. At the time the borings were made the casings were left in place and periodic readings were taken by means of a tape and float to establish the ground water level and any possible fluctuations. It was determined that the ground water level recedes almost uniformly below mean high water from the water front as far inland as the intersection of Willoughby Street and Flatbush Avenue Extension. It is probable that this condition is due to the large number of wells which have been sunk in this district by private owners in order to supply water to the various buildings.
Supporting Buildings. The supporting of buildings during the passing of the shields was resorted to twice. Under the contract it was intended to remove the Brooklyn Citizen Building, located at the intersection of Fulton, Adams and Willoughby Streets, as one tube of the Whitehall-Montague Street line was to pass directly under this property. A change in alignment was made, however, which brought the tube under only a small portion of the building and an agreement was made with the contractor for its support by maintaining the structure on screw-jacks placed in the walls. During the period of tunneling underneath, extreme care was exercised in operating both the shields and screw-jacks, an engineer being stationed constantly in the building with an engineer's level to determine the amount of settlement and to direct the work of maintenance. The work resulted most satisfactorily, no evidence of settlement being noticeable.
The second undertaking of a similar nature was carried out by the contractor on his own account because of the proximity of the south tube to the Continental Fire Insurance Company's building on the southwest corner of Court and Montague Streets. The precautions taken to safeguard this building, which is six stories in height and has a cast-iron front, were equally as successful as those at the Brooklyn Citizen Building.
In a number of cases buildings were supported where deep excavations, which extended under or near the buildings, were carried down from the street surface. Where the excavation extended under the buildings, the front walls and portions of the side walls were carried on the timbering of the cut. Where excavation was carried close to the buildings, the fronts were supported on screw-jacks and cantilever beams, the latter reacting against the rear of the building.
Because of the precautions which were taken, no material damage has resulted to any building along the entire line of the work.
Underpinning "L" Structure. The Fulton Street "L" structure extends over the entire portions of tunnel and cut and cover work from Clark to Willoughby Street and was required to be either supported or underpinned throughout its entire length. Over the tunnels, between Clark and Pierrepont Streets, the structure was supported in advance of and behind the shields in order to guard against excessive settlement resulting from a run or loss of ground at the face of the shield. The supports were placed under the column cap or at a panel point in the transverse truss depending on conditions. The bracing consisted of an "A" frame equipped with a screw in each leg which bore against a timber still resting on the street surface. As the shields advanced the frames were moved ahead and any settlement which had taken place was eliminated by raising the column through the driving of steel wedges between the column base and brick pier, the resulting space then being grouted.
Columns Underpinned. At Johnson Street and from Pierrepont to Willoughby Street the work required the permanent underpinning of thirty-nine columns in all. Some were outside the neat lines and were therefore placed on the underpinning piers as soon as these were constructed in advance of the general excavation. Others within the neat lines were supported on timber towers which were constructed in pits carried to subgrade before the general excavation of the cut was begun. These towers, ranging in height from 30 to 60 feet, remained in place until the subway structure was completed on both sides, when two new towers were built, one on each side of the gap, supported on the roof of the completed structure. The load was then transferred to these new towers by means of two 30" I-beams 45 feet long which carried an "A" frame, the latter supporting the column at its cap. The original tower was then removed and the structure under the column completed, a concrete pier being brought up from the subway roof to the column base without further changes.
Plate III illustrates the unusual underpinning which was required for two columns located on the west side of Fulton Street just south of Montague Street. In both cases the concrete pier between the tunnels was placed before the tube construction reached that point. It is interesting to note that the tunnel shields were driven past these piers on 275 and 300 foot radius curves with a theoretical clearance of only 7 inches in one case and 9 inches in the other. The underpinning was completed after the shields had passed by excavating two pits 18 feet long in which the concrete footings and side wall of the station structure were placed. A trench excavated between the pits in each case permitted the placing of the long girders and the piers under the columns, thereby assuring the safety of the "L" structure at these points while the deep excavation for the station was in progress.
Plate III. Section through Borough Hall Station and underpinning of "L" columns over tunnel and adjoining subway. (Click image to enlarge.)
Timbering Methods. In order to maintain traffic, the cut and cover method was required for all excavation carried down from the street surface. Two methods of timbering were employed for supporting the street and sides of cuts. In Willoughby Street at Flatbush Avenue Extension and at Lawrence Street, 3-inch tongue and groove sheeting was placed vertically with horizontal wales braced by bents 10 feet on centers. At Flatbush Avenue Extension, in order to allow room for the construction of the two shields used for driving the tunnels to Lawrence Street, and at Lawrence Street, to allow room for skidding them through the cut at that point for continuation of the tunnels west to Clinton Street, the lower portion of these cuts had to be unobstructed in whole or in part. Plate IV shows the arrangement of timbering at Lawrence Street for this purpose, with the shield in process of being skidded through.
Throughout the Fulton Street cut and in the shafts horizontal open joint 3 and 4-inch sheeting with vertical wales in bents 5 to 7.5 feet on centers were used. Steel plates and wedges were driven tight between the ends of each needle and the vertical wales. Diagonal braces were bolted in place on various levels of each bent to carry the live loads into the sides and longitudinal struts were placed between the needles of each bent in sufficient number to insure absolute rigidity of the entire structure. This is shown in Plate V.
Plate V. Timbering on south approach to Borough Hall Station. (Click image to enlarge.)
This type of timbering allows the sheeting to be brought down on the neat line, thus effecting a considerable saving in excavation for the contractor. Another advantage lies in the fact that it is possible at all times to find and fill any voids which may appear behind the sheeting by packing through the space which is left between the boards. Certain disadvantages are incurred through its use, consisting principally of the difficulty encountered in removing the vertical wales which encroach within the neat lines, it being necessary to back all side wall columns of the structure with concrete before the wales may be taken out. Where waterproofing is applied to the side walls, seven operations are necessary before the side wall arch is complete.
In the Montague Street shaft west of Court Street the same method of timbering was employed, but in addition each piece of sheeting was placed with a mortar backing which fills all voids, thereby holding the bank without settlement.
Connections Simplified. The work of making connections between the old subway work and the new has been greatly simplified and the City has been saved many thousands of dollars due to the fact that in the plans for the earlier work for lines now in operation provision was made for connections with lines to be constructed in the future. The judgment and foresight of those responsible for the plans for the earlier work are thus strikingly vindicated.
Under Fulton Street, in order to avoid a grade crossing, a depressed track was constructed at the time the First Subway was built, extending from Smith Street to a point outside the neat line of the upper structure at the east end of the present Borough Hall Station. The upper track connection is made at the grade of the present operating level and was provided for by installing the roof girders with top and bottom flange angles and cover plates projecting beyond a temporary wall so that the old structure could be widened by splicing a new section to the existing girders. After this was done and the roof load was carried on new side wall columns, the old wall and columns were removed. In this way difficult work over a heavily operated track was avoided.
At Flatbush Avenue Extension, on the line of the Fourth Avenue subway, two depressed spur tracks were provided. These begin to depress just north of the DeKalb Avenue Station and finally curve into Willoughby Street to a point outside the neat line of the upper four-track structure. As in the case of the low track on Fulton Street, it was only necessary to remove the concrete bulkheads to complete the junction after the new structures were built.