The South Ferry Subway Extension

 

The South Ferry Subway Extension

By Gerard Hillenbrand, P.E.

That was the subject of Met Section’s Technical Dinner meeting held on Thursday, November 15, 2007 at Con Edison Headquarters on 14th Street in Manhattan. A team of engineers presented a detailed computerized slide program illustrating the design and construction of the new South Ferry Terminal Subway Station. The engineering team was led by Sankar Chakraborty, P.E., Project design manager, and assisted by Pratap Jadav, P.E., Mechanical Systems Engineer, and J. Greg Sanchez, P.E., Principal Mechanical Engineer for the project. Mr. Sanchez should be well known to Met Section members, from his service during the 1990’s on our Section’s Executive Committee. Mr. Sanchez worked at Parsons-Brinkerhoff and was transferred to London, England to work on several subway expansion projects at that time. Attendees welcomed Mr. Sanchez back to Met Section activities with enthusiasm.

Mr. Chakraborty began the presentation by summarizing the features of the project which included unique techniques in excavation, foundation underpinning, site de-watering, instrumentation and site monitoring, surface traffic management, and archeological discovery and resource preservation, all conducted in a heavily used urban setting with minimum pedestrian and commercial disruption. All designs were subjected to extreme engineering episode criteria, such as earthquakes, fires, and explosions. This extension project is part of a $1.4 Billion downtown Manhattan transit development plan, which also includes the expanded interconnected stations under Fulton Street. The project is 100% federally funded and scheduled for completion and ready for public use in early 2009. The project was designed by the MTA Engineering Department in cooperation with outside consulting organizations in record time. For example, the structural engineering design (15% completed in house) was completed in 27 months as was the signaling equipment system. The finished station and signage was designed 100% in-house and completed in 24 months. Final landscape design for the disrupted Battery Park was completed with the cooperation of the Parks Department.

The existing downtown terminus of the number 1 subway line at South Ferry has serious deficiencies. The station is positioned on a sharp curved closed loop and can only handle five cars of ten car trains for exiting passengers. There is only one access stairway to the street and nearby ferry terminal which has been expanded recently to accommodate increased traffic to and from Staten Island. The new station is located east of the closed loop and parallel but below the existing Bowling Green station on the number 4 and 5 line to and from Brooklyn via the Jorallmon Street Tunnel. The new station is straight and has two dead-end tracks as well as three entrances and interconnections to the 4, 5, R and W Subway lines nearby. The overall design has the approval of the New York City and State Departments of Transportation, the Department of Environmental Protection, and the Federal Transportation Administration.

From the Bellmouth Switching connection to and from the Number 1 subway line, along the approach tunnel to the station, and into the new terminal station itself, the structural box construction technique has been employed. This consists of an enclosed reinforced concrete tunnel like configuration rather that the structural steel construction employed in traditional subways. The base of the structural box is imbedded into the bedrock existing below the surface of Manhattan. However, the use of this structural box technique did not avoid the usual problems inherent in urban construction, such as:

  • Physical constraints including utility lines for water, gas, electricity, steam, telephones fibre optic cables and sewers.
  • Excavation limitations were severe due to crowded, tight-built surroundings. Mechanical excavation using backhoe equipment was employed from the surface, and blasting used when bedrock was encountered. Explosions were carefully controlled, held during off-hours, and a safety net used to cover the entire blast area. The excavation walls were stabilized with secant pile structures.
  • Underpinning procedures were liberally employed such as buoys, test pits, bracing, and pile pipes filled with concrete imbedded into subsurface rock. Major underpinning was confined to one. 52 hour long weekend to minimize area disruption. Special care was used to minimize problems under switching crossovers and under the Number 4 and 5 subway lines.
  • Site conditions were continually monitored using instrumentation and computer detection systems. Detection sensors were located 200 feet around all nearby subway lines and located 100 feet around all nearby structures.
  • Water seepage into the construction site was continually monitored in a zone within a 400-foot radius of the site. Dewatering equipment was continuously available.
  • Surface traffic was maintained over the site using decking for vehicle lanes and pedestrian paths while providing access areas for the required modification and relocation of the many utility lines in the area.
  • Archeological discoveries mandate that contractors stop all work until experts (by law continually present on site) determine their significance. The items must be removed and stored, where officials of the state and city historic landmarks preservation commissions and the city Department of Parks and Recreation can conduct studies. If any material proves to be insignificant, it may be disposed of. On this project a number of old coins were discovered along with a buried 18th century battery fort wall, all of which will be preserved.
  • Surface restoration also must be complete. In cooperation with the N.Y. City Parks Department, Battery Park and surrounding areas have been completely restored along with creation of a new Peter Minuet Plaza.

Next, Mr. Jadav summarized the mechanical systems installed as part of this project. These included ventilation of the station and access tunnel, air cooling for the platform area, drainage plumbing for the track area, and fire suppression equipment for all areas. Several fan rooms provide the required ventilation power. For the tunnel two 200hp reversible axial fans provide fresh air during the intake mode and smoke elimination via the exhaust mode in the event of fire. Heat generated in the station is exhausted for four 125hp axial flow fans automatically controlled by heat detection devices. Station temperatures are continually maintained throughout the year between 67oF and 82oF by four 75-ton air coolers. No cooling towers are employed.

During normal operation two 200gpm pumps provide excess water elimination. For emergency flood conditions a 1500gpm pump is activated by water level rise detectors. Four additional stand-by pumps are also available for service during emergency periods. Fire suppression equipment is installed throughout the tunnel and station. The sprinkler system water has a special cleaning agent added to minimize destructive fire damage. Standpipe systems are located throughout the extension. Added fire suppression capacity is provided in all communications, fan and control rooms as well as automatic alarm systems to prevent unauthorized access to these sensitive installations.

Mr. Sanchez was the final speaker of the meeting and summarized his work in computer modeling fire safety conditions in subway stations and tunnels. Catastrophic explosions have never occurred in the N.Y. City subway system nor have serious fatal fires. However, the subject is not just theoretical; as last year’s terrorist attack in the London subway system showed where many deaths and casualties resulted along with widespread structural failure. The object of these studies is to develop techniques to minimize the explosive heat release causing deadly fires, to maximize egress provisions for stations and tunnels, and to maintain the structural integrity throughout the system. Also optimum design is required for the ventilation system which requires airflow capacity as high as 300,000cfm and sufficient airflow to provide 24 complete changes per hour. To achieve these goals subway tunnels should be designed with five air ducts, one duct along each wall, one duct between tracks and two ducts along the ceiling over the tracks. The conclusion of these studies is that the old assumption of zero lethal risks in current subway construction is a myth.

Maintaining structural integrity during these events is particularly important. The quantity of heat generated during an explosive fire is proportional to the square of the explosion temperature. Maximum temperatures may be as high as 2350oF in the resulting fire which may last as long as two hours in poorly ventilated tunnels. Structural steel looses its strength at 1200oF and design provisions should aim to reduce temperatures at the exposed steel surfaces to less than 1000oF. Test have shown such temperature reduction is achieved by covering exposed steel members with a two inch thick layer of concrete through which the damaging heat cannot penetrate. Under these circumstances, fire protection for steel with concrete becomes essential. The reinforced concrete structural box construction used in this extension project meets the criteria for steel safety. Train cars themselves also require added heat insulation particularly in passenger areas and the operator’s control cab. Non-inflammable upholstery and floor coverings are absolutely essential in minimizing the fire’s damaging effects.

The Met Section extends sincere thanks to MTA’s engineering team for delivering such a detailed technical report covering this South Ferry Subway extension. Attendees at this meeting qualified for one hour of professional development credit toward the continuing education requirements mandated several years ago by the N.Y. State Education Department.

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