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Challenges of No. 7 Subway Line Extension Project’s TBM Shaft Design Authors: Prabir K. Das, P.E., Vice President, Parsons Brinckerhoff Quade & Douglas, Inc. Eric C. Wang, P.E., Supervising Geotechnical Engineer, PBQD, Inc. C. C. Chang, Ph.D., P.E., Senior Engineering Manager, PBQD, Inc. ABSTRACT：As part of the New York City’s redevelopment of the far west side of the mid-town Manhattan, the No. 7 Subway Line Extension project will add 1.5 miles of twin bored tunnels to the existing subway line and two new subway stations. The underground construction has two major components: (1) Running Tunnel & 34th Street Station Cavern, and (2) Tenth Avenue Station & Times Square Connection. The running tunnel contract will have a TBM launching shaft (Shaft A), an Adit connecting Shaft A to the running tunnels, and TBM Assembly Chamber at the southern end of the project, i.e., near 25th Street at Eleventh Avenue, where tunnel excavation begins. This paper wil discuss challenges of the design and construction of Shaft A where it meets the Adit. PROJECT BACKGROUND The proposed alignment for No. 7 Subway Line Extension Project will extend the existing tunnel westward beneath 41st Street from the present Times Square Station and turn south under Eleventh Avenue and terminate at 25th Street. Under the current design, the extension will incorporate a new line station at Tenth Avenue and 41st Street and a two track terminal station at 34th Street and Eleventh Avenue with two 1800 feet under ground storage tracks to store six trains. (See Figure1a, Project Map and Figure 1b,Project Map Rendering.) The new subway tunnels and underground station structures will be primarily constructed within the Manhattan Schists and Pegmatitic rocks, and will be located in close proximity to a number of major existing structures, including subway, railroad and vehicular tunnels. A comprehensive and phased geotechnical investigation program has been conducted along the project alignment, including Site A to characterize the ground conditions for design and construction purposes. FIGURE 1A PROJECT MAP
FIGURE 1B PROJECT MAP RENDERING SITE GEOLOGY The project area is located in the western part of the Midtown Manhattan and is characterized by complex physiography and geological structures. The area forms a part of the Manhattan Prong of the New England Upland Province. The hard crystalline metamorphic rocks form ridges and softer rocks mark the valleys in the area. The intrusive igneous rocks, v.z., Pegmatities and Granite form the uplands in the form of partly domal structures. A valley striking West - North West to East - South East along 41st Street between Ninth and Eleventh Avenues has been observed along an apparent buried stream. Similar valleys have been observed in the southern area approximately between 27th and 30th Streets along the same orientation (See Viele Map, Figure 2). High water inflows are typically associated with these stream locations, indicative of the zones of weakness in the rock mass.
FIGURE 2 Viele Map EXISTING CONDITIONS AT SURFACE The proposed TBM access shaft at Site A, located east of Eleventh Avenue between 25th and 26th Streets, is currently occupied by a paved parking lot. The site is bordered on two sides by existing multi-story buildings. Specifically, a State & National Historic register-eligible six-story pile-supported gallery building is located immediately south of the site. Beyond the southeast corner, an existing one-story building is supported on spread footings. A two-story spread footing supported building is located to the east of the site. Street access to the site is available on the north side. SUBSURFACE CONDITIONS Borings drilled in the vicinity of the Shaft A encountered a general subsurface stratigraphy consisting of miscellaneous fill underlain by successive strata of alluvium, glacial deposits and foliated Manhattan Schist bedrock as described below: Overburden Borings were performed from street level through the pavement and concrete sidewalk surrounding the proposed shaft site. This surficial paving was underlain by granular fill consisting of brown to gray, silty sand with varying proportions of gravel, and occasional cobbles and boulders. This fill extends approximately 17-ft below existing ground surface. The above described manmade land is underlain by the alluvial soils including an upper organic stratum composed of dark gray to black organic silt & clay to silty clay with occasional shells. This stratum is found to possess low organic content and is normally to slightly over-consolidated, based on laboratory test data results. The thickness of this organic deposit, where encountered in nearby borings, is on the order of 8-ft to 10-ft. Organic deposits were absent in boring FD-103 located within the southeastern section of the site. Underlying the organic deposits, an approximate 13-ft thick very soft to soft Marine Clay was observed in the borings. A medium dense glacial sand layer was found to underlie this Marine Clay stratum. An approximate 20-ft thick layer of glacial till was encountered between the glacial sand and the bedrock. Till consists of gray to red-brown sand and gravel with varying proportions of silt to clayey silt and occasional cobbles and boulders. This glacial till stratum is typically medium dense to very dense, based on elevated driving resistance of the spoon sampler, or N-values from the Standard Penetration Test (SPT). Bedrock The borings encountered bedrock composed of foliated mica schist with occasional pegmatitic intrusions at approximately 75-ft below existing grade. Typical core recoveries and Rock Quality Designation (RQD) within the proposed shaft excavation ranged from 80% to 100% and 75% to 100%, respectively. Ground Water Conditions Based on ground water level measurements obtained from nearby observation wells after development, the high and low water levels at Site A are approximately 10 feet and 15 feet, respectively below the surface. The Site A shaft is located within Zone C defined on the Federal Emergency Management Agency (FEMA)’s Flood Insurance Rate Map (FIRM) as an area of minimal flooding. This elevation excludes the effort from storm surges and wave runup. Existing Structures Construction impacts of the extension project on existing structures along the proposed alignment must be minimized. Several multi-story buildings are located in close proximity to the proposed alignment. Specifically, multi-story buildings are located immediately south and east of Site A. These adjacent structures include a historic register eligible building, and both spread footing and pile supported buildings. Also an old large combined sewer and an old gas line are located close proximity to the shaft under Eleventh Avenue. DESIGN OF EXCAVATION--RIGID GROUND SUPPORT SYSTEM To mitigate ground movements and protect the adjacent structures, including the utilities,the project selected a rigid excavation support system. The geologic profile along the center line of the Adit is shown in Figure 3, for a low rock condition. The associated design cross-section across the proposed excavation support system is shown in Figure 4.Construction of the rigid shaft excavation support system will commence with an outer rigid excavation support cut-off wall keyed into the top of rock followed by rock excavation extending to the shaft invert elevation under an initial support including shotcrete and rock dowels. The “Secant Pile” shaft excavation support system was one alternative considered. Another feasible option included ground freezing with nominal steel liner plate support of the deep overburden and high groundwater level subsurface conditions. Upon completion of shaft excavation, the final concrete lining will be installed.
FIGURE 3 LONGITUDINAL (LOW ROCK CONDITION PROFILE)--LOOKING SOUTH of SITE A SHAFT &ADIT CONNECTION
FIGURE 4 CROSS-SECTION--LOOKING WEST of SITE A SHAFT & ADIT CONNECTION Design Philosophy and Assumptions The Shaft A will serve as the TBM launching and mucking shaft during construction. A multi story systems building will be constructed on the top of the shaft. During operation of the transit system the shaft will used for ventilation, egress and for system operation.Design considerations for the permanent shaft support system which optimize the performance of the outer rigid support wall and the inner shaft lining were as follows:The design would conform to the requirements of the New York City Transit Guideline DG452A, Structural Design Guidelines –Subway and Underground Structures, Issue No.1 dated December 2004. The outer “temporary” rigid support wall would be designed for associated ground and surcharge loading, while the inner “final” lining would carry only hydrostatic loading in the event of infiltration through the outer wall system. The shaft support system design considered both symmetric and asymmetric loading conditions. Construction Impact on Adjacent Structures Site A is underlain by relatively poor subsurface conditions, namely high groundwater table and soft compressible deposits consisting of organic silt and clay. Numerous STRUCTURES 2006 utilities of various kinds at varying depth are in the close proximity of this Shaft A, primarily beneath Eleventh Avenue. Thus, the site is sensitive to any construction activities generating vibrations and/or requiring construction dewatering. Typically, the vibration requirements for historic structures are more stringent than for ordinary structures. Therefore, a rigid excavation support for TBM launching shaft, or Shaft A has been specified in order to limit ground movement and dewatering requirements. The principle of ground-structure interaction has been thoroughly applied to assess the construction impact on both adjacent structures and ground. Furthermore, the confined site condition due to adjacent buildings requires selected construction equipment which can operate within limited lateral clearances. Shaft and Adit Intersection The longitudinal subsurface profile along the adit based on nearby borings indicates the potential for mixed face conditions in the crown of the adit and shaft connection. Localized ground support techniques in the form of drilled grout-filled piles creating a canopy over the crown have been evaluated to manage this condition should it arise during construction. Construction Monitoring The contracts along the No. 7 Subway Line Extension alignment must keep the construction impacts to a minimum. Impacts are likely to result from lowering of groundwater, deformation of rock mass around the excavation and vibrations related to drilling and blasting. As an integral part of construction a comprehensive instrumentation and monitoring program has been completed and included in contract documents. The monitoring program provides a mechanism for ensuring construction is performed in accordance with the contract documents and the impacts to surrounding structures are limited to meet local regulations/ ordinances, and consistent with design assumptions. CONCLUSIONS The site geotechnical subsurface conditions and the adjacent structures, will have significant impact on the design and construction of Shaft A and the associate underground construction. The construction will have impact on the surrounding ground and the adjacent structures. However, such impact should be kept to a minimum without comprising safety. The design approach for this project considers the ground/structure interaction in the analysis. Thus, it automatically presents an opportunity for assessing the impact onto the existing structures from the construction. With implementation of geotechnical instrumentation being an integral part of the construction the design of Shaft A and the related structures will be verified in the field. ACKNOWLEDGEMENT The authors gratefully acknowledge P. W. McGrade, P.E., Program Manager, S. K. Singh, P.E., Design Manager and M. Naik, Chief Engineer of MTA-NYCT for their permission and D. A. Donatelli, P.E., Senior Vice President, the Project Manager of PB Team for his cooperation and support to publish this paper. LIMITATIONS Any views or opinions presented in this paper are solely those of the authors and do not necessarily represent those of their companies, their employers or their subsidiaries. REFERENCES [1] Barton, N., Lien, R. and Lunde, J., (1974). Engineering Classification of Rock Masses for the Design of Tunnel Support, Rock Mechanics, 6: 4: pp. 189-236. [2] Baskerville, C.A., (1982). The foundation geology of New York City: Geological Society of America. Reviews in Engineering Geology. Vol. 5, pp. 95-117. [3] Das, P.K., et al. (2004). New York City No. 7 Subway Line Extension Project Mitigating Construction Impact at Shaft A. Proceedings of Structural Engineering Institute of the ASCE Structures Congress and Exposition. [4] Dunnicliff, J. (1988). Geotechnical Instrumentation for Monitoring Field Performance. [5] Hanna,T. H. (1985). Field Instrumentation in Geotechnical Engineering (Series on Rock and Soil Mechanics). [6] New York City Transit Guideline DG452A, Structural Design Guidelines – Subway and Underground Structures, Issue No. 1, December 2004. [7] Shah, A.N., et al. Geological Hazards in the Consideration of Design and Construction Activities of the New York City Area, Environmental & Engineering Geoscience, Vol. IV, No. 4, Winter1998, pp. 524-533. [8] Stumm, F., A. Chu, and J. Monti, Jr. Delineation of Faults, Fractures, Foliations, and Ground-Water-Flow Zones in Fractured-Rock on the Southern Part of Manhattan, New York, Through Use of Advanced Borehole-Geophysical Techniques, U.S. Geological Survey, Open-File Report 2004-1232, 2004. [9] Viele, E. L. (1874). Topographic Atlas of the City of New York. [10] Wang, E.C., et al. (2004). Influence of Geologic Conditions on Excavation Methodology, Proceedings of North American Tunneling Conference, pp. 185-192. |