Construction of NYC's 63rd St. Subway Line

Figure 1: Project Alignment and Contract Boundaries

The second segment of New York City’s 63rd Street Subway Line will provide a two-track connection between the Line’s current terminus in Queens and the existing local and express tracks of the Queens Boulevard Line. The connecting track sections will cross under the existing Queens Boulevard Line tracks prior to merging with them. Excavation will be done by tunneling so that service on the Queens Boulevard line will be maintained during construction. The project alignment and boundaries of the project’s three construction contracts are illustrated on Figure 1.

The high groundwater table at the site makes dewatering necessary, but conventional dewatering could not be used because of the potential impact of drawdowns on nearby chemical contaminant plumes and consolidation of peat deposits below nearby structures. Instead of conventional dewatering, slurry cutoff walls will be used to provide a relatively watertight enclosure that will limit groundwater drawdowns outside of the wall. Necessary dewatering will take place inside the cutoff walls.

More than 914 m (3,000 feet) of slurry walls are proposed for the two most northern contracts (Contracts 2 and 3) and nearly 230 m (750 feet) of jet grouted walls where access problems prevent the slurry wall construction technique from being used.

Project Geology and Hydrogeologic Conditions

Bedrock consists mainly of slightly weathered, medium to very strong, closely to widely fractured gneisses and schists with pegmatite sills and dikes scattered throughout the mass. Zones of lesser quality rock occur occasionally due to variations in weathering, fracturing patterns, rock composition, and structural features such as foliation and shear zones.
The bedrock is overlain by mixed glacial deposits, glacial till, outwash and other reworked till deposits. Stratification is complex, and significant variations in the thickness and location of the different types of deposits are common. The glacial deposits are overlain by a layer of miscellaneous fill material. There are also local deposits of peat and organic silt that were formed by post-glacial streams and creeks in marshy areas.

Groundwater flows south to southeast, with moderate gradients that range from 4.6 m to 12.2 m per kilometer (15 feet to 40 feet per mile). The outwash soils are estimated to have medium permeability (5x10-2 cm/sec) due to their poor grading and low void ratio. The majority of the soils classified as SM or GM within the glacial till deposits are estimated to have a low to very low permeability (5x10-4 to 5x10-7 cm/sec.) Rock mass permeability varies considerably (<10-7 cm/sec to 10-3 cm/sec) depending on joint spacing, orientation and degree of infilling.

Additional hydrogeologic characterization of the site, which was obtained from two pump tests and a jet grouting test shaft, is as follows:

• The soil profile does not act as a uniform aquifer and drawdown is dependent on the vertical flow component.
• The zone of influence of drawdown is limited and less than 152 m (500 feet).
• The shallow groundwater table is perched on the glacial till, suggesting that the till is semi-pervious, thereby allowing some limited vertical leakage.

Keying the Wall to Control Drawdown

Because of the ground conditions, the cutoff walls will minimize but not completely eliminate groundwater drawdowns. Slurry wall toe elevations were optimized to provide an effective cutoff for sufficient drawdown control while minimizing the amount of difficult excavation. Keying the toes of the wall sections into low permeability soils at higher elevations rather than into rock at depth was desirable from a cost and constructibility point of view, provided that the objective of limiting drawdown was met. Toeing the walls into these soils, however, was contingent upon the soils having sufficient thickness and continuity, and a significant percentage of fines. We found the conditions varied between the southern two-thirds of the connecting track segment and the northern third.

Southern portion. The internal dewatering requirements were the most demanding in this area. Wall toe sections were designed to be keyed into bedrock because sufficient drawdown control would not be provided if the walls were keyed into the less permeable soils. Some of the rock in this area has appreciable fracture permeability, however, so drawdowns outside the cutoff walls would be excessive and some type of additional cutoff treatment was therefore necessary.

Examination of the rock core and other related data indicated that it would not always be possible or practical to continue the wall through the permeable rock layers to the impermeable rock layers. Instead, some sections will require toes that terminate above the permeable rock layers. This condition required cutoff treatment in the rock by a grout curtain, similar to that used by dams founded on rock. Our assessment of the rock cores and field geohydrological tests indicated that the upper 4.6 m (15 feet) of rock will require grouting.

We established the following criteria for the construction contractor to follow in order to evaluate the cutoff wall
toe elevations in rock during construction and make any necessary adjustments:

• Drill exploration holes beneath every panel, with a maximum exploration hole spacing of 3.0 m (10 feet).
• Conduct packer pressure tests in every exploration hole (because the quality and permeability of the rock is not consistent). When the packer test permeability exceeds the allowable limit, the rock is pressure grouted.

Northern portion. Here, the glacial till stratum thickens considerably. There are a number of boulders, the rock becomes deeper and the excavation dewatering requirements are less demanding. Keying the walls into rock in this area would:

• Extend the time required for excavation
• Impact the overall construction schedule
• Prolong traffic control requirements
• Increase the risk of problems with the wall construction
• Provide diminishing returns at significantly greater costs.

The upper layers of the glacial till are known to contain lenses of clean permeable sands. The wall toes were extended below these lenses. All wall toe elevations are being adjusted in the field, however, based on soil characteristics immediately below the planned wall toe elevations. Soil drilling, sampling and laboratory testing are specified below every panel. When the soil immediately below a panel is “clean” or has a low percentage of fines based on grain size distribution, the wall toe is being extended deeper.

Drawdown Outside the Slurry Walls

The cutoff walls were expected to result in small drawdowns outside the walls due to underseepage since the walls could not be toed into an ideal, continuous impermeable material. PB began the cutoff wall design, with respect to groundwater control, by establishing the pattern and magnitude of drawdowns based on estimates from case histories, analytical studies and observations made during the pump tests and the jet ground test shaft.

Settlement by the estimated drawdowns and their patterns could be computed at different locations from the cutoff walls. For areas near contamination plumes, we also evaluated contaminant transport and dispersion from induced changes in groundwater flow directions and gradients. Based on a combined assessment of the potential settlement and contaminant transport, we computed maximum allowable drawdowns at various locations around the site, then established the required cutoff wall thickness, permeability and toe depth.

We used 2-dimensional finite element analyses of flow in vertical cross-sections using the dual formulation for hydraulic potential and stream function for the analytic studies. Fixed groundwater heads on either side of the cutoff wall were applied at appropriate locations, soil and rock permeability assigned and the resulting drawdowns computed. The wall geometry was altered until the computed drawdowns approximated the target maximum allowable. These analyses included the effects of rock mass permeability and the grout curtain below the wall toe.

The construction contractor is required to maintain groundwater elevations outside the walls. Compliance is checked using strategically placed piezometers outside the walls with specified limits on drawdowns at each monitoring location. The allowable drawdowns are small, generally 0.6 m (2 feet) maximum.

Internal dewatering is essential for constructibility of the required excavations, and maintaining groundwater levels outside the cutoff walls is extremely important. Therefore, the contractor is required to conduct deep well pump testing inside the enclosed walls at critical areas before excavating and to demonstrate that internal piezometric levels can be achieved and that they do not induce excessive drawdowns or wall leakage. The contractor is also required to remediate the cutoff walls when the observed external drawdowns exceed the allowable limits.

Construction is Under Way

All three construction contracts are underway with cutoff wall construction. The most difficult problem is, as predicted, excavating the walls through or into glacial till, a dense sand with boulders. Rock coring beneath every panel has confirmed the expected variations in rock quality and permeability. Production grouting of the rock beneath the wall toes is commencing in sections where the walls terminate at a rock surface.