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Extending the Subway


Seth Pollack* who was a consulting geotechnical engineer on the project, won the 2008 Glossop Award for his work designing caverns in an urban setting the 7 Line Subway Extension Project, New York

Geoscientist 19.4 April 2009

Much has been written lately on the design of mined caverns in New York City. A number of metro projects are currently in planning and design stages, aimed to increase capacity and coverage of an ageing transport system that has seen little change in recent decades. The forerunner of these numerous projects is the 7 Line Subway Extension. Some major engineering geology challenges went into the design, and now construction of this major underground infrastructure excavation within an urban setting.

The 7 Line Subway Extension project represents the first large span excavation to be mined under Manhattan in nearly 50 years. The city hopes that the extension will act as a catalyst to revitalise the west side of Manhattan, currently dominated by industrial warehouses and the Hudson Rail Yard. The 2.4 km addition extends the 7 Line from its current terminus at 42nd St Port Authority (at 8th Ave.) west to 11th Ave., and then south down to 26th St.


Arup was engaged to design the initial ground support for all mined portions of the project. This included assembly chambers for the dual tunnel boring machine, five cross-passages between the running tunnels, and a 350m long, 21m span station cavern. As the contract was let by the city as a “design-build” arrangement, Arup worked directly for the contractor and was able to follow the project from concept to construction in just a single year.

This is not the typical way in which most major infrastructure projects are carried out in the United States. It is much more common for the designer to hand over the construction drawings to the city or municipality, which will then appoint a contractor through competitive bidding. Following this, a separate construction management team is appointed which deals with inspection and field issues. We hope that the 7 Line project can set a standard of precedent for using design-build contracts in similar situations, as it can be very advantageous and cost-effective to keep the designer involved through construction.

Main cavern

The centrepiece of the project is the main cavern station located at 34th St. The geometry of the station includes interlocking, or “crossover” caverns on either end of the station. These have the same span as the main station, but are lower (as they are not intended for public use). There are several stub tunnels, which will become future entrances, and two shafts for ventilation.

The construction sequence for the main cavern has been divided into a top heading and bench arrangement. The top heading itself is further divided into multiple drifts, which are blasted using emulsion explosives. The picture shows the top heading when broken out to the full design span of 21m.

What lies above

There are several design and construction challenges associated with building a large cavern in an urban environment. The first is that they are generally situated near to the surface, so as to allow access from street level. The result is that the rock cover above the cavern is lower, and in the case of the 7 Line Project, less than one diameter. This means greater potential for ground settlements above the excavation, and as we were working under real estate that has one of the highest per-square-metre costs in the world, this could have had major legal implications, should the construction have resulted in any building damage.

To achieve a successful design that addresses all the challenges listed above, the place to start is the ground investigation. For the 7 Line Extension, over 200 borings were drilled, and all the usual suite of lab tests carried out on cores. We also conducted a large number of acoustic televiewer scans, packer tests, thin sections, and hydraulic fracturing.

Manhattan’s geology is very different from London’s, where tunneling is likely to involve poorly consolidated clays, or complexes of alluvial gravel, sand and clay, often with overpressure. Two different rock types occur along the 7 Line Project alignment: a mica schist, commonly referred to as “Manhattan Schist”, (below) and a granitic pegmatite. This schist comes in two different forms; the first has little jointing and foliation fabric dominates the rock-mass behaviour, while the second displays multiple mappable joint sets. Consequently the rock takes on a more “blocky” aspect, where fractures that have formed parallel to the foliation are dominant, but the rock mass behaviour is influenced more by the strength of the joints than of the rock itself. The granite typically has three joints sets, the most common being oriented vertically, and can range from fine grained up to coarse (pegmatite). These rock types are roughly equally distributed along the cavern alignment.

Manhattan Schist
One major challenge facing the engineering geologist is how to make the step from conceptual geologic to numerical model. Numerical modelling is commonly used to verify the support design that is produced by using one or more empirical classification systems (Q, RMR). So often, the role of the geologist ends after producing the interpretation leaving the engineer to create complex models without fully understanding geological constraints. Finding the optimal design solution requires a combination of two things - a basic understanding of numerical modelling (from the engineering geology side) and an understanding of ground behaviour (on the engineering side).

The jump to modelling cannot be made straight from concept stage – a common mistake. Matching the anticipated ground behaviour to the software package best suited to capturing the resulting failure mode and ground movements takes a series of steps. On the 7 Line project we adopted a methodology that would ensure this, the first step being development of a basic geologic section across the area of interest. For the station caver, five sections were produced, based on rock mass classification. The section contains information on rock type, joint condition and orientation, and any major structural features as interpreted from local borings and acoustic televiewer data.

Widespread use of the acoustic televiewer instrument has given engineering geologists a wealth of critical information regarding joint-set orientation and spacing. This is especially useful in urban environments where direct measurements on outcrops are not possible, leaving the televiewer as the only means by which this valuable structural data can be obtained. From the raw data a stereonet can be produced to give joint-set orientation. Another useful feature in most stereonet programs is that poles found within a certain set window are called out and can be used (with joint-depth) to give typical joint spacing for specific sets. This information is then applied to the geologic section.

Drill and blast operation being carried out with an Atlas Copco Rocket Boomer, one of three jumbos on the job.

Continuum v. discontinuum

Numerical modeling can be a great tool for the tunnel engineer, if used properly and in the right situations. Different types of program (finite element, discrete element) are only appropriate for certain ground types. The challenge is to match the ground type to the modelling software that best captures the failure mechanism and ground-support interaction. Ground behaviour can be simplified down to either continuum or discontinuum. In a continuum, the rock mass will behave isotropically and the strength of the material will be the controlling factor on stability. Such behaviour can be assumed, if the rock mass is generally intact with few joints, or (on the opposite side of the spectrum) if the rock mass is so jointed and broken up that the influence of the joints loses significance, and strength is again isotropic (imagine a gravel in the extreme case). Somewhere in between these end members lies the realm of the discontinuum. The rock mass behaviour in this case is structurally controlled, and the shear strength of joints becomes the limiting factor in the stability of the cavern.

From the discussion above, it will be apparent that scale plays an important role in this – that is to say, the block size, compared to the span of the excavation. For this reason one must have a method of estimating block size - preferably from a borehole, as outcrops are rare in the city. Palmstrom has suggested a methodology for estimating the joint volume from the observed fracture frequency and orientation in a rock core. Joint volume can then be related empirically to a block diameter based on data collected by Palmstrom.

The concept of “continuity factor” has also been introduced1 to provide guidelines on rock mass behaviour when an estimate of block diameter can be made and excavation span are known. This is essentially a measure of how many blocks will form in the roof of an excavation. In the table below, this number has been associated with the most appropriate type of numerical model for the situation.

Nothing in rock mechanics is ever just black and white, so there are areas of overlap; but using the above as a guide can give good justification for selecting different software programs to be used on the same project.

The next challenge faced by the engineering geologist is providing values for the many input parameters required to produce a numerical model. Few can be obtained straight from lab tests, and it is often necessary to use empirical correlations and previously published values to supplement expensive lab testing (and sometimes poor results).

For a typical model, over 20 inputs for the rock mass may be required. Several of these main input parameters also require values for two or three secondary parameters. For example, the value of “rock-mass cohesion” requires Hoek-Brown mi, GSI, and an estimate of the minimum principal stress. It can be seen how variation within a model can quickly escalate if not controlled. One method adopted on the 7 Line Project was to produce Design Notes for all geotechnical parameters required in the various models. Each Note gave specific guidance on determination of parameter values and ranges if applicable. By standardising the way in which parameters are determined, model accuracy is increased as is the ability to compare results between models.

All of the preceding data were then summarised on a one-page spreadsheet so that the flow of pertinent modelling information, between engineering geologist and modeler, was as smooth as possible (without sacrificing time or accuracy).

Ground truth

Design does not end when a set of drawings is handed over to the client. It is critical to verify the design continually by geological mapping, and taking measurements during construction. In providing on-site engineer services to the contractor, we were in the position to do just this, carrying out rock-mass classification after each blast, and comparing to the design values. Many hard-to-quantify parameters can be assessed during construction, including: joint persistence, large scale joint waviness, and joint roughness. Such parameters have a huge impact on support design, and in the models; but they remain very difficult (if not impossible) to define accurately during the design stage from a 50mm core-sample. Roughness is assessed in situ by a profile comb, then compared to published profiles and JRC values to back-calculate the Jr value used in the Q system during empirical and numerical design.

Finally, being involved during construction has allowed us to build up a significant statistical database on such parameters such as joint roughness and large scale waviness, which will prove invaluable during the design and modeling of future excavations under Manhattan.


  • Stille, H and Palmstrom, A “Ground behaviour and rock mass composition in underground excavations” Tunnelling and Underground Space Technology 23 (2008) 46-64.

* Arup, New York 2008 Glossop Award [email protected]. Seth Pollak is a Tunnel Engineer for Arup in New York. He is currently assigned as a Tunnel Shift Engineer on site for the construction phase of the project. This feature was based on the presentation he gave to the Engineering Group at the 2008 Glossop Lecture, held at Imperial College on 6 November 2008.