Mass Transit - Cut and Cover Tunnels
Class 5 - Session 2

I have some videos showing the construction activities below the street level, but before we view those, you need some basic orientation. The site here includes an existing subway facility, and this is a cross-section, a very schematic cross section through the existing subway facility. At the low level, there are two express tracks. At the upper level, there are two local tracks, and it is also a station. There are platforms serving the local tracks. The purpose of this project is to construct a pedestrian walkway, or mezzanine, or passageway, underneath this existing structure. That passageway will provide access from the existing transit facilities to the new Path terminal at the World Trade Center site.

Of course, to get underneath that existing structure, there is some underpinning required. The existing structure will be underpinned using drilled in piles sometimes referred to as mini piles. There are two piles which are outside the train area. And of course, that's very desirable. You want to try to install your underpinning facilities without any Interruption to the train traffic. However, it is necessary and was necessary to completely close the station, so that there is no pedestrian activity, no passenger activity.

However, there are the low-level express trains and upper level local trains in full service at all times. These external mini-piles can be installed without interference to the train movement. They can either be installed from the street level, which is very often an attractive possibility. However, up in the street, this is a very highly trafficked street so that you need to accommodate the street traffic. You may have found a solution that avoids interference to the train traffic, but you must also accommodate the surface traffic.

Mini-piles installed from the street might be done at night, when lanes can be closed and an area can be dedicated to the construction equipment. It may also be possible to install the drilling equipment in the platform area. That is attractive in that it does not impact either train traffic or street traffic, but you're now working under a low headroom conditions. You need special drilling equipment, and the mini-piles are geared up to work under those conditions. You can see, it's a balancing act between a finding the most cost effective devices to install, but at the same time, minimizing impact to street traffic and train traffic.

There is, however, one support pile, which runs down the center of the structure. This cannot be installed without interfering with the trains. The way that is generally done is during a weekend shutdown. The transit facility is taken out of service from late Friday evening to early Monday morning.

Of course, you must work very efficiently. You must design that effort so that the train traffic can be reliably replaced in service on Monday morning, without impacting that Monday morning rush hour. There's usually a learning curve involved in doing that. You need to develop failsafe procedures where you can halt the work, perhaps, prematurely, so that you can be certain of restoring train movements on Monday morning.

You need to weigh the options of working inside the tunnel, which would avoid interference for surface traffic. During the weekend shut down, train traffic may be slow, but the surface traffic in this area very busy. It may be the best solution is to work inside the tunnel during a long shutdown of the train traffic.

When the mini piles have been installed, the next step is to install the support beams under the track system. To do this is not a simple job, because the track has to be freed from the invert structure, which is usually concrete, and the beams have to be inserted and the track re-supported all during a short window when the train is taken out of service. Again, there are details which facilitate this kind of work. It's beyond this lecture to get into those details. But I do want you to understand, at least in a broad brush way, this is the approach to underpinning an existing rail facility, and you'll be able to see elements of it in the videos.