Tunnel Tour questions:

Is such a tunnel technically feasible?

Such deep-bore tunnels have been completed or are under construction in several European cities. Urban automobile tunnels have recently been built in:

• Lyons, France – the East-West Tunnel
• Marseilles, France – the Pardo-Carenage Tunnel
• Baden-Baden, Germany – the Michaelstunnel
• Brussels, Belgium

and are currently planned, under construction, or already in use in several other cities in affluent, developed countries elsewhere. The 6-mile long Versailles Tunnel in Paris is a very good example, since it is a brand new, cylindrical, deep-bore, autos-only tunnel. The Automobile Association Limited of England describes the Versailles Tunnel as follows:

• Costs have been radically reduced by building the tunnel for light vehicles only. This means dual 3-lane carriageways can fit in a single bored tunnel…. The flexible boring machine used can cut through terrain from wet, sandy soil, to hard-packed extremely resilient lime. Unlike Boston's "Big Dig," some 90% of construction is underground at a depth making surface noise and vibration imperceptible. Lighting and other road features are designed for safety and comfort to woo motorists from the alternative…routes available.

The first section of the Versailles Tunnel is scheduled for completion this year. It's cross-sectional shape is typically a 32-foot diameter cylinder. A relatively inexpensive deep-bore automobile tunnel would need to have that cross-sectional shape of a cylinder to minimize construction costs. It is important to emphasize that the tunnel system shown in the Tunnel Tour was definitely diagrammatic. It showed inbound and outbound lanes side-by-side, rather than stacked over and one another within a cylindrical tunnel, as they would actually be. The notion was to make the drawing have diagrammatic clarity, so the logical functioning of a tunnel system could be more easily understood.

The Freshgate Tunnel would probably be a similar design to the Versailles Tunnel, and would also be only for light vehicles (cars, taxis, vans), as are all the existing Charles River parkways. An American version of such a tunnel would likely be somewhat larger to comfortably accommodate SUVs and small vans, and since I believe American highway lanes may tend to be wider than European ones. Of course, since the speed limit in this tunnel would probably at most be the current speed limit of the existing surface parkways (maximum 40 MPH) the tunnel lanes could perhaps be a little narrower than interstate lanes. Here is a cross-section of what the Freshgate Tunnel might typically look like.

The Freshgate Tunnel would probably need to have a larger diameter in some sections where six lanes of traffic ("dual 3-lane carriageways") would be insufficient for rush hour. For instance, where the new tunnel would handle the traffic load of both Soldiers Field Road and Memorial Drive (from roughly just east of the Anderson Bridge to just west of the Western Avenue Bridge), it would need more lanes than six. But six lanes would probably be sufficient for about three quarters of the tunnel's length.

Rather than build a larger diameter cylinder in the sections where six lanes would be insufficient for peak traffic volume, it may be less expensive to simply build two parallel tunnels of the same diameter as all the rest of the tunnel's length along this part of the route. That way the identically sized tunnel-drilling/tunnel-making machinery would be used throughout the entire construction enterprise, instead of needing an entire second set of equipment for a differently sized diameter section. Here is the business end of a tunnel boring machine.

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