Red Line Baltimore underground station: an underground station provides ample space for passenger amenities.

Maybe it was the gigantic tunnel boring machine that over-ate itself with fine granulate until its boring head had to be rescued in a big open dig in the middle of Seattle. May be it was the infamous Big Dig in Boston with its perennial delays and cost overruns. Or is it that almost any topic elicits antagonistic views split along party lines.

Whatever the cause, many Americans seem to buy the argument that tunnels are boondoggles. Being against transit tunnels is sold as prudent reasoning even where hardly any alternative exists, like the case of the rail tunnel under the Hudson River. In Baltimore, a proposed light rail tunnel was declared the flaw that sank the cities first and only major transit project in nearly a quarter century.

Meanwhile the Swiss successfully completed a 34-mile long rail tunnel under the Alps in which trains will begin to run next year and the Japanese build the world's first long distance Maglev high speed rail, much of it in tunnel. Why is an American Transit tunnel a "fatal flaw"?

Granted, tunnel construction is a pretty specialized field and much of it occurs out of sight, which is the whole point, of course. So maybe a closer look at how such a tunnel is actually built is in order. Why are tunnels being built in the first place, how exactly does it work to dig your way deep down under city streets, what are the risks and are they worth their high cost?

Anybody who has used the subway in New York, DC, London, Paris, or Baltimore's very own Metro Subway will probably have done so without asking all those questions. In those "real subways" tunnels and underground stations, escalators and whole systems operating below the street are standard affair and taken for granted.

Should the train ever stop between stations passengers tend to get nervous because trips are expected to be expedient, efficient and include stops only in stations. Few however, no matter how nervous, will dwell on tunnel construction: was it built like a big trench and then covered up again, or was it dug mole-like under the streets and buildings?

Even fewer will ponder the question if people before the construction asked themselves do we need a tunnel at all and can we afford it? Instead, subway users enjoy that the train can go as fast as the tracks or curves allow, not as slow as buses on the surface streets.



Here, in this article, though, let's ask all those questions! If they are dug as if by moles, where are the molehills? Are tunnels waterproof? How safe are structures above those tunnels? Who exactly digs them, surely not miners in hard hats? And with rising sealevels, will many subways share the fate of New Yorks subway when water filled tunnels in certain areas when Hurricane Sandy struck?

Train tunnels are old news: Welsh tunnel WV

Train tunnels are old news, especially in the old eastern seaboard cities like Baltimore. (Passenger railroad was invented there!)

Baltimore boasts one of the nation's oldest train tunnels which was finished in 1873 and still carries Amtrak and MARC trains under Bolton Hill and Sandtown-Winchester. The 1895 Howard Street Tunnel, originally a passenger tunnel, is still in use today for freight trains.

Only the much more recent Metro tunnels constructed in the late 1980s used a version of what is is common practice today, mined construction with large tunnel boring machines. open excavation for the stations.

Technology for the tunnel boring machines (TBMs) has greatly advanced in the last 30 years making new tunnels safer and more efficient to build, namely by being able to better control the movement of soil and groundwater during the mining process. But stations are typically still built "cut and cover" style, i.e. as a large open excavation like for a building with a concrete deck as cover on which the street or buildings can thenbe constructed.

Trains running through the streets of downtown were considered not particularly practical as early as 1895 when the Howard Street tunnel was built instead. Similarly today's transit planners sometimes conclude that surface trains would encounter too much interference from pedestrians, bicyclists, trucks and cars, especially in the city center or the narrow streets of historic districts such as Baltimore's Fell's Point. Such interference would make transit agonizingly slow and their progress unpredictable.

Because this conclusion isn't new or specific to just one city, there are plenty of examples of underground light rail in the U.S., including legacy streetcar/light rail systems running in tunnels like in Philadelphia or Boston. New, modern low floor trains in tunnels under downtown in San Francisco, Seattle , and Pittsburgh, or under a university campus in San Diego

Unlike Metro which requires the infamous "third rail" for power, light rail gets electric power from overhead wires which are safely out of reach. Light rail picks up the electric power through a roof-mounted "pantograph," while Metro picks up power through a "contact shoe" on the side of the train that connects to the third rail.

This allows light rail to come up to run on the surface where traffic is less dense. It can cross streets and run across plazas just like streetcars without any fear of electrocution. Metro, by contrast, has to be above grade on bridges once it leaves the tunnel except for runs where it can be fenced in from both sides like the median of expressways.



Light rail trains are usually shorter than Metro, and modern trains have low floors that are level with a sidewalk. Metro requires platforms that sit high above the track. Modern light rail cars are also rarely wider than buses so they can navigate better on streets. These differences make it nearly impossible to run light rail and heavy rail metro trains in one and the same tunnel or station but they do represent some savings in favor light rail tunnels.

To make a "mixed-use" LRT-metro tunnel station work one would need hybrid solutions with two sets of platforms per station, two sets of rails to allow the narrower trains to reach the same platform edge, and two power supply systems, expensive, impractical and also inconvenient because of the disruption that these measures would bring to whatever service would already occupy a tunnel. In a tunnel, light rail can run free of interference and traffic lights just like a real subway.

Underground light rail station in Strasbourg, France

(photo: ArchPlan)

Back to how tunnels get built: Even though the tunnel is mostly located under streets, open construction would be very disruptive for traffic, businesses and residents. one would need hybrid solutions with two sets of platforms per station, two sets of rails to allow the narrower trains to reach the same platform edge, and two power supply systems, expensive, impractical and also inconvenient because of the disruption that these measures would bring to whatever service would already occupy a tunnel. In a tunnel, light rail can run free of interference and traffic lights just like a real subway.Back to how tunnels get built: Even though the tunnel is mostly located under streets, open construction would be very disruptive for traffic, businesses and residents.

Modern TBMs, by contrast, can dig their way through any type of soil or rock with rotating blades mounted on a gigantic shield at the front of almost 300-feet of equipment that look like a freight train.

Once this assembly line "train" is through, it leaves an almost complete tunnel behind, really quite amazing.

Still, construction cannot be entirely removed from sight or avoid surface disturbance altogether especially if one stays away from the new gigantic 60-ft diameter boring machines like Bertha which got stuck under Seattle. With two more standard 22-foot bores about 12 to 18 feet apart and 40 to 150 feet below the street, one bore for each track, a station is difficult to do, although not impossible.

London visitors may recall tight stations that have the shape of a tube (hence London's underground is called "the Tube") with the two platforms narrow and connected via small pedestrian cross tunnels.



Open excavation for a station box in Manhattan with

firs cover being lowered.

If one wants larger, more spacious and easy orientation modern stations, one needs to build them by digging from the surface. Even though limited to the length of a station, this creates a pretty big disturbance during construction and requires careful "maintenance of traffic' plans, especially where a station is built under a major artery. If one wants larger, more spacious and easy orientation modern stations, one needs to build them by digging from the surface. Even though limited to the length of a station, this creates a pretty big disturbance during construction and requires careful "maintenance of traffic' plans, especially where a station is built under a major artery.



Utilities are placed below the road and then excavation

can continue under the traffic above.

Once an underground line is in operation, however, pedestrians, bicyclists and drivers above as well as transit riders below will gladly reap the benefits of unimpeded travel for decades to come. Typically, slurry walls will be built through drilling and fill along the edge of the station box and then each half of the road excavated deep enough to be able to work under a cover that allows traffic back on. Imagine this being done for each half of the street so some lanes can always remain open.Once an underground line is in operation, however, pedestrians, bicyclists and drivers above as well as transit riders below will gladly reap the benefits of unimpeded travel for decades to come.

The blades of the rotating boring shield can cut through rock, but where they encounter silt or urban fill, the surrounding soils need to be stabilized during the boring operation. A pressurized liquid mix is applied to the loose soil right at the shield.



Image of cut and cover station construction

in street with "slurry wall" When the 300-foot-long machine completes a tunnel bore it leaves behind stabilized soil and watertight precast concrete panels that provide an essentially finished tunnel only lacking the rails, the power, the communication and the ventilation. This dig-stabilize-line approach avoids groundwater pumping or lowering of water levels, two operations that often lead to sinkholes and structural damage above due to the potential for movement of the soil around the work area.

Just like moles, the tunnel machine pushes the dirt – approximately 80 truckloads per day (from two machines) back toward the tunnel entrance, if each progresses at the typical rate of about 40 feet per day.

The dirt is really a ground-up slurry of "spoils" that can be pumped, often to the same point where the boring machine was initially assembled and "launched", typically one of the tunnel entrances or "portals". From there, the rock will be processed and trucked away. (For lines that never surface, TBMs are launched from open station boxes).

To account for rising sea-levels, all possible entry points for floodwater must be located above recognized flood elevations as well as mandated "free boards" that account for wave action or for sea level rise.



One of the costly requirements for tunnels and underground stations is ventilation. Long gone are the days when the trains pushing the air through tight tubes was considered sufficient ventilation. The deciding factor for sizing ventilation is smoke evacuation in case of a fire.





So what about the cost and the question of whether or not a light rail tunnel makes sense for transit in second tier city? Light rail cost can go from as low as $ 42 million per mile (surface) to as high as $400 million per mile if tunnel is involved (Toronto). The Baltimore Red Line had a total cost of $ 2.9 billion for 14.1 miles or slightly over 200 million per mile. A lot of money, for sure. How to determine if its too much?

Two tunnel boring heads as seen from an open "station box"

(Seattle light rail)

Until recently the Federal Transit Administration had a complicated formula, the cost effectiveness ratio, that answered the cost benefit question in a decisive manner. In the numerator sits the cost, in the denominator the benefit, expressed as the product of ridership and travel time savings. If the ratio came out above a certain value, a project was doomed and not even further considered for federal funding. A recent new requirement from Homeland Security demands all ventilation openings to be safely out of reach; with that ventilation grates in the streets are gone as a solution as well and full size structures need to occupy valuable ground outside the right of way if it is a street.So what about the cost and the question of whether or not a light rail tunnel makes sense for transit in second tier city? Light rail cost can go from as low as $ 42 million per mile (surface) to as high as $400 million per mile if tunnel is involved (Toronto). The Baltimore Red Line had a total cost of $ 2.9 billion for 14.1 miles or slightly over 200 million per mile. A lot of money, for sure. How to determine if its too much?Until recently the Federal Transit Administration had a complicated formula, the cost effectiveness ratio, that answered the cost benefit question in a decisive manner. In the numerator sits the cost, in the denominator the benefit, expressed as the product of ridership and travel time savings. If the ratio came out above a certain value, a project was doomed and not even further considered for federal funding.

In other words, if a project garners enough travel time savings and, through the more attractive travel time, enough additional riders, even large cost increases can be justified, provided that the local funding share would go up accordingly.



A tunnel with concrete panel liners after boring and before

rail installation and finishes.

A look around the world may be helpful as well. One will find a good number of light rail systems using downtown tunnels to achieve the efficiency of metro in downtown and the flexibility and lower cost of light rail and streetcars in the outer areas. A few light rail tunnels in the US were already mentioned. For those who don't want to rely just on math to decide an important infrastructure project, the Obama Administration relaxed the all powerful formula a bit and allows less quantifiable benefits to be considered as well. It should be noted, that the Baltimore Red Line did pass the strict numerical test as long as it was in place.A look around the world may be helpful as well. One will find a good number of light rail systems using downtown tunnels to achieve the efficiency of metro in downtown and the flexibility and lower cost of light rail and streetcars in the outer areas. A few light rail tunnels in the US were already mentioned.



A schematic image of the entire TBM assembly line with the boring

head left (Herrenknecht).

It is a truism that historic cities like Baltimore, Boston, Pittsburgh or Philadelphia cannot widen their typically narrow streets without destroying their city. In Europe LRT tunnels can be found in Austria (Vienna), Germany (about 20 cities), the Netherlands, Belgium, Italy and France. I am fairly well acquainted with the system in Stuttgart, where all lines of an extensive light rail system were moved underground in the core area of the city of over 600,000 residents, a solution that has made Stuttgart's light rail system comparative to full metro systems but allowing running trains far into the suburbs for much less than metro would cost. The Stuttgart also includes the steepest tunnel grades for passenger rail in the world.It is a truism that historic cities like Baltimore, Boston, Pittsburgh or Philadelphia cannot widen their typically narrow streets without destroying their city.

For surface transit to work reliably and reasonably fast, this would require drastic impacts on vehicular traffic through turn restrictions, transit only lanes, signal priority and station facilities that could clog up sidewalks or storefronts.

Whether DOTs are stuck in car-only mobility mode or subscribe to the "complete streets" concept, they are usually not eager to cede much of the scarce street right of way to the transit system. The situation is a bit easier in western cities in the US, where cartways are wider.