Archive for the ‘technology’ Category

Far From Boring:
Meet the Most Interesting Tunnel Boring Machines

Thursday, January 25th, 2018

Right now in Los Angeles, Elon Musk is playing in the dirt with his shiny new toy, a second-hand tunnel-boring machine (“TBM”) named “Godot”. He hopes to revolutionize car travel by building highways underground, in “going 3d”.

Musk’s antics have already drawn criticism from transit-planning experts. Jarrett Walker points out that Musk’s ideas are inherently anti-urban and anti-transit. Alon Levy shows that the technological claims Musk makes about tunnel technology are bogus: Musk seems to believe that there are a lot of low-hanging fruits to improve tunneling technology, whereas in reality, the technology is already very advanced.

People tend to think it’s the tunnels that are the most expensive part of underground systems like metros, but thanks to the already existing TBM technology, they often represent only a small portion of the overall cost. Sometimes as little as 10%.

The most expensive parts of subways are the stations. In my view, modern boring technology becomes interesting when we can use it not just for the tunnels in-between the stations, but to build the complete system, including the stations, cheaper.

Many tunnel building innovations have been developed by various companies to deal with real-world constraints. They give us TBMs like these:

The Giant TBM

Musk says he can build tunnels cheaper if he just makes them smaller. But in reality, it’s not small TBMs that are the future, but big ones. The cost of a TBM doesn’t get much higher as you increase its diameter. Tt is therefore cheaper to build one very large tunnel, rather than two smaller ones. So giant, linear tunnel building factories have been constructed, with some reaching up to 17.6m in diameter.

The two phases of tunneling: pushing forward & installing the tunnel lining (check out the full video here)

A large diameter tunnel becomes really interesting for the construction of a metro, if it is large enough to fit not only the tracks, but the station platforms inside it. This saves costs because it’s unnecessary to excavate large caverns, possibly dug from above requiring the purchase of a large amount of land.

A large diameter tunnel was used for Barcelona’s line 9/10 project, where the tunnel is so big that two tracks and two platforms can be stacked above each-other, inside a single tunnel.

These arge tunnel can also house other necessary infrastructure: siding tracks to park trains at night, ramps so trains can cross over from one level to the other, evacuation paths, power substations — all items requiring space and cost.

Other projects used very large TBMs to build tunnels hosting six lanes of highways on two levels, plus emergency evacuation and ventilation, all inside a single tube (Madrid, Seattle). There are also water or sewer tunnels.

State Route 99 tunnel (source)

One interesting project in Kuala Lumpur, called “SMART Tunnel”, combines a four lane highway tunnel with a storm drain. During major storms, the road is closed and the whole tunnel is used to carry stormwater.

Other projects combine rail and road in one tunnel. In Wuhan, China, a twin-tube tunnel under the Yangze River, currently under construction, combines a highway on the upper deck with a metro line on the lower one.

Wuhan Metro line 7 tunnel (source)

The Vertical TBM

Let’s say you’ve built your metro line using a giant TBM, and installed platforms inside the tunnels — you’ll still need to access those stations from the surface. Traditionally, you would dig some access shafts. This may be slow, labor-intensive, and complicated if there’s ground-water. But in the interesting new world of tunnel technology, there’s a TBM for that: the “vertical shaft sinking machine”.

Vertical Shaft Sinking Machine (source)

It’s a machine that consists of an excavator at the bottom of the tunnel that removes the earth, and machines at the top that build the tunnel lining rings that are being pushed down from above.

digging and adding rings (source)

The trick here is that rings are installed at the top of the shaft and then the whole shaft is pushed down. It leaves a lot of the complex technology at the surface. It also means you don’t end up with a big machine at the bottom, without an escape: most TBMs can’t go backwards because they’re bigger than the tunnel they’re building, so they need an opening on the other end to escape the underground (or be taken apart at the end).

At the bottom, the excavator can work under water, so the shaft can have the same ground-water level as the surrounding environment, until the desired depth is reached and the a concrete seal is poured at the bottom.

pouring the bottom seal (source)

With this machine it would be relatively simple to build access shafts for elevators, to access our hypothetical metro line.

The Diagonal TBM

We could now build our subway line deep underground inside giant tunnels, and vertical shafts down to provide elevator accesses. The thing is, if you want a lot of people to access your station, you need escalators, which move many more people per hour. This means we want tunnels to be neither vertical or horizontal, but built at the 30 degree angle of escalators.

In 1997, Saint-Petersburg opened an extension of its line 5, very deep underground. But one 102m deep station, Admiralteyskaya, wasn’t opened until more than a decade later, because they couldn’t figure out how to build a connection to the surface. For all this time, trains just ran through this ghost station but didn’t stop there, since there was no connection to the surface.

The problem was that there are a lot of museums and heritage buildings nearby and the ground is composed of a soft soil. The usual solution to freeze the ground and treat it like rock was deemed too risky, as the ground expansion from the freezing could damage the buildings (oh also, they ran out of money, and they had trouble finding a plot of land to use for the station).

In the end, they came up with a tunnel boring machine that digs at an angle. The machine takes away soil and immediately replaces it with a tunnel, minimizing movement of the ground. This allows digging without affecting the environment.

Side view of the TBM (source)

In a way, building tunnels barely more than 100m with a TBM seems crazy. Setting up a tunnel “factory” only makes sense when you have a lot of tunnel to build. To make this economical, the TBM was made as re-usable as possible. Once the TBM reaches the bottom, most of it is disassembled and moved to the next site. The only thing that stays underground is the tunnel shield, the big protective metal sheath at the front of the TBM under which the tunnel lining is assembled – because it’s wider than the tunnel.

View down the angled shaft (source)

In the end, three angled tunnels were built, at lengths between 105m and 160m allowing three new stations to open. The Admiralteyskaya station opened in 2011, about 14 years after the line itself.

The Rectangular TBM

One issue with traditional TBMs is that they build tunnels with a circular cross section. But the internal cross-section of tunnels usually needs to be rectangular; generally we want a flat bottom, walls going straight up, with some relatively constant ceiling height.

If we use a circular TBM to build a tunnel, we have a bunch of wasted space on the sides. This can be especially an issue when space is at a premium, or if we want to build tunnels as close as possible to the surface.

This is especially true for underpasses, which have to be as close to the surface as possible and which also have a relatively small height (as little as 2.2m) but require a good amount of width (4m and more). Traditionally, these would be constructed using cut-and-cover: the road would be dug up, the tunnel placed in, and the road rebuilt on top. This can be a major disruption to the surface roads above.

To deal with all of these issues, some tunnelling technology companies have started to offer a new kind of tunnel boring machine: the rectangular TBM.

the rectangular TBM (source)

The idea, proposed by a German company but already built and used by a Chinese one, is to have a small rectangular digger. They can be used to build short, low-depth tunnels, in space-constrained environments, while minimizing surface impacts (which may also reduce costs).

A rectangular TBM digging an underpass (source)

Rather than including a facility for assembling the tunnel lining (out of multiple segments) inside the TBM, complete rings are inserted at the insertion shaft, and the whole tunnel is jacked forward one segment at a time.

This technology was used to build a metro access at the Havelock Metro Station in Singapore. The company hopes that this technology could be used one day to build whole stations.

A station built using this rectangular TBM concept could be very interesting: unlike the ‘giant’ TBMs mentioned before, this technology could allow building stations close to the surface, in much more spatially constrained environments, and allowing a passenger platforms to be side-by-side rather than stacked.


When building transportation systems, constraints are usually about physics, geometry and cost, not the technology (see my criticism of the “Gadgetbahn”).

I think that we have to be careful of primarily technology-based “solutions” to transportation problems. Musk is trying to sell a quick-fix technology that in real life won’t be able to escape geometry and physics. His approach makes little sense, his understanding of cities is poor … but also, his technology is boring.

On the other hand, technology can provide interesting tools to advance projects anchored in the real world, and push geometric, physical, time and cost constraints to its limits.

For more reading on tunnel technology, I recommend the website TunnelTalk, and the Tunneling Products Page of Herrenknecht, which I’ve both heavily linked in this article.

What’s a Gadgetbahn?

Sunday, December 3rd, 2017

For some time, I’ve been meaning to write about German transportation systems like what’s an S-Bahn or what’s a Stadtbahn.

With the recent news out of Quebec, I figured I’d instead talk about a transportation concept that doesn’t actually transport anybody at all: the Gadgetbahn.

The word is a portemanteau of the English “Gadget” and the German word “bahn”, which means rail or train. A gadgetbahn is a speculative transportation concept that proposes to solve planning and financial issues via some sort of magical techno-fix, likely some technology that doesn’t even exist yet.

Classic examples of gadgetbahns are: monorails, “personal rapid transit”, maglevs, or the newest addition to the family, the “hyperloop”.

Proponents of these technologies may be referred to as gadgetbahn enthusiasts, or more derogatorily, “pod-people”. They often promise the sky in terms of reduced cost and increased speed and comfort, often with little consideration for capacity, risk, safety or realism.

Back to  Quebec: the prime minister decided he wants a fast transit link between Montreal and Quebec City, but not a train, because “we can do much more modern things now”.

Instead, he wants something “futuristic”, something from the minds of Quebecers. (note from editor: Sorry to pop his bubble, but Quebec is not exactly known for having a long history on the bleeding edge of transit technology.)

The problem of the gadgetbahn isn’t necessarily that it’s a techno-fix. It’s that it is a technology for the sake of technology, a shiny gizmo to brag about, with little regard to solving the actual transportation problem.

The transportation minister clarified his position himself: he wants anything you can conceive of, any project; innovate, come up with new ideas!

And somehow, some consortium sprung up ready with proposals, renderings, promises and deadlines to build a high speed monorail (“MGV”), and all they want is a quarter of a billion dollars to develop it.

Cost, Speed, Comfort
– The problem is geometry, physics and the Right of Way

The beauty of proposing a gadgetbahn is that since it doesn’t exist, proponents can make up all sorts of quasi-magical properties for their technology, which supposedly make it superior. Since there aren’t real-world examples, proponents can use the most optimistic theoretical scenarios they can come up with, and compare them with the actual performance of projects that have been built and which are bound to the constraints of the real world.

For example, the high speed monorail promoters claim their system is cheaper than conventional rail, because they could just use existing highway medians, with an elevated rail system where vehicles are suspended from above.

The “MGV” (monorail a grande vitesse), high-speed pods on elevated tracks, suspended from above

When you look at its basic structure, compared to conventional rail, this monorail essentially differs in the method of propulsion: instead of two wheels on two rails below the train, we have two wheels on one rail above the train.

The main conceptual difference between conventional rail and ‘MGV’:
the location of the wheels

Like for any gadgetbahn, the claim is that this new technology provides more speed, more comfort at lower cost.

But in the real world, these three aspects are always intricately connected and subject to tradeoffs – due to simple geometry and physics.

Want faster transportation? Then you need very straight tracks. Don’t have straight tracks but still want high speed? Then your trip will become a barf-ride, so less comfort.  Want to build cheaply in an already existing highway median? Well the highway curves are made for cars going at 100km/h, so your choices are:

  • slow down (less speed),
  • run inside the existing geometry at higher speed (less comfort),
  • straighten the curves (more expensive).

These problems will always come together. At the end of the day, you’re still pushing a metal can full of people at high speeds you can’t get out of issues of geometry and physics by changing where you put the wheels.

Going further, once it appears the basic problems have been overcome, the next issues become capacity, safety, energy and access (stations).

For example, Elon Musk is proposing to build tunnels with his “Boring Company”, which will supposedly be cheap, because the tunnels will be relatively narrow. This reduces capacity, because only smaller vehicles will fit in the tunnel. To compensate, he may propose that vehicles will run super close together – which will represent a huge safety issue unless they run slowly. A small tunnel with a lot of vehicles that are barely smaller than the tunnel diameter, running at high speed may become a death trap in case of emergency, if there isn’t enough space and facilities for egress. If somehow he manages to solve all these issues, there’s still the problem of getting everybody into this tube of his.

All these considerations come down to the one fundamental constraint on which everything else depends: the right of way — how much space do you have available, how straight is your path, how and where is the downtown access, and how much space is available at stations.

The Maglev

An example of this issue is the maglev technology (essentially magnetically elevated monorails). The concept has been around for ages. The Germans and Japanese have been developing the technologies for a long time. In the early 2000s, the Germans were able to sell their Transrapid technology to China for the Shanghai Maglev airport connector, a 30-km line. It was a pilot project, with the hope to eventually cover the whole country with maglev network.

The Maglev was seen as the next generation of trains, mostly by being faster. But there was also the hope that it could even be cheaper, by putting the whole track on stilts, without having large elevated bridge-like structures. But in the end, conventional high speed rail and maglev require similar geometries, a similar kind of infrastructure, and it turns out that the speeds are not that different (Shanghai Maglev up to 430km/h, conventional rail up to 350km/h, both with a maximum experimental speed of around 600km/h).

At more than 300km/h, a lot of energy is spent to overcome the air drag of the trains themselves. Issues also become noise, and the required straightness of the infrastructure. And the issue that with stops, decreasing travel time by increasing maximum speeds becomes marginal. Overall, speeds above 300km/h tend to become uneconomical, no matter the propulsion system.

From a distance, elevated conventional high speed rail (left) and maglev (right) appear strangely similar – the requirements on the tracks are due to physics and geometry, so the propulsion method alone won’t give one system a clear advantage over the other in terms of infrastructure cost

All of these issues come together to make the technological choice a bit of a wash.

In the end it makes more economic sense to build from conventional technology which has been developed for a longer time, has multiple vendors, has existing infrastructure that can be built on top of. This way, you can upgrade lines for high speed service, while being able to run the new high speed trains on existing tracks inside cities or to other cities. This provides a huge economic advantage.

So indeed, although the Shanghai Maglev works, in an economic sense it’s a failure. It convinced the Chinese to focus on high speed rail: ten years after the opening of the Maglev, the country still had the same 30km of maglev, but built 20,000km of high speed rail.

(Oh and btw, I’ve taken the maglev, it’s not a very smooth ride; it rumbles like a rollercoaster)

The Real Issue: Economics

The real issue of using a gadgetbahn to solve a transportation problem is not really technical. It’s that it re-frames the problem to build an infrastructure as the problem to develop a new technology — now you’ve got two problems to solve!

The technical problem should really be solved through private investment, not public funding. Thus I view the ‘offer’ to develop the “high speed mono-rail” for “only” 250M$ with a great amount of distrust. If the idea is viable and the people behind it are competent, it should have attracted private investment, as there should be a potential to make profit selling the technology. After all, the proposal has been around for 23 years.

When proposing a completely new technology in order to solve a specific transportation problem, the major problem is Risk:

  • Technological risks: Will it even work? Will it deliver on claims? Will it have sufficient capacity?
  • Cost-related risks: How much will it cost to develop? How much will one kilometre cost once the technology developed?
  • Regulatory/safety risks: Will it be safe? What will safety requirements be? How fast will we be allowed to run?
  • Risks because timelines are not understood: How long will it take to develop? How long will it take to plan & construct?
  • Operating Risks: How much will it cost to operate? How much will it cost to maintain the infrastructure? Will automated/unattended operation actually be possible?
  • Risks because we need to build complete systems at once, rather than incrementally updating existing infrastructure.
  • Also, new technology means relying on a single vendor, which is again extremely risky, both in terms of cost and availability of the technology in the future: Will the vendor exist in the future? How much will they charge us once we depend on them, given the lack of competition?

For any technology that’s already existed and been researched for decades, and that may have tens of thousands or hundreds of thousands of kilometers of infrastructure built today, all these question are much easier to answer. There will be much less risk, and thus less cost.

Be distrustful of Gadgetbahn Concepts

Any time somebody in power proposes to solve an infrastructure problem by first developing some new technology, or by using some proposed technology that hasn’t been delployed yet, we should be distrustful.

Often, the proposal may simply be an excuse to not invest in infrastructure today, because tomorrow some techno-fix will come along and solve all our problems ‘for free’. This tactic may work especially well if the proposal includes an appeal to some futuristic dream or a nationalistic project.

The gadgetbahn may really just be a big diversion. For example in Quebec, the most realistic scenario to get a fast link between Montreal and Quebec is the “high-frequency train” proposed by VIA rail, which would link the cities with conventional rail. After repeatedly proposing high speed rail for the last 40 years and getting no support from the governments, VIA decided to propose a system that’s not true high speed rail – but it would use dedicated passenger rail tracks, allowing somewhat faster speed than today, and no interference with freight.

This would hit an economic sweet spot for VIA, allowing them to almost finance and build the system themselves, with only relatively little public support.

But the main problem is the access to Montreal – VIA hoped to use the Mont-Royal rail tunnel to access Gare Centrale. But with the REM light rail project pushed hard by the prime minister, the tunnel would be converted to light rail, an incompatible technology, which will cut VIA off. In some sense, the prime minister’s announcement that he wants some gadgetbahn to Quebec City means VIAs rail project would become obsolete, and the conflict would become moot.

So is it really just a diversionary tactic to hide the regional rail planning problems in Montreal?


Interesting related reading (and with thanks for some inspirations) by Alon Levy:
“Loopy Ideas Are Fine, If You’re an Entrepreneur”