Are tunnels actually dangerous during an earthquake?
Metro Light Rail Tunnel Entrance
Los Angeles is the talk of the tunneling industry thanks to one Elon Musk, whose plan to revolutionize metropolitan transport with a network of tunnels has become perhaps a bit more controversial than he may have expected.
Proponents of the plan seem to praise it as the right solution to a major problem that Los Angeles has, or at the very least, a step in the right direction. Others are interested in the idea but have concerns about the feasibility of the project, given issues such as the cost of tunneling (it isn’t cheap). Still others think it’s a terrible idea, particularly for Los Angeles, a city located in a part of California subject to 10,000 earthquakes per year, even though many are too small to feel.
On the surface, the earthquake argument sounds like a very reasonable safety concern.
Earthquakes can cause gratuitous damage, and one might expect an earthquake to be a death sentence for anyone trapped in a tunnel. And yet, L.A. has a subway system that has survived 92 years (or about 920,000 earthquakes). In fact, Los Angeles Metro just completed a brand new tunnel for its upcoming underground light rail system. Seattle, which is also prone to earthquakes, is finishing up a major tunneling project to replace the Alaskan Way Viaduct, and one major reason for it is because the viaduct is no longer considered to be safe in an earthquake. Meanwhile, Elon Musk fully expects the permits for his plan may be a pain to obtain, but he does not seem to doubt that they will be granted.
Musk put it best himself on his Twitter:
Earthquakes tend to have the biggest effect on the surface, like waves on water. That's why LA can have a (lame, but getting better) subway.— Elon Musk (@elonmusk) January 25, 2017
His words, L.A. Metro, not ours.
Historically, tunnels have always been safer during earthquakes than most places on the surface of the earth.
“Tunnels are the safest place during an earthquake because tunnels move as one unit with the ground,” explained Murthy Krishniah, executive director of Transit Project Delivery for L.A. Metro.
Washington State Department of Transportation used this analogy on its website to answer earthquake safety questions about its new tunnel in Seattle:
Imagine a plate of fruit-filled gelatin dessert. Tunnels are like the pieces of fruit at the base of the gelatin, while above-ground structures are like the fruit toward the top. If you shake the plate, the movement becomes more exaggerated as it flows up from the base of the gelatin. In an earthquake, this translates to tunnel movement measured in inches, while the movement above ground might be measured in feet.
This is the same idea that helps prevent bridges and skyscrapers from collapsing during an earthquake. Moving with the shaking helps negate the forces that would otherwise topple such structures. Add to that the fact that the shaking itself tends to be much weaker underground compared to the surface and it’s easy to see why engineers that inspect L.A. Metro tunnels after large earthquakes have not yet found significant damage.
In the book Tunneling and Tunnel Mechanics, author Dimitrios Kolymbas quotes a report from the M 6.7 San Fernando earthquake of 1971:
The earthquake caused an outage of electrical power that caused the tunnel pumps to stop. Amid the attendant confusion and anxiety, the miners made their way to the locomotive and drove 5 miles out of the tunnel in pitch darkness. This means that the rails were not significantly distorted to cause a derailment. However, Southern Pacific Railroad tracks on the surface were distorted and broken.
Tunnels are not infallible, of course.
The entryway to the tunnel is still at the surface, and could mean falling debris depending on how it’s constructed. In addition, a tunnel could suffer varying amounts of damage if the earthquake is extreme, or if the tunnel intersects or is near a fault line. Engineers, however, are able to design tunnels to withstand pretty severe earthquakes.
The tunnel Seattle is building, for example, is strong enough to withstand a M 9.0 temblor. It is watertight to make sure people can be evacuated safely, has flexible joints to make sure the tunnel can move with the quake if needed, and all its parts have been analyzed for performance in seismic activity. Some projects also inject grout into the surrounding soil to guard against liquefaction (when the soil loses strength and falls apart, acting like liquid and causing instability for structures around it).
Being underground during an earthquake may sound counterproductive, but with the help of physics and intelligent engineering it’s actually one of the safest places to be.
Alaskan Way Viaduct and Downtown Seattle, Washington.
Attribution: Joe Mabel
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