Following Elon Musk's attempt to introduce underground transportation systems a question is raised. Is tunneling safe in Los Angeles?
Musk founded a construction company (Boring Company) to accomplish his idea of innovative transportation. His concept included shooting electric cars that could reach up to 150 miles/h through underground tunnels. This would alleviate the traffic issues. In order to succeed, his idea has to be sustainable when large temblors strike the earthquake-prone area of Los Angeles.
During an earthquake, structures on top of the ground suffer more damage than those beneath as ground movement is usually amplified and high stresses are provoked.
Nevertheless, in some cases, tunnels have to address faults zones. At least 12 active faults exist below the surface of Los Angeles. At some time, underground structures will have to face potential displacement at the direction of the fault. For this reason, tunnels are lined with airtight material to guard against water and gas leaks caused by the fault's displacement. Framework damage cannot be prevented when a large fault is activated. "At a fault crossing, it's not really possible to design [a tunnel] in such a way that, once that earthquake actually happens, the day after the earthquake, everything's going to be fine, and the trains are going to be running," Jonathan Stewart, UCLA professor of geotechnical engineering says.
Moreover, underground structures are vulnerable to another phenomenon caused by earthquake, called liquefaction. Liquefaction occurs in saturated soils that experience loss of strength and stiffness when an earthquake occurs. During liquefaction, the soil behaves like a liquid and therefore heaviest structures tend to sink while lighter structures tend to rise.
In Los Angeles, there are a lot of areas that are prone to liquefaction. Currently, liquefaction hazard is addressed in the area of Metro's Westside Purple Line extension. Officials utilize a common technique to improve the soils by mixing it with grout or other materials, making it less vulnerable.
Last but not least, tunneling in Los Angeles faces another difficulty due to the underground presence of methane. Earthquakes can trigger methane leaks that could enter a tunnel threatening the health of people.
Despite all the severe issues that tunneling addresses, Jean-Philippe Avouac, geology professor at Caltech, states: "I'd be less nervous about being in the subway than I would about being in a high-rise in Downtown LA."
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