Comment by abalone | Hacker News Reader

Comment by abalone | Hacker News Reader

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> air allows mines to use a very interesting way to communicate

This is a very important point to remember about subterranean tunnel systems. It is exactly what came to my mind when I watched the Boring Company video about a huge network of 3D tunnels.[1] The tech press, which had probably never even covered a construction project yet alone tunnels, was basically like "what about earthquakes"? But tunnel collapse is not the primary safety issue.

It's fires. Smoke and toxic gases from fires spread very quickly through tunnels.

I am a huge fan of the concept, by the way, but I want to emphasize that most fatalities from traffic tunnels have been from fires (apart from ordinary traffic accidents).[2][3][4][5] And Elon Musk has stated that what makes this vision feasible from a cost perspective is smaller tunnel diameters. Which makes air "communication" all the more accelerated and safety critical. Thus any vision of tunneling without detailing fire safety, evacuation systems and firefighter access is significantly incomplete, as these can add significant cost and fundamentally constrain designs.

[1] https://www.youtube.com/watch?v=u5V_VzRrSBI

[2] https://en.wikipedia.org/wiki/Caldecott_Tunnel_fire

[3] https://en.wikipedia.org/wiki/Gotthard_Road_Tunnel#2001_coll...

[4] https://en.wikipedia.org/wiki/Kaprun_disaster

[5] https://en.wikipedia.org/wiki/Mont_Blanc_Tunnel#1999_fire

> Smoke and toxic gases from fires spread very quickly through tunnels

It's not just the confined-space aspect that causes the fire gases to have such high lethality (although it's certainly extremely significant), it's also the rate of gas production.

Most solids and liquids don't actually burn; they emit flammable gases when exposed to heat, and it's those gases which actually undergo combustion. These gases are created by endothermic processes (pyrolysis and evaporation), and the more heat is available to convert unreactive solid/liquid combustibles into reactive gases, the faster the fire will burn. Therefore, less heat loss (to the outside environment or to non-combustibles) will enable a higher combustion rate and the fire will produce more heat.

Tunnels are quite good at retaining heat, so most of the heat produced by a fire will feed back right into heating combustibles -- enabling devastating heat release rates in the tens to hundreds of megawatts. The heat release rate is roughly proportional to the fire's fuel consumption rate; which will be roughly proportional to the rate of lethal gas (CO/HCN/CO2) production and oxygen depletion.

The radiant heat flux alone can make the fire unapproachable by even appropriately-equipped firefighters -- the massive production of superheated, oxygen depleted, CO/HCN-containing gases can kill -- far beyond the range that the heat flux is deadly.

Great points. An interesting addition is that Musk has also stated that the tunnels would be vacuums (to 5-6 atmospheres) in order to seal against the water table.[1] It's interesting to consider the impacts on fire safety, both in the reduction of risk of spread (a positive) and the difficulties it presents with evacuation from compromised vehicles (a negative).

[1] https://www.ted.com/talks/elon_musk_the_future_we_re_buildin...

Can't watch the video now, but are you saying they are vacuums or pressurized? If they are vacuums, the highest vacuum you can have is -1 atmosphere, I.e. No air.

5-6 atmospheres would be PRESSURIZED, not vacuum.

Also, a vacuum would pull in water from the water table, you'd need pressure to keep it out.

Vacuum would be good for fire prevention, things can't burn if there's no oxygen. Pressure, on the other hand, will greatly amplify fire, if you just pressurize air. If you want to avoid that, you have to pressurize with a neutral gas like nitrogen, which would be expensive if there is any regular amount of gas leakage.

In either case, you'd have to build compartments that were pressure/vacuum proof (think a submarine or spacecraft) -- and in either case, leaving the pressure vessel would pose dire risks to occupants.

I'll try to watch the video when I'm back on WiFi.

From the transcript, this is what the "5 atmospheres" thing meant -- it was about the pressure differential, and that whether the inside is at 1 atm or 0 atm isn't a big deal if the tunnel has to be underground (in which case it already has to stand quite a bit of outside pressure!):

> Exactly. And looking at tunneling technology, it turns out that in order to make a tunnel, you have to — In order to seal against the water table, you've got to typically design a tunnel wall to be good to about five or six atmospheres. So to go to vacuum is only one atmosphere, or near-vacuum. So actually, it sort of turns out that automatically, if you build a tunnel that is good enough to resist the water table, it is automatically capable of holding vacuum.

Thank you for clarifying. I misspoke earlier slightly, but the result is the same: when you build a tunnel strong enough to withstand the water table, it can hold a near vacuum. This helps transport and energy efficiency.. but also presents interesting issues for fire safety.

The minimum pressure of a perfect vacuum is 0 atm, not -1 atm as you say [0].

[0] https://en.m.wikipedia.org/wiki/Vacuum#Measurement

Sure, but it's -1 atm gradient, which is how we commonly measure pressure. I.e., when you fill your car tires to 32 psi, that's 32 psi over atmospheric. When you're talking about pressure, it's the gradient that matters.

It was taught as psia, -g, & -d (or absolute, gauge, and differential) when I was in school, so I read your -1 atm as psid. Your car example would be 32 psi gauge assuming STP, or ~36 psi absolute.

The miners are below the surface and not at 1 atm, so any gauge measures taken there would be psid.

The tunnel becomes something like a stone oven right? I imagine you'd have to let it cool off before trying to approach it.

From the Wikipedia page of the Mont Blanc tunnel fire:

"Some victims escaped to the fire cubicles. The original fire doors on the cubicles were rated to survive for two hours. Some had been upgraded in the 34 years since the tunnel was built to survive for four hours. The fire burned for 53 hours and reached temperatures of 1,000 °C (1,830 °F), mainly because of the margarine load in the trailer, equivalent to a 23,000-litre (5,100 imp gal; 6,100 US gal) oil tanker, which spread to other cargo vehicles nearby that also carried combustible loads."

It was days before the tunnel cooled enough for recovery crews to enter.

With this sort of heat release rate and amount of combustible material, fire doors alone are useless. Even if the fire doors miraculously remain intact and gastight, the walls and doors will eventually conduct enough heat to make the temperature in the fire refuges untenable for human life.

Exactly. The walls act like firebricks.

From the fire perspective, I suppose the positive aspects of musks proposal would be:

Cars are transported on skates, so the engines aren't running during the fire, reducing noxious fumes in the tunnel.

Cars are controlled automatically, reducing the chance of collision.

Since the skates for he cars are controlled automatically, they can be stopped/rerouted much earlier, avoiding bunching cars near the fire.

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