So as a civil engineer with experience working in tunnel construction I've been following Elon's Boring Co. for a while now but honestly I keep getting frustrated when I read these media releases of his because it really shows how little he seems to understand about tunneling or what some of the big challenges are. A number of people on this thread have brought up Elon asking tunneling execs about whether TBMs are thermally or electrically limited. The honest answer is neither, and the true limiting factors are often project specific. Modern TBMs often have enough power to literally tear themselves apart if not carefully monitored. The text book example of this would be Seattle's Bertha which boasted 50,000 HP and proceeded to crack its main ring bearing when it struck a buried steel pipe, setting off hundreds of millions in repair costs. And as far as power goes, large and mid size TBMs are often connected right to the electric grid with their own dedicated substations. A big limiting factor of most TBMs is cutting tool life. In extreme hardrock conditions cutting discs often last less than 24 hours before needing replacement. The Follo line in Oslo required 24,000 cutter changes over 22KM of tunnels with rock strengths up to 300 MPa or 43,500psi. A single standard 19in disc cutter has a tungsten carbide blade and the whole assembly weighs about 425 pounds. Musk has mentioned he wants to find a way to automate the replacement process but has not yet released any details I'm aware of on how to do this. Its also worth noting that when a TBM is going through soft ground the space behind the cutting head needs to be kept sealed and pressurized in order to avoid collapse of the tunnel face. Replacing cutting tools in soft ground is a serious headache where the only way to do it without collapsing the face is to pump in high pressure air to force muck away from the cutterhead than swap tools out through a series of airlocks and workers operating under hyperbaric conditions to make any needed repairs. Its such a pain that tunnelers will try and avoid this by creating planned safe zones along the tunnel route to do repairs. They will drill from the surface down to the tunnel depth and pump in large amounts of concrete grout to artificially create an area of hard ground where you can safely depressurize the face of the TBM. Another big risk and cost factor in tunnel is dealing with water. A large number of TBM rescues occur when a machine gets flooded out due to high water inflows. Machines expected expected to tunnel through high water pressure environments need to have a complex system of seals and dewatering pumps. The TBM currently digging the Rondout Bypass tunnel under the Hudson River north of NY City can seal against 30 bar (435 psi) of hydrostatic pressure and has a 2,500 gallon/minute dewatering capacity. On top of this the plan is to use probe drills inside the tbm to drill holes about 100 ft in front of the tbm and than pump high pressure grout through the holes to fill all the cracks and crevices in the rock face to limit the amount of water that can flow into the tunnel. (When the original section of tunnel passed under the Hudson using the drill and blast method crews had to pump out 4 million gallons of water a day even under hyperbaric conditions.) Lastly on this thread and elsewhere there seems to be the idea that tunneling companies are this big psuedo cartel raking in massive profits and ripe for disruption like Musk did to the aerospace industry with SpaceX. That's really not the case though, construction is a notoriously competitive and small margin industry. Much of the small diameter utility tunnel work like what Musks suggests he'd expand into is done by small mom and pop companies in a highly fragmented market. The most recent tunnel project I was involved with was for building about 700ft of 5ft diamter drainage tunnel as part of a larger project. My company subcontracted the actual tunnel boring to a small firm with experience in the area. The company is just the owner and a few dozen employees and they do around $20 million in work a year. Even the mega-projects that are done by huge multinational construction companies are done on incredibly small margins. Skanska who was the tunneler for NYC's Second Ave Subway which famously cost $1.6 billion/km reported Q3 2018 profits of just 1.8% on USD 13.2 billion in revenue, mostly in real estate holdings and had to write off more than $100 million in losses for its US construction division. Lockheed Martin for comparison posted over 10% profit for the same quarter, and even Ford managed 2.6%.

One of the more memorable moments of my career came when I attended a prebid sitewalk of the Queens-Midtown Tunnel in NYC. We were in the ventilation ducts of the tunnel inspecting steel brackets that needed to be replaced. To call them ducts is a bit of an understatement though as they were 6.5 ft high and the width of the tunnel. They were supplied with air by 46 fans 20 feet in diameter. The engineer from the MTA who was leading the walk through made a point of saying that should there be a car accident in the tunnel below the fans would automatically switch on to full blast in order to pull as much smoke out of the tunnel as possible and give first responders a chance to get to the accident. He went on to explain that this would mean that over 3 million cfm of air would come through the ducts at around 150 mph and that any crews working in the ducts at the time were considered "acceptable losses" compared to all the people that may be trapped in the main portion of the tunnel. Fortunately the job was completed without that scenario actually occurring.

Besides the obvious "Just because he succeeded in one field does not automatically mean he will succeed in another related field", I would point out that even if Elon didn't know much about rockets when he started SpaceX he did hire a bunch of rocket scientist to help him. As far as what I've been able to determine he's hired only one Professional Engineer with tunneling experience so far: Mike Wongkaew who he poached from Mott MacDonald. And he only hired him in April long after Elon had started boring his Hawthorne tunnel and had pretty much filled fleshed out his whole electric skate idea. All the other engineering staff at the Boring Co. seem to have come from SpaceX and have Aerospace backgrounds. And tunneling is not rocket science (pun very much intended). So when I read various stories like this one about the Boring Co. it strikes me that Elon is spending a lot of time learning things that are already known, not talking about solutions that have already been developed, and not fully anticipating some of the known second and third order problems with the ideas he's coming up with. A few things I've noticed from the Boring Co. press to date: Elon has stated repeatedly that tunnel boring machines must stop and build a tunnel ring segment before beginning their next drive, but that is only the case for single shield machines in very soft ground. Double Shield machines can dig and erect tunnel segments simultaneously, and Gripper TBMs can bore and place tunnel support such as rock bolts, ring beams, or shot-crete simultaneously; which allows a second crew to come in a place the permanent tunnel lining later at their leisure. Single shield TBM systems like the Boring Co.'s Godot (which is a refurbished Lovat EPBM) are most often used in soft ground applications where the only thing for the TBM to push off against is the already constructed tunnel behind it. However a major problem with digging to fast with one of these machines in soft ground is that you can open up voids in the soil or send pressure waves through the ground and cause all sorts of problems with subsidence and sink holes at the surface. When digging in dense urban areas tunnelers have to deploy at considerable amount of monitoring equipment on the surface to make sure they aren't upsetting the foundations of any buildings. In particularly tight areas its not unusual to stop the machine after each advance and wait to make sure there aren't any vibration or settling issues before you resume tunneling. In soft or mixed ground tunneling you are constantly trying to keep pressure at the face of the TBM at equilibrium with the surrounding ground, your pushing through a medium with constantly varying mechanical properties and you have to make sure you're extracting material at a rate exactly matching the rate you are displacing it with your TBM and that the material is being extracted evenly across the whole face of the TBM. The whole move fast and break things maxim of Silicon Valley doesn't hold that well when the things you are breaking are occupied building falling into sinkholes. Now its worth noting that I don't want Elon to fail and its certainly possible he has come up novel solutions different what the industry as a whole has developed. But if he does he hasn't made it public yet and he now has two major public-private partnership deals to build tunnels in the second and third largest cities in the United States, so one can be forgiven for raising and eyebrow and wondering if Musk jumped the gun a bit. Even SpaceX took years of iterative improvement and R&D before they successfully reused a rocket. Last thing I'll say in this post is that tunneling is still an incredibly dangerous activity. New York City's Sandhogs, who I consider among the best urban tunnelers and miners in the world, have a saying: "A man a mile", that is that for every mile of tunnel that has been dug in NYC an average of one man was killed in the process. When you have people working in extremely confined spaces next to thousand ton machines accidents are all but inevitable. If Elon really follows through on his plan than at some point someone working for him is going to get killed in one of these tunnels. Killed not in the pursuit of some noble goal like colonizing Mars but for a for profit transportation gimmick that started out as a joke while stuck in traffic.

Modern standards for tunnel ventilation are less concerned with emissions than they are with smoke from fire events which the tunnels also seem likely to have trouble meeting. These tunnels would likely be held to NFPA-130 "Standard for Fixed Guideway Transportation Systems" which is the same fire code subways are held to. For comparison in the event of a fire in New Yorks Second Ave Subway each tunnel segment can circulate air at a rate of over 2 million cubic feet/min through ducts that exhaust out large multistory ventilation buildings.

Manhattan and The Bronx and northward are notorious in the construction community for its incredibly hard rock so even with TBMs you have to drill and blast out your access shafts, caverns, and cross tunnels. Famously New York's Water Tunnel 3 has claimed 24 lives since the start of construction in the 1970's. Water tunnel 3 is also 800 feet deep below Manhattan and includes numerous caverns and shafts for valves and other equipment. They made a pretty good documentary on the sandhogs a few years back. Link to the trailer below. https://www.youtube.com/watch?... Edit~ I should also point out that London sits primarily on soft clay and running ground which is why the tunnel shield was invented and pioneered for early London railroad tunnels. Cities like New York sit on hard bedrock. A major issue with hard bedrock is that you tend to get what is referred to as squeezing ground when you dig through it. This is when the built up stress of all the overburdern and surrounding rock cause the rock to violently burst at the newly created sides of the tunnel. Its usually recommended not to use a segmental tunnel machine in ground like this because the squeezing rock can trap the machine. Instead the common technique is to use an open face machine and than come in immediately behind it with rock bolts to counteract the stress on the rock. However, you can still end up with rock bursting events which can definitely kill someone if they're not careful. See link below for an example. https://www.youtube.com/watch?...

"Ahead-Of-Schedule Project Not Finished,

Left: Ban all meat

Left: Ban all GMOs

Left: Ban all vaccines. They cause autism.

Left: Ban all nuclear power

Left: Ban all suburbs and rural areas. Force everyone to live in cities

Left: Ban all new construction. Ban the modernization of historic homes

I’m a faggot.

I‘m an incel faggot.

They didn't tell him not to talk. They told him to stay home So he couldn’t talk.

Both of those examples the worker and solder have discretion.

Oh ok, the police recognized him as a murderer and therefore left him on the ground to die? That's what you believe? Lol.

Or is it that the police should be allowed to execute anyone, without even a fake mock trial, who opposes them?

Nobody wants the type of police force you want. The police work for us, actually we want people like you arrested for wanting to kill people for the littlest things. You are a danger to society and the type of order we want.