Former tunnel boring engineer here.
There is a ton of room for innovation around tunnel boring machines built by the major manufacturers (primarily Herrenknecht and Robbins), around reliability, durability, ease of serviceability etc. I cannot emphasize enough how modern TBM's require an unbelievable amount of engineering attention, repair labor, spare parts infrastructure etc, similar to many super-early-stage fragile prototype technologies. Unfortunately TBM's are no longer early stage, but for some reason the technology is frozen at just-good-enough-to-barely-work."
However I don't know if the economics will work out to fix any of these. Here are a couple of the big problems:
* Most TBM's are semi- or fully-customized for a single job. This raises machine costs. It'd be better if there were only a small range of small-medium-large TBM's that work ~everywhere.
* Most TBM's are fully assembled in the factory, smoketested aka turned on to make sure they work, disassembled and shipped to the job, then reassembled and used. This is not efficient, surely we can figure out a better way.
* Most TBM's are entombed aka thrown away at the end of the job, because getting them out of the hole is expensive and difficult.
* Changing the cutterheads is labor intensive and dangerous and requires highly trained very expensive humans, and it's slow. While you change the cutterheads, your billion dollar toy is sitting there doing nothing.
* TBM architecture is highly dependent on geology. A slurry faced TBM that works in mixed soils is a totally different beast from a hard-rock TBM. It would be cool to have one machine that works in many geologies, perhaps with minimal or automated modifications.
* TBM's require lots of care and feeding from a small army of humans. This raises job costs.
* Topside support infrastructure such as slurry plants and ground freezing machinery comes from different vendors, often even from different countries. E.g. it's common to buy your topside slurry plant from the MS company, in France, while your ground freeze vendor might be Tachibana from Japan. Often each subsystem's engineers on site literally don't even speak a common language. Hilarity predictably ensues. Vertical integration would pay huge dividends here.
Ideally Elon can mass-produce TBM's that just work out of the box for most jobs, and that are easier to work on. Then we can laugh him out of town for his stupid "put cars in tunnels" ideas and use his miracle machines to build sensible train tunnels.
Here's Crossrail's video explaning how their TBMs work. This gives enough detail that you can get an understanding of what's going on.
There's so much variation in geology. The Seikan tunnel to Hokkaido had huge variations in soil and the project went very slowly. Eurotunnel is mostly through chalk, and went well. The Gotthard Base Tunnel in Switzerland was a long grind through hard rock well above water level. That's a slow process, but was reasonably uniform. SF's BART tunnel, unusually, was built onshore in sections and lowered into a dredged trench. NYC's Water Tunnel #3 was in hard rock, and that project went very slowly.
Many things can go wrong. Hard rock may not be competent rock capable of supporting itself. The Tom Lantos tunnels south of San Francisco were through a small mountain of loose shale. One tunneling project in Japan hit an underground river.
The most advanced underground rail project today is probably the Tokyo-Osaka maglev line. 43 kilometers of this is already operating, and most of the route is in tunnel. Tokyo to Nagoya is scheduled to open in 2027. The connection through to Osaka was scheduled for 2045, but the Government has decided to accelerate the program and get it done sooner. Here's what the ride looks like. 500km/h. Working now.
They are often custom built (at least partly) and thus not mass produced. While we won't see that for rockets any time soon, it believe he managed to bring down the cost of building rockets and continues to do so (he uses his own money after all and does not have the support of every tax-paying citizen like NASA used to during the Apollo missions).
Sure the engineering challenges are probably different for the most part but the economics are the same. Rockets are more expensive then they should be. Boring is also more expensive then it needs to be.
> he uses his own money after all and does not have the support of every tax-paying citizen like NASA used to during the Apollo missions
This isn't really true. SpaceX, like pretty much all private Spaceflight providers, has received many development contracts from NASA. These are basically NASA giving the company a bucket of money to develop launch capabilities to certain requirements.
SpaceX has _absolutely_ been supported by the US taxpayer, and likely will continue to be. It definitely has been _much_ cheaper for the taxpayer than Apollo, but we haven't gotten a free lunch on this one.
They had to prove that they could do it before they saw a dime. The NASA has a project and gets funded without having at least a prove of concept (e.g. A single rocket that can do the job) as long as congress likes it. I'm not saying that this is bad, but this way he earned the money.
Furthermore, unlike the Apollo missions, sending satellites into orbit served a practical purpose. Sure the apollo missions were really cool and have helped science a lot but their practicality was a lot less.
I know Cincinnati's subway failed because of some bad tunnelling that caused foundations to sink .. well that and the great depression. If they had started earlier or designed it better, Cincinnati could have a Chicago like transit system today. Unfortunately it took them years to get a tram and there are people still trying to shut that down instead of expand it! Unbelievable.
I know in Seattle, many of the tunnels (like to Northgate/UDistrict) have actually been fully bored. The new stations are still not due to open for a few years, mostly because the majority of time isn't spend on the tunnels, but installing track, electrics and building the underground rail stations.
If Elon could make tunnelling 10x faster, requires much less Human labour and reusable, this will dramatically cut down the cost of building SubWay, or other underground Networks. I am not sure if such tech could be used to build like a underground Shopping Mall in Cities. But if we could build a Walk way under every Car Road would be great for urban cities.
I have a naïve question about subterrean _housing_.
With the real estate skyrocketing in some cities, and scarcity of terrain, I wonder why there is not much more underground liveable habitations (for example, near an expensive city center with no space left).
I think this is really just Elon's goal, and the talk and fluffy marketing video are just that. Fluffy marketing and PR for his companies.