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Comment by 1024core | Hacker News Reader

> They aim to have no impact at the surface by digging just about 20 feet (6 meters) in the ground.

Is 20 feet enough? I would imagine that a lot more depth would be required to support the above-ground structures.

A TBM doesn't just dig a hole. It installs tunnel ring segments to support the ground above. 6 meters down is no big deal. That's mostly to get below other buried infrastructure.

Here's a good explanation of how TBMs work from Crossrail.[1] They use two types of TBM. There are also hard-rock TBMs, used in deep jobs like the Gotthart Base Tunnel. For some hard-rock jobs, a tunnel lining isn't necessary.

Most of the problems in tunneling involve water. Mud, leaks, underground rivers, etc. The Boring Company doesn't mention water much. So far, the Boring Company just seems to have bought a small TBM and is learning to use it.

[1] https://www.youtube.com/watch?v=z38JIqGDZVU

They've mentioned water when asked about Hyperloop, to point out that standard tunnels are waterproof to a specification that they could also support the level of vacuum necessary for Hyperloop, without modification.

The SpaceX rep mentioned this in the recent Hawthorne city council meeting, and I believe Elon said the same thing in one of his presentations.

It's not keeping water out of the tunnel once built that's hard. It's building the tunnel through water and mud that's hard.

Sure, but groundwater flow has to be accounted for. Hard engineering can lead to very bad outcomes. The Soviets tried to ignore the effects of disrupting thermal equilibrium in their attempts to build cities in permafrost areas. Their solution was prodigious volumes of concrete. It failed badly. These days geotechnical engineers try to work more harmoniously. For example, in areas with saturated soil one approach is to excavate an equivalent weight of soil to that of your construction, thus the building effectively floats, and there is no disruption to the local dynamics.

I cannot see that Elon can get around the need for proper geomorphological study here.

In the video two comments up, they explain that a different kind of TBM is used for waterlogged boring. Bentonite is used to keep pressure on the tunnel face instead of the bored material, the slurry is then separated with a centrifuge and piped back to the face/out of the tunnel.

I was also thinking about Crossrail, and how it's going to cost $20+ billion for 73 miles of track...

Some of the engineering is terrifying - less than a metre separation between the tunnel they are boring and adjacent tube lines!

https://www.theengineer.co.uk/issues/october-digi-issue-2/yo...

Look at the route. There are twists and turns to avoid various underground obstacles. Most station work involved acquiring and demolishing something before work could even start.

There was supposed to be a Crossrail 2, running north-south, but it looks like that's being canceled. After Brexit, London will be much less of a banking center, and traffic is expected to drop.

So that's a bonus for the environment then, what with all the concrete it would have required.

Given expected passenger numbers and the fact that Crossrail 2 (as Crossrail) would stretch to outer parts of London with a high share of care ownership, the reduction in car traffic would probably outweigh emissions created for concrete.

This was a question I had as well. I could easily see pilings for a multistory building going more than 20' under the surface.

However, they don't appear to want to tunnel in arbitrary places but more along roads. Roads have a relatively fixed amount of infrastructure "under" them (around here that seems to be a sewer pipe and cable/fiber/copper cables along the edges (not very far down though, at most about 10')

I can imagine a very efficient public transportation system if you combine the self driving aspects with elevators. At what would now be considered a bus stop would be an area where a 'pod' can be summoned. You tag your card and your destination. After a short delay, the elevator lifts and on it is a 'pod' with seating for four. You step in, sit down. The doors close, the elevator lowers, and the pod drives along easy to navigate tunnels to your destination. Then queues for the nearest elevator, rises, and the doors open. Done.

The beauty of such a system is that you can do autonomous faster because the lanes/signalling/weather conditions are all normalized to the tunnel. There are no 'manual' drivers or bicycles or pedestrians. Its a much more constrained environment so fewer things to worry about. It can re-use the right of way under the street so there isn't any need to use emminent domain to seize property for a right of way. All of the 'stations' are above ground and can re-use existing bus stops so very low cost.

Hostile acts are an issue. If you carry on a backpack bomb into your pod and blow it up you're going to block that tunnel until maintenance can clean it out. And people sleeping/living in pods would no doubt be a nuisance. Same thing with summoning a pod that the previous occupant had just been massively sick in or defecated in. Probably needs a button on the summon station to say 'send this pod to maintenance' but there is an interesting denial of service attack then of people sending all the pods to maintenance. Perhaps some operator / NOC ability to look record/evaluate the users would be required.

That's a neat idea. I can imagine most of the potential problems can be solved by requiring a credit card to sign up, and image recognition software - if the pod is supposed to be empty, then the onboard camera can check if it really is (could also measure the pod's weight, use infrared and ultrasound "vision"), and alert a human operator otherwise. Bombing a 4 person pod does not make much sense to me, since there is not that much damage that can be done underground, with the exception of starting a bomb right beneath a huge building. I'm not sure a potent enough bomb can actually be carried around though.

The problem with following roads is that you don't want to do a 90 degree turn at 200 km/h or whatever speeds they're imagining here. From what I understand it's supposed to be more of a hub system rather than a point A to point B system, i.e. you drive to the nearest entry point, go down with the elevator and have it ship you to the exit that's nearest to your destination. This means it will probably mostly be straight tunnels, similar to a subway which will eventually have to go under buildings.

Nope, modern subway stations aren't necessarily deeper than that. Plus TBM's bores are much smaller diameter than traditional holes. Plus they don't need the peripheral infrastructures needed with subways systems just elevators and I'm assuming emergency stairs.

Musk said something at TED about it being related to tunnel diameter and I imagine this test track having a pretty small tunnel compared to say a subway.

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