Having the benefit of working for an Austrian construction company specialising in TBM boring and how professional they are in their preparations, planning and execution of tunneling, I have my doubts that Mr Musk said he could bore at 5–10 times faster than current companies as they have spent at least 50 years refining these techniques so even going 2 times faster than today has major obstacles to overcome.
Currently boring at 20m per day is considered doing really well using TBM’s such as these;
Traveling at 18m per day you need to place 12 precast concrete rings per day which look like;
each of which is comprised of 7-9 segments, which look like this, before they go into the tunnel:
These segments all need to be transported into the tunnel (remember, there’s only one way in and one way out, thus only one vehicle at a time is possible) using multi service vehicles like this which can be driven from the front and the back. Typically we try to carry one ring per trip.
When you are 5km into a tunnel this can easily take 2 hours per trip including unloading carrying one ring to the TBM. So in 24 hours you might manage 12 trips /12 rings in a day.
Perhaps a little more is possible if your team is very slick but the longer the tunnel the less likely you will be able to improve on this no matter how fast the TBM moves. Therefore the timely supply of rings becomes a limiting factor.
This is also why metro stations tend to be 3 - 5km apart to allow a new access point for concrete segments and allow the TBM to start off again on a new 3 - 5km stretch.
However, if you are lucky and you are tunnelling in ideal rock conditions you can use a “gripper” TBM which does not need concrete rings to support the rock/prevent water ingress.
Is this the answer to going faster?
No.
Why?
The downside of this TBM is that rock is naturally very hard and slows progress to about 10 - 20m per day.
Gripper TBM’s also have to stop frequently to change their disc cutters;
These disc cutters do all the hard work breaking the rock and wear out needing regular stopping of the TBM for replacement. Not easy at the front of a TBM when they can weigh 100’s of Kg each so a person cannot carry them.
How else can we speed up a TBM? Use slip form instead of precast segments? This technique is used for circular high towers;
However they can only achieve 5m per day so this is not the answer either.
Another method of tunnelling for small diameter tunnels (up to 3.5m) is pipe jacking where concrete rings are pushed into the tunnel using huge hydraulic rams but it is only effective for about 1km before another shaft is required as the rams can only push so much without damaging the rings.
This is not the answer either. Too small and no quicker.
I don't have the answer to going 5 times faster so I will not be getting a job with Mr Musk for now to get TBM’s to go faster. If I did have the answer it would revolutionise tunnelling.
However, in Mr Musk’s recent TED interview regarding the Los Angeles tunnel project he definitely mentioned reducing the cost of Boring by a factor up to 10 which may be much more possible.
Scale is always a way to save money so it would pay to have huge factories reducing the cost of concrete segments.
Bore through material that can be recycled as aggregate and/or sand and used in the concrete segments is a vague possibility but a bit of a stretch that you would be that lucky.
How else to save / raise money?
You also end up with HUGE amounts excavated material on tunnelling projects which cost money to dispose of.
If a lot of tunnels were excavated at once you might be able to build one of these off the Los Angeles coastline and earn some additional revenue for the tunnelling project. \U0001f600
TBM’s also use a massive amount of electrical power to drive them so electricity costs could possibly be reduced by using wind/green generated electricity.
Another cost not mentioned is that when you bore a tunnel is that you have an empty circle that is difficult to drive in.
See all the concrete and track at the bottom? A lot of work (and money) goes into getting that back into the tunnel. Perhaps a method could be developed to mix the excavated material with cement etc as the TBM is moving forward to reduce the need to bring material out only to have to bring it back in again.
In summary there is currently no silver bullet to reduce tunnelling costs but re-using the excavated material would seem to provide the most practical avenue of providing additional project revenue.
Efficiency is the key to tunnelling and is the constant mantra whilst works are underway. I suspect that the tunnelling project that Mr Musk referred to in Los Angeles suffered a lot from poor efficiency and support logistics which are easy ways to lose a LOT of money.
Another way to lose money is to choose the wrong TBM type which is catastrophic or even worse getting it stuck by inexperienced teams.
Therefore, having the right people in the TBM and experienced supervisors planning and executing the works is the single best way to save money.
This is true on most construction projects of any type but is most often where cost cutting occurs.
From what I read I bet Mr Musk does not often cut corners with his SpaceX staff so he should continue this simple tactic with The Boring Company if he really wants to save money.