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Elon Musk Posts New Video of 'Boring' Equipment and Company's First Tunnel (cnbc.com)
Elon Musk has posted a new video and several pictures of equipment that will be used to start digging tunnels beneath Los Angeles. There's a picture of boring machine segments that are being lowered into the start tunnel at SpaceX, a front view of the tunnel, an inside view of the tunnel, and a picture of the front of the boring machine that will cut through underground rock. Additionally, the video shows a version of the "skate" that will cary cars through the tunnel at a speed of 125 mph. CNBC reports: The project is one of Musk's latest ventures, which was inspired by a desire to alleviate "out of control" traffic in Los Angeles. He aims to first dig a tunnel from SpaceX headquarters in Hawthorne, California, to the nearby Los Angeles airport. Musk frequently flies from Los Angeles to the San Francisco area, where he runs Tesla. Eventually, he envisions a deep, multilayered network of underground tunnels spanning the city.
An Elon Musk project where the engineering "details" haven't come out yet? Tell me it ain't so.
Simultaneous lining and boring has been a done deal for decades. I refer you to the Channel Tunnel.
I don't know what kind of bedrock they would be boring through but they all come up against something they didn't expect. Doesn't matter if it's sedimentary or metamorphic rock in the case of the English Channel or igneous rock in the Swiss alps. Add to that an area that is prone to earthquakes....piece of piss, right?
I think I've seen enough of Elon Musk to determine that he is full of shit and preys on people who know precisely fuck all about engineering...or anything else, for that matter. That includes the politicians who give him vast subsidies for whatever is the latest delusional bollocks he happens to come out with.
Absolutely correct.
As someone who worked on the Channel Tunnel at the dirty end, I can confirm 'simultaneous' lining and boring has been carried out for the last few decades.
However, there will always be an cyclic element to the operation as the Tunnel Boring Machine (TBM) has to push off of something to get thrust forward and that is usually done by hydraulic jacking off the leading edge of the lining behind the head. The TBM usually thrusts forward to the extent of its jacks, then grips the side of the tunnel and retracts the jacks to allow segment erection before jacking forward once again. The 'downtime' also allows for regular maintenance for replacing cutter heads, drilling forward exploratory holes, lubrication etc.
The TBM also has a 'maintenance train' running on rails behind it, of up to 200m long of connected sub modules, which the head pulls along as it jacks forward. The train contains the logistics such as power packs, transformers, spoil handling, temp storage, grout pumps, spare parts, mobile workshops etc. As the lining has to be the full bore of the tunnel, it has to be broken down into sections that can pass through the centre of the train and then be re-erected behind the TBM cutting head. This erection of sections has to be carried out as close to the head as possible to minimise the length of unsupported ground behind the head for both worker safety and technical reasons i.e. the ground could collapse and trap the TBM from the sides. Add in any water issues or soft flowing ground and the erection has to carried out in a sealed/semi sealed environment as well. As a gap will also exist between the final outer surface of the lining and ground, this has to be pumped full of weak cement mix or 'grout' to fill the voids, both providing a waterproof annular surrounding for the tunnel as well as avoiding any point loads from uneven ground on the lining.
With the TBM head (for a 8m dia tunnel) weighing around 200 tons (with a main bearing of around 30 tons) and lining segments to be handled of 1-2.5 tons ea, (maybe 8-12 segments per metre of lining) you're not playing with small numbers to shift around and construct. Appreciate that Elon's tunnels will be smaller diameter but current tunneling methods are continuously being optimized and the TBM design is matched to the predicted ground conditions, therefore there is next to no chance of increasing tunneling speeds by a factor of 10x as is quoted..
As someone who has worked in hard rock tunnels 10,000 feet underground, where the rock was under some much pressure it was continuously 'spitting' small flakes at you, to shallow 'soft rock' tunnels where you worry about the TBM taking too much ground out and causing sinkholes to appear on surface, I have to call Elon Musk out as full of shit