This is a really (not so) boring story about Elon Musk’s new venture, the Boring Company. Musk, who is the modern day Iron Man founder of Paypal, Tesla, and SpaceX; wants to shuttle people across Los Angeles in subterranean tunnels. Specifically, he aspires to transport your vehicle aboard an electric cart on an underground track at speeds approaching 200 kilometres per hour.
Musk has said that he wants to create a 3-dimensional network of tunnels to alleviate congestion. He also said, “there is no real limit to how many levels of tunnel you can have.” His strategy is to build up to 40 layers of underground streets to clear up congestion. He also said that more efficient boring machines would make tunnels orders of magnitude cheaper to construct.
Will this endeavour ever succeed?
On any typical day I’d say that the man is insane: but then Musk has built self-driving Tesla’s and rockets that land themselves. I’m only a journalist trying not to get fired.
To put this project in perspective, consider the notorious Boston Big Dig. The project began in 1991 and wasn’t completed until 2006 at a cost of $19.7 billion dollars. The main objective of the project was to cut a 5.6-kilometre long tunnel beneath downtown Boston to reroute Interstate 93.
In the fifteen years consumed constructing the tunnel, ten lanes of traffic were built. The new tunnel displaced 16 million cubic yards of dirt. That is 541,000 truckloads of filth, or 16 stadiums worth of mud. The dig also consumed 3.8 million cubic yards of concrete. You could build a sidewalk from Boston to San Francisco three times with that volume. The venture also installed more than 26,000 linear feet of steel reinforced concrete slurry walls. These walls formed the walls of the underground highway, and supports for the above ground interstate 93. That is eight-kilometres of slurry walls, the largest application of the construction technique in history.
Slurry walls are reinforced concrete structures typically built in soft water saturated soils. Slurry is used to support excavations that are later reinforced with steel and filled with concrete.
The dig also consumed steel in such volume that it could have been formed into a one-inch steel bar and wrapped around the equator.
It gets better.
Major construction projects devour vast quantities of resources, but the logistics can get much worse. The project relocated 46 kilometres of gas, electric, telephone, sewer, and water lines maintained by 31 companies. It also installed 8000 kilometres of fibre optic lines and 321,000 kilometres of copper telephone cable.
No doubt, his customers will still need to breath. Boston built a seven building ventilation structure to aerate the tunnel, one of the largest in the world. This structure was further assembled within a cofferdam. Cofferdams are temporary water tight structures constructed to restrict water prior to construction.
Of course, this project needed manpower. The dig employed 5,000 staff at its peak and 150 cranes. The work was divided across 118 construction contracts and 26 drilling agreements.
Elon Musk is famous for his tenacity and fabulous wealth. It could be a safe bet that in a few years he will get it all started.
A look at what’s involved
Not withstanding his outlandish claims, this is where tunnel-boring machinery stands today. Drilling in urban environments usually involves soft ground excavation near the surface. There are three types of soft ground boring machines: Earth Pressure Balance, Slurry Shield, and Open-Face. All of these machines use thrust cylinders to advance forward by pressing against concrete segments. Cutting heads are typically a combination of tungsten carbide, carbide disk, drag picks, and hard rock cutters.
The Earth Pressure Balance machines uses controlled drilling and hydrostatic pressure to maintain balance. Often additives are injected ahead of the cutting head to strengthen the earth prior to excavation. Additives can be polymers and foams, designed to solidify the ground into a solid plug. Earth Pressure Balance machines use an Archimedes screw as a drill head.
The Slurry Shield machine uses pressurised slurry that applies hydrostatic pressure to the excavation face. These machines are very sophisticated because the contaminated slurry must than be recovered and filtered. The filtration is rarely accomplished on location or even within the excavation.
The Open-Face machine can only be used in soft ground that is self-supporting. This limits its use to ground with low water inflows and high rock strength. The ground is removed using a backactor arm or cutting head. The machine is advanced by periodically jacking the shield forward. The ground is then cut and the process is repeated.
An enormous liability
One major obstacle of urban excavation is ground subsidence, sinkholes. Which is bad for morale. Towards this end Earth Pressure Machines and Slurry Shield machines are ideal. Not surprisingly, there is enormous liability involved. Shallow excavations are at high risk of instability, collapse, and deformation.
Sources in the industry have stated that 60 feet per day is the average speed of a machine. This is assuming that the crew is running two or three shifts and excavating 20 hours per day. On good days rates can be as high as 200 feet can be achieved. On bad days, don’t ask.
One unresolved issue within the industry is production techniques. While machines can excavate up to 200 feet in a day, transporting the whole assembly line is a serious challenge. Workers need to fray with the advancement of support wiring, conveyer systems, and plumbing to raise production. Changes in the strata only compound the logistical complexities of running the enterprise.
So when might we see Musk’s vision come to life?
At publication, the Boring Company has produced no usable tunnels. The boring story is that Musk is at work excavating experimental tunnels beneath the campus of SpaceX. When and where any production tunnels would be bored is total speculation. What is not speculative is that serious engineering challenges will need to be resolved before any production can begin. Drilling entails high risks in urban areas, and with high risk comes even higher costs. One thing that Musk is notorious for is transforming high risks into high rewards.