World costs of tunneling and Elon Musk Boring Company

Pedestrian Observations has a comparison of world subway, tunnel and rail costs.

There are updated comparisons of construction costs.

In non-English-speaking developed countries, the typical range for urban subways is $100-300 million per km, with a few outliers in both directions.

In the US the cost of tunnels and subways is about $1.5-6 billion per mile.

1. Elon Musk's Boring company is looking to cut the diameter of the tunnels they dig, moving from the traditional tunnel diameter for passenger vehicles of 26 to 28-feet to a 12-foot standard diameter which would be sufficient for the Tesla skate. On the surface, this might not seem like a lot, but cutting the diameter by 50% cuts the cross sectional area by a factor of four. This is significant as the speed and cost of tunneling is largely driven by the amount of cross sectional area to dig. Being able to cut out 75% of the time associated with digging comes with enormous cost savings.

2. Traditional machines dig, slowly and incrementally, then stop to install reinforcements to the walls. The Boring company will install the reinforcements continuously thus eliminating the need to pause operations. This integration is expected to increase the speed of the overall process by as much as 50%.

3. Current digging machines are nowhere near their power and thermal limits, and is looking to 'jack up the power' to the digging machines. This will increase the speed by a factor of 4 or 5 on top of the other improvements.