Answer: Turn it into building material for low-cost housing.
He’s already revealed what he wants the company to do with the rocks it digs up while tunneling, so it should come as little surprise that the head of The Boring Company, Elon Musk, has a plan for the displaced dirt as well.
Via a May 7 tweet, Musk announced that said dirt will be repurposed as bricks for low-cost housing.
New Boring Company merch coming soon. Lifesize LEGO-like interlocking bricks made from tunneling rock that you can use to create sculptures & buildings. Rated for California seismic loads, so super strong, but bored in the middle, like an aircraft wing spar, so not heavy.— Elon Musk (@elonmusk) March 26, 2018
The idea appears to be in its infancy, so the actual feasibility of bricks made from Los Angeles dirt is yet to be determined. However, given the fact that Musk has delivered on previous "out-there" promises like Boring Company flamethrowers, there is a good chance that Musk will make it happen.