Removing the dirt is around 15% of a boring company project cost. Musk's people figured out they could mould the dirt into bricks and sell them for a dime apiece. A liability could be turned into an asset if it works.
https://electrek.co/2018/07/13/elon-musk-boring-company-bricks-dirt-tunnels/
https://twitter.com/boringcompany/status/1017769225555972096
https://electrek.co/2018/07/13/elon-musk-boring-company-bricks-dirt-tunnels/
https://twitter.com/boringcompany/status/1017769225555972096