Where did they put all the soil when they dug the London Underground?

When tunnel boring machines came into usethey used some of it to mix with clay slurry to create the solid walls. Some may be left as is as an earth pressure balance shield. As for the rest, landfill is the most common, they also recycle parts of the soil makeup like the silica and minerals but where you ask.... hmmm, this is stumping me.......