[Earthink]

Hi, I have a client that has these massive planter boxes built into a three tiered wooden patio on a sloped property. These are so big that if empty, two people could stand in them and have 3'-4' walls extend above their heads.
They were filled with soil but keep sinking every year because they are bottomless, which will never end because run off will continue to sweep the soil along. Right now the soil level sit's almost 1'-2' feet below the top in some boxes.
So, I need to come up with a clever idea of how to sink a liner of some sort (suspended) into these boxes (odd angles and shapes of course) to create a basket, if you will, to hold soil and plants. Removing the existing extremely root bound soil isn't an option.
The boxes are lined with heavy duty rubber liner and there is electrical wiring running through them for the lights...I'm thinking we just leave the soil in there and hang something above it without compromising the boxes themselves. Any idea's or any suggestions on some materials we may use?

[phototropic1]

Well, what are they trying to grow in those things? If it were anything other than trees, I suppose, you could build a bottom for them several feet down from the top in the same manner that you'd frame a deck.

[Fine Edge]

That would be my suggestion, build another 1/2 planter / wall below the existing planters to stop the constant run-off of soil.