Green or white (trash)

So we've been renovating and renovating and renovating. Years of renovation. Tearing down and building back up.

And in that process, we've tossed debris left and right.

But something about the bathtub ... the concavity, the depth and width and height - it sparked an idea.

So we hauled out to the back, and voila! An extra one-third of my planting space is born.

One thing nags though - is this a "green" solution? Or just plain white trash?