I have a mini bog set up in a very small preform spillway from Lowe's. It's sitting up on a milk crate in the pond, and has about 100-200 lbs of gravel in it, plus plants.
Well, I was wondering why the gravel level kept lowering, even though no one has taken any gravel out. One side of the preform has gotten so low, the water is about ready to pour over the edge, instead of the spillway.
Today I discovered that the edges and parts of the preform that are not directly supported by the milk crate have been "melting" over the edges, sagging down under the weight of the gravel.
I know plastic is maleable and stretchy, but I sure hope it has a high breaking point, 'cause I don't want a leak - or to have all that gravel and silt spill into my pristing pond.
More stuff to buy: Milk crates from Wal-Mart ... they have a rectagonal one that will fit under the preform.
More stuff to do: Empty bog and move 200lbs of gravel, and reset it up on the new crates.