Pond Shelves - Recommendations Please?

I am constructing a 16 sq m butyl-lined wildlife pond. I have built shelves
200mm (8") deep and 500 (18")mm wide. Because the pond is quite large and
will need many marginal plants I intend to plant direct into soil on the
shelves. I will try to shape the soil under the butyl to provide a small dam
to retain the soil.

Is this reasonable? Any recommendations?

thanks

Davy