Hollow.
Sorry if the other explanation wasn't clear.
I fix the foam to the walls first, now as most ponds aren't built true square there may be a gap that needs filling in the corners. It can either be filled with a slice of foam or bridged with a fillet. (the fillets also aid water flow round the pond as someone else has said.)
The fillet is cut say 4,5, or 6" wide and the back face has 45 degree chamfers sanded or cut off it with a stanley knife. When this is fitted to the corner it fits flat and flush if you see what I mean.
Tried to find some pics on my pc but can't at the moment. Also there is no need to backfill with more foam, cement, concrete or anything.
I used to fix bulkheads into RIB's (rigid inflatable boats) that used to get hammered against waves at 30 knots so have some idea of the forces involved and the strength that can be achieved with a PU/glass combo.
Don't get me wrong, Kingspan is ok to use but the stuff I use is more workable in my opinion. No need to take foil off, easily sanded, cuts better without lumps coming off it and bends round corners like Beckham can bend a free kick! This is me in an interceptor pit with a 1m diameter pre lined with 12mm PU.
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