I need to make sure that the enormous pit, aka ex-pond, drains before I start to fill it in with rubble, sand etc. The space will then become useable.
Bought a 400mmx12mm drillbit to drill exploratory, hopefully draining holes.
I now know how thick it is. 12" concrete base. The centre has a sunken bit about 8" across and the concrete at the bottom of that is only 6" thick. Managed to drill about 10 holes in total yesterday - but it still didn't drain. Could be compacted rubble below the concrete, could be clay, could be anything. Ideally I'd remove a chunk a couple of feet square and make sure it can drain, but short of explosives I can't see any practical way of doing that. 12" is too thick for any cutting saw, too thick to break up with an SDS drill or even a jackhammer. Absolutely ludicrous.