Frankly, I expect the wooden floor rotted, and they filled up the cellar with rubble and rubbish, and poured concrete on top. The rubble has probably settled since then, and the concrete might be wedged against the walls and staying in position out of habit. It might crack or subside. It is very unlikely to be reinforced or to meet modern standards.
Reminds me of the hotel I worked on in the 80's- it was converted in the 60's and the basement was dug out and a cast iron drain line put in . When we were searching for the drain smell source and found a 4 inch uncapped end we also found where the tons of dirt/rubble went - into a lower cellar @ one end of the building where the uncapped drain was - just managed to get @ it across piles of rubble going up like a stalagmite to the hole where it was all poured in . If skips had been around then ( most smallish builders had a lorry) who would have spent £000's shifting muck away when it could be lost on site.
Hmm, comments aren't promising. I'm going to see whether I can find more plans on the building. Apparently it was Grade 3 listed when the pub was converted so there might be some documentation about it.
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