Filling deep holes in the floor

[Two Sheds]

I have a 1960's house which used to have a hot air ventilation heating system, that was partially removed by the previous owners.

By removing the heater and fans, they exposed three large holes in the kitchen floor that had provided access to the (sheet steel) ventilation ducting.

Instead of filling the holes they covered them with chipboard and lino. I'm now looking at having the kitchen flooring replaced, prior to installing new kitchen units.


The holes in the floor are of various sizes (the largest is about 50x50 cm), however they are all 70cm deep.

I'm wondering how best to fill them in(?)

70cm of solid concrete seems a bit much, so I'm thinking well-packed rubble, gravel and a layer of cement? (if that sounds good, what depth of cement?)

Also would I need to remove all the steel ducting (which still lines some of the holes) beforehand?

(The concrete walls of the one hole which isn't still lined with steel feels quite dry - but I'm also wondering if I need to put damp-proof membranes in the holes?)


Thanks in advance for your advice.

 

[23vc]

Soil, mot layer, sand, dpm, concrete, screed I’d have thought. Or without the screed. Don’t think you can leave the dpm out.