Where do you get this whole swimming pool thing in any case?
You have excavated large hole in the ground to allow for your ground floor make up. Lets say we reduce levels based on the following conditions.
The home owner has a finished floor level of 0
To gauge the new extension floor to the existing, bearing in mind the make up advised by the OP was sand blinding say 20mm, insulation say 75mm and a concrete slab say 100mm (in my opinion should be more)
You now need to reduce the external ground levels by 20mm + 75mm + 100mm = 195, lets call it 200mm.
You now have an reduced level dig of 200mm (yes I know the exiting ground levels will play a part in this.
You excavate and cast you foundation, say a further reduction in ground levels of 700mm = foundations at 0 minus 900mm.
You construct your sub-structure blockwork up the DPC level, lets sat 150mm above the finish floor level of 0 = +150MM
This is where the problem comes, you lay a layer of sand (Blinding) to protect the VCL from any potential damage (Not a great idea) The VCL is lapped up the inside of the substructure blockwork to DPC level - This equated to an up-stand of VCL of approximately 350mm.
You install your nice dry insulation 75mm thick
You cast a concrete slab on top your nice dry insulation say 100mm thick (We haven't even discuss the water contamination form the concrete.
You now have a 350mm deep swimming with your 75mm insulation at the bottom and 100mm of concrete over it.
At this point the the works carried out to date are open to the elements. It pours down with rain as advised by the OP and there you have it -
A swimming pool.
The same thing can apply, membrane and VCL wise in both scenarios, as can the whole water ponding thing.
No - The concrete slab is cast on type one compacted stone. You could hose it down with a hosepipe all day long. Once the structure is completed and watertight the VCL in laid over the slab thus stoping any moisture from being on the nice dry warm side of the fabric. The wet slab below will happily sit their and dryout / keep getting wet without effecting the internal fabric.
You can still lay another membrane prior to screeding in both scenarios.
If you instal an additional VCL over the slab there is no possible way of the water escaping as its now trapped between the VCL below & the VCL above - You can't have two VCL - Its a one way street.