Posted: 10/05/2016 at 23:39
Hi, I don't think there will be any issue with filling the bottom of the hole with hardcore, and it will make the ground above more free draining. The only issue could be that in very wet weather, you might find water seeping in from surrounding area where the draining isn't so good, so you will have a patch which always fills with water when it rains heavily and for a sustained period of time, like winter period. You could end up with a bit of a bog garden!! To be honest, there may be no way of knowing until you fill the hole in and see what happens. You should also be aware that after a period of time, the ground may sink as the soil and hardcore settle, so unless you want a dip in the garden (which again, may cause a puddle as it becomes a sump), be prepared to throw in some more top soil to bring it up to the same level as the rest of the garden.
I'm thinking if you plan to line it with a membrane and fill with rubber bark that it will simply fill up with water and because it will be lower then the surrounding area, it will become a huge puddle as the water collected in the membrane won't have anywhere to run to. I think you would be better raising it slightly above the level of the surrounding ground (even 6/12 inches will be enough) so you can allow water to drain out.