Hi @LP1,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Firstly, where were you planning on installing a border?
Defining the border with Garden Edging is a great place to start, as it will define and enclose the area, allowing you to calculate the volume of materials you'll require much better. You can use a variety of things, such as concrete garden edging, steel edging, retaining blocks or timber sleepers, depending on your preference. Once you've defined the border, you can move on to filling the area.
The best thing you can do for drainage in that low area would be remove the concrete, as it is blocking the ability of water to drain down into the soil. Obviously, this would be a lot of work, but it is certainly worth considering.
If you couldn't do this, then instead of using road base, I would suggest you start with a layer of drainage gravel. It is still compactible, although not quite as compactible as road base, but the larger stones will mean there are wider channels between the stones, which will allow for better drainage. I would suggest putting a 50mm layer of drainage gravel at the base, compacting it, then adding road base over the top, before using paver sand over the top for the final adjustments and levelling. The paver sand should not be compacted, but rather levelled with a spirit level or screed.
The drainage gravel and road base should be poured in 50mm lifts, compacting as you go to ensure it is solid from top to bottom. For 190mm of height, I would lay 50mm of compacted drainage gravel, with 100mm of compacted road base and 40mm of paving sand on top.
So, essentially, you would follow the guidance in How To Lay a Base For Pavers, just add additional drainage gravel beneath to assist with drainage.
Let me know if you have any further questions.
Jacob
