excavation

[paving]

if my drive is 20mtrs long and 10 mtrs wide and i am diging out 500 mm how much hardcore do i need to fill back up

[jake-the-peg]

This site recommends a 50 - 150 mm sub base.

http://www.pavingexpert.com/faq_subbase.htm

how much you need will depend on the size of the hardcore pieces

[gazzawazza]

20 x 10 x .15 = 30m3 of hardcore. Those big bags are about 0.8 of a m3 (check with your builder's merchant) which would make about 40 bags which seems a lot. Are you sure it's 10m wide? That seems very wide for a drive.

[Top_Gooner]

I would recommend a hardcore layer of 250-300 depth, for trafficked areas. Ensuring all/any topsoil has been removed. This must be rolled or vibratred to ensure good compaction. I would think you may need to add perhaps 5% to the bulked measurement to allow for compaction.

Assuming 250mm x 10m x 20m = 50 cubic metres plus 5% gives a total of 52.5 M3. Which seems a lot.
I would order this in bulk and not by the tonne bag as it would be considerably cheaper. A decent builders merchants or indeed, a local reclaimation/skip company would probably be your best bet. don't forget to shop around as prices vary considerably.