Soakaway tips

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Need to dig a soakaway, to drain about 20m2 of block paving. I’ll fill it with rubble rather than buy the plastic crates, as I’ve got a big pile of it already.
Reckon a cubic meter is good for this? Or can get away with less or need more? It’s not clay, more sandy clean soil.
How small would you break the hardcore up? Half bricks kinda size ok? Get smaller as you get to the top?
cheers.
 
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You can do rubble filled for less than 25sqm but a hollow soakaway is a better job - see here for good advice https://www.pavingexpert.com/drain08

Part H of the building regs also has the calculation for volume.

For rubble filled the old NHBC is best https://nhbc-standards.co.uk/2018/5...-below-ground/5-3-11-surface-water-soakaways/

The rubble is to create the storage volume of the soakaway so you want consistent size - smaller bits will fall down into the rubble and reduce the storage volume.

Typical spec stone is type 3 which is no fines, around 40mm. But as long as it's free draining anything will do, bigger bits just means better storage volume ... in the range 30 to 80mm should be ok. Give it a rinse and get rid of the little bits/dust.

Line the pit with geotextile and then dress the top with something to stop soil falling in.. NHBC suggest concrete cap.
 
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Hardcore size don't matter, it's not a filter. Whilst bigger gaps potentially mean faster percolation, the issue with large pieces and voids is the potential for the pieces to move and consolidate causing the surface to dip.

Most important is to line the sides and top with a thick quality geotextile to delay the ingress of fine grit and debris.
 
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