Joined: July 2004
||Posted: 01 Aug. 2019,11:43
4/40 and 4/20 are not quite the same thing, although they perform an almost identical function. In essence, they are clean, no-fines aggregates used particularly with permeable pavements.
As JWILL points out, it's not easy to source small quantities of these aggregates, even though we are being moved towards ever greater use of permeable pavements. The BMs only stock what sells regularly and in quantity: just for now, that rarely includes a DTp3 or equivalent, although some of the more clued-up landscape suppliers DO provide bulk bags and small quantity loads.
Your plan for a permeable parking space needs better planning. You would need to use at least 150-200mm of a no-fines aggregate, and then dress it with a suitable clean gravel. A fairly typical spec for parking a small/medium car would be a construction grade geotextile beneath 150-200mm of 4/40 overlain by 100mm of 4/20 and then surface dress with 30-40mm of 6mm or 10mm clean. This would provide some attenuation of accumulkated surafce water, but that clay sub-grade may present a problem that would require some form of overflow or outlet pipe to cop with exceptionally heavy precipitation.
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