Forum: Other Pavings
Topic: What exactly is mot type 3 hard core?
started by: Cazzie

Posted by Cazzie on 21 July 2019,15:03
I want to build a tiny raised parking space where I currently have a yard. It will be approx 5x2m. To avoid having to go to planning I would like to make it permeable. Apparently this means the cheapest option is MOT type 3 hardcore - possibly finished with some gravel on top. My main question is can anybody tell me exactly what constitutes MOT type 3? Is there a specific size range and material? My parents have a load of hardcore they've just dug up and I was going to sift through and rescue what I could to use in my drive but not sure what I'm looking for.

If anyone knows too - my plan is to dig down through the concrete yard that is currently there to soil, pop down a permeable membrane inside retaining walls and then infill with the hardcore. The soil here is clay though - so how permeable this is actually going to turn out to be I will only find out once we dig down. If it doesn't drain well can I just connect an overflow pipe back to my rain water collection pipe which is close by? Does this sound like the cheapest and easiest route to getting the job done?

Many thanks!

Posted by jwill on 22 July 2019,14:01
Google will tell you exactly. Isn't it like 60mm down to 20mm no dust?
Posted by Cazzie on 22 July 2019,15:08
Cheers jwill - tried googling it forever but still don't seem to have a clear answer. It's also called 4/20, and is said to be '40mm with no fines' but I don't know what this really means :blush:
Posted by jwill on 22 July 2019,18:04
You might struggle to source that amount in the quantity you will need. It's not something a merchant would stock in a bulk bag. You'd either be taking 10t delivery or collecting loose in tipper from quarry/works. An easier option would be look to drain the surface water somewhere permeable. Alternatively if there is no restrictions on having gravel within first 5m of boundary then a type1 subbase with geotextile and decorative gravel would satisfy planners. Ok on paper type1 is not permeable but realistically you little 10m2 patch isn't going to create any significant run off with a gravel surface
Posted by Tony McC on 01 Aug. 2019,11:43
< DTp3 >

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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