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Topic: Covering drainage cells, Slabs on top of cells, is it a good idea< Next Oldest | Next Newest >


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Joined: Sep. 2017
Posted: 06 Dec. 2017,21:52 QUOTE

I am installing 3 attenuation cells under my new block paving drive, is it appropriate to cover the tops (500mm wide) with a line of 5 in No. 600 x 600 x 38mm paving slabs before back filling, this could reduce the depth of hole dug, reduce disposal, & amount of backfill required. ( I do have a tendancy to do 'belts & braces') and the working space available is very limited. The vehicle using the drive will be a 3.5 t. transit. Or would it have consequences I haven't thought of?

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Tony McC

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Posted: 09 Dec. 2017,10:49 QUOTE

How does the use of flimsy 38mm flags reduce the dig depth?

The usual spec is for a minimum 500mm of cover over and above the geo-textile wrap. Now, if you're using a typical block paving spec, you'd have 250mm (or thereabouts) in the build-up, so the extra 250mm is simply additional capping which will help ensure the stability of the paving over the attenuation cells.

If your plan was to put the wafer flags over the geo-textile (they would need a sand cushion at the very least to minimise rip risk) and then the 250mm build-up, I'd be concerned that any movement of the flags as the structure beds-in over the first few months, would be reflected through to the pavement surface.

There are sound engineering reasons why the typical spec is for 500mm cover, so to reduce that with a standard crate could be asking for trouble. There *are* shallow crate systems that act as a virtual sub-base replacement (see here) but they are engineered to be used at shallower depths and so are stucturally stronger than standard crates.

Further, by reducing the overall depth of the crate, you could be compromising its efficacy: will you have sufficient penetration of the free-draining sub-layers of the ground?

Personally, for the sake of 250mm, I'd rather play safe and do the job properly....but the choice is yours.

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