Drainage under permeable driveway - Too shallow for correct cover

[Lonemascot]

Hi

I'm about to be laying the sub-base of a 150m2 permeable driveway for a building project that I'm currently working on. I have a bit of an issue with the drainage for the foul water. The invert is very shallow and is only 300-350mm down from finished ground level over the whole driveway. Ground level gradually falls in a similar slope to the driveway.

The architect's Building Regs drawings state that drainage runs under driveways are to be capped with 75mm concrete over 100mm min gravel cover around the pipe. Essentially what that means is the concrete capping will be pretty much right beneath my block paviors in places.

The drawings don't provide me with a build up cross section of what the permeable driveway should be (I've not done one before) but from reading the main site it seems that the sub-base is generally deeper so instead of digging down the usual 250mm I'd be looking at digging down to literally invert level. Will that cause me an issue?

Cant go deeper with the pipe.
Cant build up the ground level due to DPC levels.
I'm basically stuck with laying a driveway around the drainage with very little cover.

I could entirely encase the pipe in a 100mm concrete shell but its a good 30m run with 2 IC's and various branches to take into account as well. I'm not feeling this idea so much.

Has any one got any ideas on a better solution?

Thanks

[lutonlagerlout]

other than what you have said a traditional drive construction with linear channels going to a soakaway with aquacells

this is one of the big problems with permeable paving, lots of services are only around 300mm down so effectively it cannot be done right

LLL

[seanandruby]

At that depth they should be encased in concrete. If the architect is allowing it then maybe you could dig half bore and replace with no fines concrete over and sides of pipe, this will keep it permeable and then place the one size no fines sub base. A composite layer of geotextile to a carrier drain to soakaway.

[Lonemascot]

I had thought about going down that route but finding a space for a soak-away 5m from a building is going to be an issue.

I am especially keen not to use the permeable blocks seeing as this system is a lot more expensive than traditional blocks. I've priced materials at £3000 for a traditional system with soak-away and nearer £7800 for permeable!

The other thing I was thinking of was a sort of hybrid system but its not documented anywhere as being SUDS compliant (or at least from my searches).

Basically, use the manufacturers minimum depth of 200mm of no fines 20mm sub base, geo membrane then the 6mm gravel, but then bed on normal blocks (cheap) which drain to the edge of the driveway into gulleys/gravel which in turn drain to perforated land drain running parallel to the drive. This effectively would allow water to drain into the sub-base making it a giant soak-away or an under-the-drive rain garden.

I guess this would only work if the drive was relatively level and if the sub-grade drained effectively.

what do you reckon?

[seanandruby]

Do a permeability test on the sub grade first.

[Lonemascot]

Footing trenches dug to 1500mm were bone dry, even overnight. Soil investigation for some piling we've got going on the site suggests the water table is 10m+ below ground with clay being found at about 12-13m. The ground drains very well indeed.

[seanandruby]

Sounds ideal for permeable.

[Lonemascot]

Today I spoke with the local highways officer and he was quite positive about the hybrid solution I suggested.

Building Control have suggested that I should bed the drainage on 100mm shingle and fill half way up the pipe, then just cap the top of the pipe with concrete to a depth of 100mm from pipe crown for protection. Essentially this gives me the depth needed to lay my blocks and dig down deep enough either side of the pipe to get the type 3 in.

[Tony McC]

I'm finding all this to be dramatically over-engineered.

The cost differential between permeable-type blocks and conventional is a couple of quid or thereabouts. It shouldn't be, they should be the same price because they are no more difficult to make, and actually use less concrete per m² than conventional, but the sell in smaller quantities, so th eprice is, almost inevitably, higher.

But the bulk of the cost difference between permeable and conventional paving comes from the sub-layers, nit the blkocks themselves, and as you are basically constructing a permeable sub-base, why would you then over-complicate matters by using conventional blocks which *will* lose the essential jointing to the open-textured bedding/sub-base and so become loose and useless?

It seems a recipe for failure to me.