The Brew Cabin

Great site! Just what I was looking for!

I am about to dig out my old concrete drive, expand the area, and lay new. The old drive was laid in a real patch and make do fashion, no more that 50mm thick, and as best as I can ascertain, there is little sub-base, just the bear earth. That said, there has not been a great deal of subsidence. We are on a hill, and the drive slopes in the region of 1:7. The subsoil is clay with a fair amount of natural rock thrown in (maybe a 8:1 clay/rock ratio) and tends to be very firm in itself. Rainfall is fairly high (edge of Exmoor) and runoff in the form of seepage from the field above is enough for riveulets to form across the current drive after sustained rainfall.

We are not yet fully decided on the final surface, currently toying between tarmac and/or pavers. We would prefer all pavers, but the cost may be prohibitive (RMC Priory is our preferred, Charcon's Woburn Rumbled otherwise). Total area is approx 130sq m.

The drive is regularly used by 2-3 vehicles, one being a long wheelbase Landrover. Because of the site layout, in and out requires 2 or 3 point turns, so the surface needs to withstand the skewing forces as well as straight up and down. We also need to occasionally take your average modern tractor (which seem to get bigger by the day) over the surface for hay deliveries and the like. Therefore, you could be looking at 5-10 tonne loads at times (maybe 6 times a year).

Most of the literature suggests a 100mm subbase, and for pavers, 100mm to allow for block and bedding layer. I suspect that because of load etc, I should be looking at a 200mm sub-base. I'm planning on excavating as required, back filling with the old drive material (probably providing approx 50-75mm depth), and topping up with 40mm scalpings, which will be compacted.

Is 100mm ok, or is more required? If we choose to go with tarmac as the surface, what depth of base and wearing layer should we aim for, and if pavers, should they be 50, 60mm or greater? Would you advise flexible or hard construction? The entire drive area is surrounded by walling to act as a fixed barrier to any paving. We have thought of mixing pavers and tarmac, so that the pavers form an edging, maybe 500mm wide around the tarmaced area. Which is easier to fit first? The other concern with tarmac is that in the winter, any rainwater tends to freeze on the surface, creating a 1:7 ice rink. Can tarmac be finished with a decent rough surface to help with grip?

And, finally (phew!) beyond the driveway to the field, we plan on using grass grids (like RMC's uni-green) to provide a natural but hardwearing surface. What base should be used for this, given that the tractor and land rover need to pass over this. This is another 100sq m.

Thanks very much in advance.
Dave


(Edited by dmeehan at 3:37 pm on Aug. 5, 2002)

Hi Dave,

where to start?

Well, if you opt for block paving, from what you've told me, I'd strongly recommend a 150-200mm sub-base, and using proper DTp1 rather than them owld scalpings, which can be highly variable in quality. You could, I s'pose, excavate, and then use the better stuff from the existing driveway as a capping layer, but make sure you have at least 100mm of well-compacted DTp1 on top of a good 50mm of the owld stuff.

I can't say this too often, but the biggest cause of settlement and ponding with block pavements is sub-standard sub-bases. It's a false economy to save a hundred quid or so on cheap, nasty "hardcore" rather than the proper stuff, especially if you're anticipating exceptional loads (for a domestic driveway).

So, 150-200mm sub-base, then your 35-50mm bedding layer of grit sand, and then a 60mm paver. There's no need to use an 80mm, not for the occasional tractor, but, if you have any other HGVs, such as a CH oil delivery tanker or the septic tank emptier, then it might be worth moving up to the 80mm block.

But what about blacktop? I'm wary about blacktop on a steepish gradient for the very reason you mention - icing in winter. You can get a 10mm wearing course that will give better grip, or you could have a hi-grip surface dressing added, but I don't think it would work out to be cost effective against a tumbled block paving such as the Priory/Woburn offerings.

Spec for bitmac would be, again, a 150-200mm sub-base, with a 65mm base course of 20mm dense and a 25mm wearing course of 10mm dense.

And then your grass pavers. These need to be laid on a free draining medium or they just turn to mud and gloop. You need a decent sub-base, with adequate drainage, so perhaps 150mm again, with a drainage composite if the ground is wet. You need a grit, not a sand, but a 4-6mm grit on which to lay the grass pavers, and then they need to be filled with a quality sand/grit/soil mix, not any owld topsoil you can dig out of a field.

To be honest, I think you need someone to look at the site and give you the benefit of their experience. You've a lot of factors to take into consideration, and, with the grass pavers, this is not a standard job, needing a good bit of branewerk before breaking the ground.