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Posted: Tue Oct 19, 2004 10:54 pm
by johnsimkin
Sir, I'm attempting to block pave my drive which is on a gentle slope and on clay. When dug out to 200mm it was quite firm but now with several days of heavy rain it has gone soft on the lower half. I've just dug a collection drain (not yet finished) which has remained dry. I used a geomembrane (TDP 115) and 50mm of scalpings compacted to give me a decent surface to walk on when installing the edge course. Although the scalpings are quite tight the sub grade is still soft and visibly moves underfoot. Logic tells me that this is bound to be the case with only a thin layer of scalpings so far but I'm still worried about the sub grade. I've followed your site very carefully and note that this problem is not uncommon but I'm not sure that I've yet found the solution. As it's not going to dry out and more heavy rain is expected what can I do? Is it safe to carry on and do the edge course in this condition? How long should I leave the edge course to dry before installing the rest of the base? Will another 150mm of scalpings cure it? Is this what flexible construction really means? I'd be very grateful for any advice, and lost without it! Sorry if I've posted this in the wrong place-I'm new to this. Fantastic site. Thanks.
Posted: Wed Oct 20, 2004 1:06 pm
by Tony McC
This is a very common problem during the wetter months, and I know from experience just how bad it can seem, but it's one of life's mysteries how the sub-base and the completed pavement suddenly become firm and sound, almost while your back was turned momentarily.
The key thing is to ensure that there's no slutch on the surface of the sub-grade before placing the sub-base material. Obviously, you're past this stage, and have done everything right by placing a geo-membrane and using a 'blinding' of aggregate to keep the site clean while the edge courses are laid. Don't worry overly about the drainage of the sub-grade and sub-base - if the ground normally drains itself (ie, it is not prone to waterlogging) then it will do so again, once all the construction work is completed.
So: for now, get the edge courses finished and then you can crack on with the rest of the sub-base. If you're using concrete bedded edge courses, then 24 hours is adequate, but to be honest, if you're careful and don't put a plate compactor or roller near the edge courses for that first 24 hours, then you can get the rest of the sub-base in place immediately after completing the edge courses.
Using 150mm of sub-base should be plenty for a residential driveway, and you should notice a significant degree of firming-up once the sub-base is compacted, but don't be alarmed if there are still one or two spots that seem to move slightly. Given all the disturbance and the saturation of the sub-grade, it can take a week or two for normal 'stasis' to be regained. As long as it is only 'slight' movement, you can carry on with the rest of the paving work.
However, if you find that there is 'trampolining', where half-a-square metre or more of compacted sub-bases seems to 'bounce' when you walk across it, then remedial work might be required. If you can, give these areas an extra few days to see if they stabilise - they often do - but, if there's no real improvement after a week, then digging out and use of a cement-bound capping layer beneath the mebrane may be what's needed. This is a route sometimes used on site when we have to complete an area to schedule and don't have the luxury of hanging around for a week to see if the conditions heals itself.
As I said back at the start, the way that the pavement firm up is still a cause of awe for me, after 30-odd years of seeing it happen to pavements we've built. Maybe there's mason pavior elves that only come out at night, I don't know, but I do know that even the muddiest, slutchiest, quagmiriest bog can suddenly become firm and traffickable once the sub-base starts to function. 