Laying a drive on chalk - Advice appreciated

[mfox999]

Hello, I am a newly registered amateur. I am in process of creating new driveway. I am on a hillside, and new drive runs across it. I have put in reraing walls, back filed the highest part with bulding rubble (some of my internal brick walls from another on-going project!) and backfilled the rest with chalk ( needed to get rid of it from back garden). The minimum depth of chalk is 150 mm, going up to 1m. the chalk has been compacted well in layers and has formed a solid mass. I am a bit high so looking not too increase height too much. Given I have such a good layer of compacted chalk, can this act as the sub base, do i really need another 150mm of compacted stone? any thoughts appreciated

Mike Fox

[seanandruby]

I am wondering about chalk as a sub base. Although it compacts well, and drains well it tends to hold moisture. i would be concerned about frost heave. down here in the south we have it as a sub grade so would be cost effective. we tend to concrete of it for footings, as it is solid virgin ground. For a drive I'm not certain. I would be tempted to lay a sub base. Iam sure this was discussed before so may be in tha arhives index.

[Tony McC]

The sub-base creates "an underlay": it's more than just a load-bearing layer; it evens out any discrepancies and creates a level surface on which to lay your blocks/flags/tarmac or whatever.

If the chalk backfill is as well-compacted as you say, the you might not need 150mm sub-base: you could get away with just 100mm, but I'd still be happier to see a sub-base than to lay directly onto the backfill..

[mfox999]

Thanks for replies, I can certainly say it was well compacted, I have just dug it up and relaid it over a larger area to reduce overall height. I originally wet it as I was compacting it, it forms a solid mass thats difficult to dig. I have just come in from compacting the relayed chalk, again wet. Messy sticky mess. Sean, i see what you mean about it olding water, it takes ages to dry out, and when waterlogged will move in a plastic fashion. I will take your advice and go for 100mm sub base. It will be a better base than my house is built on, bricks straight onto the chalk! Is there anywhere I can compare costs of various surface finishes out there anywhere? my new drive is 160m2 and i fear going to cost me more than i bugeted to finish.

thanks
Mike Fox