Yet another sub-base query

[DougieP]

Hi,

I found this site this morning and have been wasting hours of my employers time getting to grips with the basics of laying a patio - fantastic site!!!

Anyways, my query... Ive got a new build house thats built on former contaminated land. The back garden resembles a ploughed field at the moment and the site manager has told me that (due to the former contamination) the top soil is about 1.5m deep throughout.

Im looking to build a 3m x 12m patio, and will probably use a Marshalls Riven slab (I was previously considering Indian Sandstone but what Ive read on this site has put me off a bit).

The advice seems to be that for a standard patio on normal firm ground no sub-base is required, but as I am laying onto pretty unstable topsoil I was wondering if I would be better to plan for the sub-base? Or would a sand/cement layer beaten with a whacker still be sufficient?

Any advice gratefully received.

Dougie

[lutonlagerlout]

top soil is about 1.5m deep throughout.

sounds a lot of topsoil,have you dug a bit out to have a look?
if its that deep you wil have to wait a year or so till it has settled or whatever you do will sink mate,the way a fresh grave does in its first 9 months
other than that you would have to do a concrete raft then lay the slabs on that
check the ground dougie 1.5 m of topsoil will probably drop 4-8 inches over a year,no amout of subbase will stop that
cheers LLL
PS i would take indian sandstone over concrete slabs any day,its just not as good as yorkstone (3 times dearer)

[Stuarty]

I highly doubt it will be 1.5 of topsoil, more likely 1.2 of subsoil and then another 300 of topsoil. But as said above, it will need time to settle.

Ive just finished a job today that sank 8 inches since being formed, 7 months ago. It was vital that it was perfectly level - we were creating a croquet lawn. The part that sank was 4 inches higher than the level after compaction by the digger to allow the area to settle. Just shows you no matter how big a machine you run over it, it still sinks down

[DougieP]

Thanks for the advice guys, Ive taken it and will leave the job until next spring.

Expect more pestering around this time next year!!

Dougie

[mudslinger]

sorry about this question but am about to lay paving slabs in back garden but unsure of the sand requirments as in is the sand mixed with cement or not? also the edge slabs will they move after awhile or do i need to bed them in a different way many thanks

[bobhughes]

Mudslinger

Not sure why you posted on this thread but in any case you need to go to the main site and have a good read before you do anything else.

Good luck with the project.
Bob