Patio over swelling clay - Question about jointing flagged patio

[willdean]

Hi,

I'm about to lay (have started groundwork and drainage) a patio of about 6m x 8m. I was planning to use fairly ordinary 'riven' style slabs, in a 'random' layout with approx 10mm pointed mortar joints.

The ground is VERY HEAVY clay, and is notorious for its lively behaviour (neighbour's extension requires underpinning within 6 months, all the victorian houses covered in cracks, Edwardian paved area in my garden all lumpy and bumpy, etc)

I am starting to have some doubts about the wisdom of trying to build what is effectively a 40mm thick unreinforced concrete sheet of this size on this ground. My first concern is that all the pointed joints will fail very quickly, and I shall be endlessly cutting them out and repointing (or at least feeling that I should be...) I suppose if it moved very badly, I'd lose the fall/drainage as well.

So, should I:

a. Stop fussing and get on with it like my wife tells me.
b. Use a different jointing technique (e.g. sand with chemical binder?)
Or
c. Use some kind of additive in mortar joints to make them more flexible.
Or
d. Do something different altogether (flexible block paving for example)
Or
e. Lay the whole lot on top of a reinforced concrete pad (only joking!)

What do y'all think?

Thanks,

Will

[alan ditchfield]

I take it by saying it has a lively behaviour you meen you have a shrinkable clay soil (or plastic soil), the best way of solving this is to remove any nearby trees and install some seriious drainage system otherwise you are looking at serious reinforcement.

[willdean]

Thanks for the reply.

Yes, I used term 'swelling', but I guess that's just the winter version of shrinking

There aren't any trees nearby, and the patio will be enclosed by an 'L' shape of the house.

Although the site is covered in silted-up clay-tile land drain, I'm skeptical about drains which run within the clay, though maybe they're better than I think.

There has been a patio here for years, but it's of the 'completely insane' paving variety, and it's impossible to say whether its unevenness is the result of heave or its dismal construction. (I think the latter is favourite)

[alan ditchfield]

Shrinkable clay soils only exist in certain parts of the country and are exagerated usually by tree roots removing large quantities of water at certain times of the year, i would be sceptical that this is your problem if there are no nearby trees. Try and contact your local planning office and ask them if this soil is in your area they will know because it will have caused many problems in the area before. If it is not then all you may need to do is install a deeper sub base under the patio and make sure it is well compacted.

[willdean]

Thanks for that - reasonably encouraging!

Assuming that I shouldn't have a *major* problem, would you still think it was worth persuing something more flexible than mortar for jointing?

Thanks!

[Tony McC]

Setting aside the 'lively' sub-grade issue, there are two possible solutions that spring to mind. Firstly, if you're dead set on using riven flags, then make it a rigid construction by laying them on a 100mm bed of concrete. At 6mx8m, you could create two bays @ 6x4m with a movement joint between the two, and be fairly confident that if there was to be any movement of heave, the slabs would move independently and take the flags with them. It is possible to create a movement joint that follows the 'stepped' jointing found on a riven patio, so that you don't have any full flags bridging the joint.

Secondly, you could opt for a flexible pavement, such as block paving. If you really prefer the 'stone' look, you could use some of the stone setts that are available. These are laid in exactly the same manner as concrete (or clay) block paving, and behave in just the same way, moving as the ground moves and accommodating any minor change in level.

Cost wise, there's not much between the two options, so it's a matter of which you prefer and which one you feel most confident in constructing.

[willdean]

Thanks Tony. Although you say 'setting aside the sub-grade issue', surely your advise to lay on 100mm of concrete is *because* of the lively sub-grade? Or have I missed something else?

I think we do, on balance, prefer the look of the slabs, rather than the block paving, even when one avoids the concrete-and-herringbone Tesco's car park effect.

I'm not going to able to mix 2.4 cube of concrete and lay 24 sqm of paving in one go (one bay's-worth), so I will need to think hard about all this.

[Tony McC]

The sub-grade issue I referred to was that of whether the clay was swelling for whatever reason. If you say the sub-grade is lively, then it's lively, and there was nothing I could add to the previous comments regarding reasons for the liveliness.

Anyway, if you go for the slabs, one option would be to cast the concrete slabs in one go and then bed the flags onto full mortar beds on top of the slabs, doing as much or as little as you feel like in each shift. Obviously, you'd not be able to align the movement joint with the flag jointing, but even with flags spanning the MJ, the amount of paving and jointing that coule be directly affected is greatly reduced.

[willdean]

OK, thanks very much. I don't think I can stand the scale of the groundwork involved in a concrete slab, nor the difficulty of getting ready-mixed concrete to it, so I might look again at block.

I don't know of any way of telling if the ground is a really problem or not (presumably one can do expensive things with scientists and consultants), so I might as well assume the worst.

Thanks for the help.

[alan ditchfield]

You can ussually have your soil tested for less than a tenner at your local uni or agricultural college if you have one, just to set your mind at rest, if not Tony s advice would be the best.

[danensis]

As someone with a house on two crossing beds of highly shrinkable clay I can tell you that tree roots are not the only problem. In the summer the soil bakes hard as concrete, and cracks appear which go down for half a metre or more. In the winter (or when it rains) the cracks close up and the ground heaves by several millimetres.

[alan ditchfield]

Its not the tree roots that are the problem it is the water they take out in summer and the water they dont take in the dormant season that exagerates the problem, as you say it is the lack of water that shriks the soil. When the trees are dormant there is usually more rain and the two together are a bad combination when plastic soils are involved.