Right.

I’ve just laid a 6sq m natural limestone patio in the garden. To get to a layer of rock beneath the topsoil, I had to dig foundations that were about a foot deep, which I filled with Type 1 MOT. It was hard work but I’m glad I did a “proper job” of digging down to rock.

My girlfriend now wants a garden path in the same limestone slabs, but she wants me to dig shallower foundations, in order to save a bit of money on the amount of sub-base material needed.

I’ve said I’m not happy about laying a path onto 100mm of aggregate then, basically, topsoil, and that over time it might subside. She says I’m being too fussy and it’ll probably be fine.

So: Is it overkill to dig 30cm down to rock in order to lay a garden path? Or is laying a shallow hardcore layer onto damp, clay soil a recipe for disaster?

Who’s right?

murder girlfriend, bury under path, saves on backfilling which is her wish surely, and no more arguments. everyones a winner? HTH

Who’s right?

It ain’t you, Captain Overkill

basically, topsoil

damp, clay soil

Which is it? Big difference.

Topsoil = you are right
Clay = your girlfriend is right

You dug down to rock to lay a garden patio?

How fat is she?

Try this approach. Report back on success or early death.

[video]https://www.youtube.com/watch?v=XFWeoxrhbE8&list=PL7884111A46A4A515&index=10[/video]

She wants you to do a job, it gets done your way.

She wants a job done a certain way, she does the job.

The one who does the job dictates how its done.

You dug down to rock to lay a garden patio?

How fat is she?

Ahem. There may have been more than one going in.

Which is it? Big difference.

Topsoil = you are right
Clay = your girlfriend is right

It’s clay soil. You just saved her life.

So: Is it overkill to dig 30cm down to rock in order to lay a garden path?

yes

Our new build had a small patio out back, that was layed on about 10cm’s worth of hardcore and sand, that’s held so I did similar when extending it.

Do it her way, then insist that the subsoil is prone to sinkage and she can only walk on it in her slippers.

Then hide her slippers.

Sex is over-rated anyway.

So: Is it overkill to dig 30cm down to rock in order to lay a garden path?

Depends. How many sheets of rebar did you use?

100mm is ample. It’ll be 150mm deep anyway given the mortar and slab depth on top.

Just apply the principles of pathology.

Dig 150mm down MOT1 then a thin layer of sand for levelling then the slabs is fine. You’re not planning on driving a car on it are you?

You’re more likely to reach subsoil type material below the topsoil, not rock directly below topsoil.
If the 300mm is a loamy topsoil then it would be better to remove it, but if you have a few inches of loamy topsoil covering dense clay then just remove the loamy topsoil.
You could always look at putting in a geotextile layer under the 100mm that your other half suggests just to ease your mind.

Regardless of all the above actual site conditions always dictate what you need to do….

Just to clarify the poster above: in a true geotechnical sense of the words (to the letter of BS5930:2015) Topsoil is a descriptive term and can be use to describe a surface clay soil.

If you lay in directly on the clay there is a chance overtime the clay at the interface with the aggregate will soften due to water ingress and then the repeated action of applying pressure to the slabs could cause movement. However, having just looked at a very similar situation in a carpark, I would suggest foot traffic alone would not be sufficient to cause this in any great rush.

To be honest a geo textile membrane to stop the aggregate pushing in to the clay and properly compacting the aggregate with a wacker would be a sufficient.

I assume that if you put a foot of type one down for your patio that you properly compacted it, preferably in two layers, otherwise you will likely see some settlement over time, potentially more than a properly compacted thinner layer with a membrane.

A wacker will not be sufficient to compact the aggregate. I’d consider getting in a Bomag 120 just to be safe. A nuclear densometer will be useful for offering immediate test results of the compaction before progressing further.

Yeah she is right as it is a garden path and not a motorway. Your way would mean disposing of cube of excavation for every 3sqm of path and buying a similar amount of ballast.

Just put down a 70-80mm thick base of 6:1 ballast/concrete and then lay on 15-20mm thick 4:1 mortar bed.

You can add an extra level to the decision tree.

If the jobs is to be done your way, you do it.

If it is her way she can do it, or pay someone to do it out of her own money (if that’s how your finances work).

So: Is it overkill to dig 30cm down to rock in order to lay a garden path?
Depends. How many sheets of rebar did you use?

😆

You’re more likely to reach subsoil type material below the topsoil, not rock directly below topsoil.

Yes I’m sure you’re right – I don’t know the technicals. But at a depth of a foot I can’t dig any more with a shovel – I’d need a pick.

If you lay in directly on the clay there is a chance overtime the clay at the interface with the aggregate will soften due to water ingress and then the repeated action of applying pressure to the slabs could cause movement. However, having just looked at a very similar situation in a carpark, I would suggest foot traffic alone would not be sufficient to cause this in any great rush.

To be honest a geo textile membrane to stop the aggregate pushing in to the clay and properly compacting the aggregate with a wacker would be a sufficient.

Really helpful thank you.

I assume that if you put a foot of type one down for your patio that you properly compacted it, preferably in two layers

Oh yes. And yes.