Problem Soil

[68-1093879142]

Hi,
I've started laying my patio, and am using 6 different sizes of Calder Brown. I've taken advice from my local builders merchants, and am using 'Washed Grit' (looks just like sharp sand to me!) as my bedding layer (I'm not adding any cement to stiffen), which I'm compacting with a whacker plate to a depth of 50mm. My concern is this: I've now laid about two thirds of the flags, but haven't mortar'd any of the joints (will do this at the end). Recent bad weather has highlighted a few 'spongy' areas in my compacted grit - I'm not talking soggy, but walking on it causes a mild flex. We live in an area with a high content of clay in the soil, and I think the sponginess is being caused by the saturation of the clay beneath the washed grit. So now to my questions: when dry the sponginess isn't a problem. So if I lay the flags and mortar in the joints when dry, water won't be able to penetrate from above - but will the problem sponginess still persist beneath the patio (i.e. seepage in from surrounding ground)? Will the mortaring be strong enough to stop the flex, or will the flag still move slightly and result in cracked joints? I'm concerned that if I continue without further advice, I may only have to take the entire Patio up in the future to dig down to more solid material and use a proper sub-base (type 1?) underneath the bedding layer.....
The amount of movement when a flag is on the grit isn't massive - max of around 2mm - and the problem isn't everywhere - will my jointing 'lock' the flags, stop the movement, and let me get away with not having to redo it all again?!?

[84-1093879891]

You really can't rely on mortar to stop flags rocking if the bedding is loose. It puts undue and levered pressuree on the mortar joint, which then cracks, allowing the flag to move even more. :(

The easiest solution is to lift the few flags that are rocking and stir a trowelfull or two of cement into the bedding before re-seating them, and consolidating again. This will stiffen the bed, and so prevent the flag rocking and so the eventual mortar jointing won't be compromised.

I'd strongly recommend buttering the receiving edges with mortar before re-laying the lifted flags, and then point the joint immediately after consolidation. This will more or less guarantee the previously loose flag is held firmly in place.

Let me know how you get on.

[68-1093879142]

Thanks TonyMcC - it's been raining cat's and dog's for the last 2 days so I'll be able to check for any spongy areas and fix as you've recommended. As the majority of flags are fine, I'm relieved to not have to start again :biggrin:
I guess I could have saved myself a lot of bother if I'd mixed cement in with the grit from the outset - we live and learn!