Ed_

I dug the hole 8 weeks ago, so it wasn't oozing then, it was dry. Now it has 20cm of water which seems to be an equilibrium. Whilst i'm generally unconcerned about it not working perfectly, as overflow will run off safely down the gentle slope, I dont want to create problems I don't understand. SI report found perched water at around this depth, but the boreholes were quite far away. I think you are suggesting 200mm crate with 400mm of cover? This makes sense to me - thanks.

So i dug a hole, it drained away quite nicely - 40cm in <4 hours. All good I thought. Just been back to check and record it more officially and there was 20cm of water in the hole. Scooped a bit of soil out in case it was clay settling at the bottom but made no difference. I suspect I have perched water, as it hasn't rained much recently and it drained away in November, plus it is higher than surounding ground so unlikley to be the water table. If this is the perched water table, at a depth of 60cm, a soakaway seems unlikely to pass, as the minimum depth crate I can find is 40cm which if placed at 60cm depth only gives 20cm of cover which is probably a bit low. Does anyone know? My next plan is to cast around a bit to see if there is a better spot, where either the soil drains better or the perched water is lower. Would another option be to make the soakaway larger so it has enough volume even with 15cm of water in the bottom from the perched water table? Or has anyone done a rain garden with no overflow?

It seems to go away. No signs of flooding or waterlogging. The gradient is minor but still that would prevent any sort of flooding, the water would just run off. In the end there is a road at the lowest point, and if it gets that far it will enter the drains.

Houses. This is an urban area. All the houses up the hill, so far as I can tell, drain to back garden soakaways.

The foundations will be down to the clay, it's a basement so will be a sort of raft. Soakaway can be downhill and I could put it 15m-20m away I think. You are right, best to dig a hole and see what happens first.

It says to a depth of 300mm below the invert pipe. That will be fairly shallow, within my made ground layer. From part H the depth seems quite clear, but when I read about soakaways and percolation tests, a lot of the Information suggests I should conduct the test in the undisturbed layer, ie the clay below the made ground. I feel reasonably content that doing the test in the made ground will give me a representative result, and if successful a working system like the existing one, but I am hardly an expert.

No...

100mm. Attenuated to 2l/s, I think.

Doing a demolish and rebuild. Existing rainwater went to a soakaway, which unfortunately had to be removed. I have 1-2m of made ground over clay. Clay is, apparently, essentially impermeable. Don't bother with a percolation test they say. All well and good but I've been quoted £25-30k for connection to the surface water drain - 8m away across a quiet residential road! I'm sure I can find a cheaper quote, but got me back to thinking about soakaways. Clearly the existing soakaway worked, I'm on a very slight slope and I guess the water percolates through the topsoil/made ground downhill, slowly adsorbing, evaporating and eventually ending in the river. The advice on a percolation test seems to be to dig it at the level of the soakaway. If I choose a broad flat soakaway design, I'm sure I could bring this above the top of the clay layer and reasonably expect it to drain adequately. Is there anything wrong with this logic? It seems percolation tests aren't exactly very well controlled.

My understanding is that having the insulation on the outside is better for condensation, as it means the concrete is warm. Insulation on the inside and the concrete is cold so the condensation point moves to inside the wall, potentially.

I've been round the internal v external debate so many times. You are right but I can't work out what to do about the cold bridge through the ground floor slab. How effective is a bit of external plinth insulation going to be to stop that? At least external is continuous. Maybe I am worrying about nothing. Will add a drawing later to help!

It would, it just needs to be thicker obviously. I'm limited to 100mm insulation where the timber frame joins the slab at ground floor level, to give a drip edge to the timber frame insulation. So 250mm EPS for example is quite a step out and probably won't look great, hence why I was thinking PIR as the thickness needed for PIR above ground is about the same as EPS below ground.

I am building a walkout basement, this means that down the sides the ground level will slope, from top of basement at front to floor level of the basement at the back. I'm struggling to understand how to detail the above and below ground insulation. i think I need to use EPS below ground and PIR is not suitable. As it's below ground, I need less, so 150mm EPS is ample, but that won't work above ground, so I thought I'd use 150mm PIR above ground and that works about roughly equivalent. The issue I'm struggling with is how to detail the transition given the slope? Can I just butt them up, render over the top to just below ground level and put in a french drain? Should I cut the insulation to match the slope (with lots of wastage) or just get as close as practical using blocks? I was planning on just adhering directly to the concrete.

Not really an option unfortunately, can't impact drive due to proximity to sewer in the road, and anything else is expensive. If I have to I will put the Insulation on the inside, it's not the end of the world, just trying to work out whether it is internal or external because a lot of decisions depend upon it and it's not turning out to be a straightforward decision.

The other sides are battered back, so traditional formwork. I have 1m to the boundary, so 350mm piles and 250mm insulation doesn't leave me much working room!