Drain Test
Forgot to mention this in the other update, but the BCO didn't seem particularly fussed about formal drain testing. I explained that I'd attempted a water drop test by bunging the manhole but it leaked through the clay and that was that. But, being a bit paranoid I really wanted to properly test the portion that's under the slab.
So, this is what I did, I picked up an inflatable bung, some 8mm pipe (Because that's what came with the bung) and some connectors - stuffed the thing all the way down the long run of pipe (
https://imgur.com/ngJDLLj) and inflated it. Then I added a temporary upstand at the end, because I couldn't bung it due to the inflatable pipe.
Anyway, it worked great and I filled the new waste connections up to the brim and watched as they didn't move a mm over an hour. So that gave me a lot of confidence to carry on forwards, and also a baseline to test against once the hardcore was down. At some point I'll do similar to test my new gully connection and everything before backfilling it all.
Oversite Preparation & Sign Off
Keep on going! Pics here:
https://imgur.com/a/KV0gcrX
(For some reason, I didn't take many pics of this stage but it's not hugely technical!)
The first thing was to dig out the oversite (And how I wished I'd taken some off when I had the digger at the start). Because part of the "plot" had already been built on, I suspect the soil had previously been backfilled - to save any drama, I decided to just dig all the way back down to the sand layer. I also rediscovered the dreaded lead water pipe, so I simply reburied it with building sand. This was actually done before the drains above, and you can see on those pics where I've dug down.
After the drain sign off and agreement about the pea shingle approach for them, I moved back onto the oversite.
Next thing was to haunch in the rest bends. I couldn't decide if this was absolutely required or not, but it felt like the right thing to do. For ease and speed, I used quick setting cement and ballast in a dry-ish mix and mixed each haunch separately so as not to run out of time. Nothing special here, just put cement all around the bend and tamped it down slightly.
While that started to set we moved onto the 10mm gravel / pea shingle. I'd hired a little tracked dumper and enlisted some help (Because, frankly, I have enough back trouble as it is and this project was never about killing myself) and loading and moving the pea shingle was a doddle. We literally just poured it all around the pipes and through the lintel opening and then kept going until the two pipes were covered under a mound of it. To be clear, I didn't FILL the space with shingle, just mounded over the pipes.
Next in was the Type 1 MOT. This took me a bit by surprise because it's next to impossible to dig out of the bulk bags so we had to cut them open and spread it over the front drive. But once down, it wasn't too bad. To get the height, I'd sprayed a course of bricks on the wall, as I knew I was going to be above the minimum 150 anyway, I decide to err on the low side and make up the rest of the floor with extra concrete,insulation and screed rather than risk being too high.
We put it down in layers, wacker plating in between. Never having laid Type 1 before, I felt that there were too many big stones and some of them didn't really seem to compact - the small stuff was great, almost created a tarmac type surface. But, truth be told, I figured if the last concrete slab that was there could survive 50 years on top of loose soil so long as I'd gone over mine with a wacker it'd be fine. I didn't worry too much about levels - as I say, we'd marked the wall for the edges and the rest was mostly by eye
And finally the last bulk material was building sand, for binding. I opted for something around a 2" layer of sand - initially we just chucked a load in thinking we'd be able to level it fairly easily. It wasn't too bad, but at this stage I was still considering putting insulation down straight after and I wasn't happy so I decided to chuck in two lengths of CLS at the height I wanted (i.e., dug out a strip of sand and put the timber in there) and then used a long spirit level to "screed it". Didn't take long, but the end result was far better.
Struggled a bit with the wacker plate, probably because of the small area - it seemed to just keep pulling up the sand and making a mess of the place. In the end I just compacted it in straight lines, picking it up and putting it back to the start after each line. Probably a whole load of overkill, but I think it finished off the prep really nicely and the surface is rock solid.
After this, I retested my water pipes just to make sure I hadn't broken anything!
Aand then I covered the whole lot with DPM, just need to tape up around the pipes before the concrete goes down.
Finally, I'm happy to report that a BCO came out to inspect and has cleared the work.