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Finally start digging and build a basement... early 2022


Kai casswell

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So to recap: we bought an old house with a bit of land at the back. We tried to get permission to knock it down and do a new build passiv house in the backland. The council said no no no, we lost an appeal, then put in ridiculous plans for a massive extension with basement on the existing old house, plus a large stable block on the back land where we wanted the new build. Council said yes yes yes, thats fine,  please go ahead....

 

Having never built anything major before, we hired a 9 ton digger and a small tipper truck and set about digging a hole. The basement would start over 4m back from the existing rear wall of the old house and as long as we graded the excavation where possible and used plenty of sheet pilings, it would work out fine. To get rid of the clay soil, we just spread it out on the land at the back saving quite a bit on grab lorries. We originally planned for a four week hire of the plant, but we ended up taking about six weeks. Prior to digging I made a few test trenches to check the soil and water level. We also found an in depth soil survey from a major development next door. 

 

Once we had the hole dug in the right place, and sorted some temporary drainage, we set about levelling and pouring the basement slab. This was designed to be 300mm concrete on top of a tanking membrane, with tons of rebar. We just followed the drawings and building control came a few times to check it all. The waterproofing was overseen at each stage so we could get the warranty as well. We poured the slab in winter of 2022 and had a few cancellations due to frost. In the end we made an insulated temporary roof over the basement and used gas heaters the night before the pour just to be safe.

 

The concrete went in very well, i think it was two loads of waterproof concrete, then we could start on the nudura walls. The hardest part of this section was wiring all the rebar together and getting the 20mm OD L bars in place which connected the slab to the nudura walls. These were doubled up all around the perimeter every 8" and a pig to get in amongst all the rebar mesh.

 

Assembling the nudura walls was very straight forward, once i got my head around the idea of a common seam - where due to the plan measurements, the nudura blocks needed cutting and joining together. We had done the nudura course and the rep was very helpful with all my questions. Once built up, building control came out again and we ordered another three loads of water proof concrete for the walls. Where the walls joined the slab, we used an expanding waterbar and kept it dry until we poured the walls. We hired a concrete pump for the pour and because it was a basement we just used the nudura walkways and scaffolding to allow the pump hose to get round the whole perimeter. I was concerned about blow outs- especially on the first load dropping down into the bottom of the wall so I over did the shuttering on the outside of the nudura - including 220mm x 50mm floor joists screwed all around the perimeter outer edge, plus lots of OSB shuttering at all the weak points. It all worked well and it was a major relief to get the walls filled up.

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