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Underpinning.
Underpinning extends your foundation downwards. A bit at a time.
If you underpin the back of your house to the depth you need to excavate your basement, your house will be safe.
As well as showing you how it is done and what mistakes to avoid, I will discuss insulation of that wall as well.
Ideally, you will never remove the soil from under more than a quarter of your existing foundation at a time.
Looking at the drawing above, you will dig out both #1s and complete the #1s before you start on the #2s.
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My son and I went up to Staffordshire to do this underpinning but we only did this first pin.
There are Acrow props under the chute in the image just above. But the clients sent us away and carried on and made a right mess of it. More about them below.
You can see Acrow props below. The formwork is pushed as hard as possible off undisturbed soil.
This was in Esher some years later.
Dry pack is usually sharp sand, cement and only just enough water it will bind when squeezed in your hand. Ram it in thoroughly with a piece of 4x2 timber or something else big enough but not too big with a flat, square end.
My son and I returned to Staffordshire when the clients said they had finished - to this
Building 'my way' means concreting over the underpinning as a part of forming the entire basement structure out of monolithic, waterproof, reinforced concrete.
Carried out properly, underpinning takes 3 weeks minimum because you need to wait at least 48 hours after every addition of fresh concrete before you do anything else.
You could ruin everything and your house could drop a few millimetres and develop cracks in your walls if you are too hasty.
You can see on the timetable below that almost all the time no one is on site. You are waiting for concrete to gain some strength and cool down and shrink.
If you have to go down too deep to complete in one go, it will take 6 weeks minimum to do the whole operation twice, the second beneath the first after the first is complete.
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The underpinning to the left was two storeys and the strange step half way down was specified by the structural engineer.
In the image below, my team are building single sided formwork against the underpinning.
Two things here I would have disapproved of had I seen them. I had employed a manager for several years and he employed his sons and their friends. He did this work. It all turned out well enough, but the client kept trying to cut me out of the equation. So I was kept in the dark for long periods.
The timber braces are not strong enough for single sided formwork. Single sided formwork needs Acrows.
Here, there should have been FRP threaded rods resin glued into the underpinning - creating double sided formwork. This team had used them before glued into piles, but I found a few times that just timber taking all the weight broke.
Notice they have rigid insulation board against the underpinning. I will be telling you lower down this page it isn't necessary.
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On one job we had a bit of trouble because as we dug each pin there was no foundation to speak of and original brickwork was falling in. We had to sacrifice Acrow props to hold the brickwork up till the dry pack was rammed in.
Underpinning Timetable.
days of the month
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Dig #1s. Put up formwork. First fill to within 80mm of the underside of existing foundation.
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Do nothing.
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Dry pack #1s between the underpin and existing footing.
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Do nothing.
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Dig #2s. Formwork. First fill.
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Weekend.
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Weekend.
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Dry pack #2s.
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Do nothing.
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Dig #3s. Formwork. First fill.
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Do nothing.
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Dry pack #3s.
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Weekend.
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Weekend.
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Dig #4s. Formwork. First fill.
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Do nothing.
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Dry pack #4s.
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Do nothing.
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Do nothing.
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Weekend.
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Weekend.
I would say this is the fastest you should contemplate underpinning. It could be wise to let all the underpinning relax a few days before excavating more of your basement.
You might be wise to let the mass fill concrete cure an extra day before you dry pack because, depending on the cement type and the thickness of the pin back to front, the concrete will get hot and expand before it cools and contracts.
You dry pack because this is your opportunity to make sure the mass fill is fully supporting the existing foundation above. But if the mass fill cools further and contracts further after the dry pack, there will be a tiny gap created and your house will drop that tiny amount when it gets the chance.
Some self builders only have the weekend to work and they are mixing their concrete in a mixer.
They might take all weekend to dig and fill only one #1.
Perhaps they can dry pack that one pin Wednesday evening before digging and filling the second #1.
It doesn't generally matter how long underpinning takes as long as you complete the dry pack of the last pin before you stop, perhaps for the winter.
Use Acrow props, not just timber or soil, to hold back your single sided formwork.
The wet concrete weighs about 2.3 tonnes per m³
Solid soil only weighs about 1.8 tonnes per m³
But backfilled soil weighs even less because it is still fairly loose.
If you rely on backfilled soil to hold your formwork in place it will move and your underpin concrete will be proud of your foundation and your basement might have to be smaller.
My son and I went up to Staffordshire to begin a project with the underpinning. I hired Acrow props and used them to really push the formwork against the foundation. After the first pin, the clients decided they could do the rest of the underpinning and we were sent home till we heard we could continue.
All the other underpins were 5 or 6 inches proud of their foundation, which was supposed to be space that is part of their new basement.
I asked why the Acrows hadn't held the formwork back. They said the first thing they did after we left was off-hire the Acrows.
We showed them how to use a disc cutter and diamond blade to cut through the concrete and chisel bits off. We told them we were going home until they told us they were finished.
This was in 2007. I had only been in business 3 years so I tried to be patient. They carried on like this throughout. She had her ornate wall built up the drive, which made it narrower, but then we couldn't get the concrete pump close enough. And so it went on. We fell out but one of my team decided to stay on. She moved his scaffold while he was working on it and he fell and broke ribs. Question what I tell you, by all means. Make me explain. But don't then just ignore the voice of experience.
I mentioned above that the client did not need insulation against underpinning.
Generally, if you lose heat through your basement structure, if that heat has to get through 2m of soil before reaching the open air, that soil is sufficient insulation.
Whoever does your energy calcs will have to include that assertion in their report.
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