Underpinning Walls in Old Houses

Now if you’re reading this because you have a settlement problem then stop, take a deep breath and try to relax. I know cracks, settlement and underpinning walls can induce bowel clenching fear into the strongest heart. But fear not; it’s often quite a simple repair. Note, I said simple, not easy! It is almost certainly going to be hard work, and it’s fiddly to do in confined spaces. But it is doable if you have the time and a little patience. It’s not even terribly expensive material wise, concrete and mortar are cheap enough.

Coincidentally; I recently found myself deep underneath an old house after a worried call from an old client. When I arrived and saw the state of the wall, I realised this was one rather tricky underpinning job. But I’m glad they called me, I quite like a challenge! To be honest, underpinning walls is not something I do every year, but once in a while I get the chance to happily crawl around underneath a house and prop up a problem wall or two.

Collapsed wall showing the loose fill construction…

The image above is in the cellar underneath the house and this is where I was working. This is pretty unusual, normally you’d be digging down around the outside of the wall to get underneath it. But on this house I had a head start as the cellar floor was already about a meter below the outside ground level (plus it was dry, about 13 degrees C and wet outside…).

800mm Thick Stone Walls

This job wasn’t a regular brick wall though, and it wasn’t a simple case of digging underneath and filling the resulting holes with concrete. No, this was a random, loose filled stone wall built around 120 years ago, and here’s the thing; it was 800mm or 31.5″ thick, almost like a castle!

The concrete was to be almost 800mm wide and 600mm or so deep at this point, fully underpinning the wall.

Undisturbed, walls like this can stand strong for centuries, but because of their somewhat ‘dry built’ nature, if something moves, or if you pull a stone out, whole sections can tumble down like a house of cards. Tricky work indeed.

This particular wall wasn’t sinking though, just in a precarious position due to some digging in the past to create some extra space in the cellar. Plus, some recent blasting nearby resulted in a largish section of stonework collapsing into an indoor rockery with no hope of a redeeming water feature…

Not Much in the Way of Foundations

Doubly tricky because there were no real foundations as such in those days and certainly no concrete footings. Back then, you’d just build in a very shallow trench on a layer of fine blinding (small crushed stone usually). The ground in this area is a mix of hard stone with occasional soft, crumbly areas, which is fine in most cases to build straight on.

The largely dry stacked wall was precariously sitting on a little loose fill blinding, on top of sloping bedrock…

I couldn’t just go in and dig out large 1m wide bays to underpin as normal, as I’d risk the wall above collapsing totally. I needed a softly, softly approach. With one exception (where the stone wall itself was missing), I went for half meter wide bays as this gave me the best chance of not losing any more walling during the work.

Here is a half meter wide bay dug underneath the stone wall down onto good ground…

Instead, I excavated each bay gently, rather like an archaeological dig. The aim was to remove any soft topsoil and loose material until back onto good ground; rock preferably, or as in some parts, a stable looking, ballast type subsoil. I wondered if there had been water running through this gap in the bedrock at some point in the past (hence the sand).

You always need a gap above the formwork or shuttering to get the concrete in…

Working like this meant I could use the very minimum of new concrete under the wall to support it. Often forgotten, but minimising the weight should be a factor in underpinning. This makes sense if you think about it, because more weight means more downward pressure on the ground; ergo the better the ground needs to be to support the load. And we already know this wall is in trouble!

Commonly, walls which need underpinning are sinking on some poor ground maybe, or on top of leaking drains etc. Throwing in tons and tons of extra weight via concrete could, in theory, make things worse.

Transferring the weight of the wall onto the bedrock below…

Adjacent bays are linked together using thick steel reinforcing bars…

Don’t forget to liberally splash a thin cement slurry everywhere before pouring the concrete; this reduces the suction between the new and ‘old’ concrete. Mix water and cement into a thin, easily splashed slurry and flick on with a large brush.

Underpinning Walls Traditionally

Often when underpinning walls, folks dug small pits under the brickwork and flooded them with concrete. I had a friend who used to do this full time. All his digging tools had really short handles (to make it easy to dig in a small square pit underneath a wall) and he drove a tiny little van too. He used to joke that he bought them as a job lot from a Gnome…

A textbook underpinning bay layout, but each one tends to be pretty unique. This shows digging the pits from the outside, but if there is a cellar you could be digging from the inside.

Standard underpinning, (image taken from the book “How to be Handy“, or you can read more about underpinning here…)

But you need to be extra careful under a loose fill, random stone wall (or poor brickwork), because the wall would collapse into the pits before you can get the concrete in. So I opted for a gentler way to provide the support the wall needed without going crazy with the concrete (and even then, close to 3 tons of material went into the job).

Finding solid strata is key, rather than just digging away blindly to hit a certain depth…

I decided to use a kind of hybrid repair to underpin these walls, because they were not sinking, just unsupported. Again, there was a real risk of the wall collapsing if I went crazy, digging out big bays etc. Plus I wanted to take advantage of some good bedrock not far away, which was perfect for supporting the walls.

I filled all the soft areas with mass filled concrete and supported the good areas with concrete pinned to the bedrock. After all, there is no point in removing good supporting ground just to replace it with concrete.

Once I had stabilised the ground underneath the wall with concrete, the wall was once again fully supported. Next was to rebuild the fallen area of stonework. Random doesn’t even begin to describe it! All sorts of sizes and even types of stone went back into this rough wall. And because old habits ‘die hard’, I finished off the joints with a section of truck fan belt and a light brush to leave it looking tidy (even though hardly anyone will ever see it and the rest of the walls were very, err, rustic lets say!).

Concrete in place, time to put together the giant stone jigsaw!

I don’t know why the spirit level is in the above image, I certainly couldn’t use it to build this wall. Building by eye was the only way, but I did have a string line right at the top so I could at least end up fairly straight!

All finished…

Hard and steady work for sure, but rewarding in it’s own way I think, even if hardly anyone will ever see it. I reckon it should be good for another hundred and twenty years at least 🙂

Stay well,

Ian

Originally published at https://handycrowd.com on January 7, 2020.