1950's reno with a history of subsidence - the last 12 months

[alfaTom]

As some of you may have seen from my introduction post (shameless plug here), we bought a house in the summer of 2023 knowing it had subsidence.

 

We decided it wasn't that bad. It was down as cash buyers only. Surely we know better than the lenders. Read on to find out if we were right...

 

A summary

We weren't looking to move house. We did the usual search on rightmove when bored. This house came up; the interior was old, it had subsidence but it came with a great plot of land and we could see it being our forever home.

It has remedial work for subsidence done back in 2012, the walls had helifix bars installed and the offending trees were removed. We even had a full structural report written up from a proper MIStructE geezer, and he said the house was alright if a little wonky and the garage was more likely to fall over.

 

What did the report say?

Floors are a bit wonky, few cracks here and there. Walls are a bit off but yer roof timbers are alright.

Nothing we didn't already know really, but nice to get it from a professional. No bowing in the walls at all so whatever happened was a nice even movement.

The professional said some movement could be attributed to the trees at the rear of the property or maybe some damaged drains. (The worst affected area was literally right next to the guttering downpipe😱)

 

What did you do next?

I made a post on Buildhub, and contacted a resin injection company to see if we could do some localised repairs. The resin company wanted to inject an expanding resin around the front of the property and also down the sides. This came out at more than we had budgeted for remedial foundation work, and since we were planning to do some structural work (i.e. extension) they said the second we dug near the injected areas it would invalidate any warranty which would have turned it into one big gamble.

As part of the resin injection investigation we commissioned some soil samples and drain surveys to understand the state of the ground and also see if we were right in our assumption the drains had failed.

 

Below is a summary from the drain survey. A green X on a run indicates no defects. A yellow X on a run means a minor defect, and a red X means urgent defect.

 

As you can see, there are no green or yellow, only red. 😂 Clearly the old terracotta pipes were knackered.

 

Around the same time as this and thanks to some of the helpful advice on this forum I started a period of monitoring the house. Structural engineers say once a quarter is pretty good for measuring any movement on a house to capture the seasons; I did it every day. Us buildhubbers do like to go overboard...

 

Bought the proper stainless steel measuring discs and a good set of digital calipers.

 

I input all the data every evening into an excel sheet to make it easier to spot trends. I also made sure to do this at a similar time each day to try keep temperatures as consistent as possible. Variations between days were fine; I was looking more to see if we had a trend of cracks widening.

 

I did this for about 5 months - not long enough to get a load of data, but I stopped in winter when the SE told me we'll have to pile the extension which meant my whole theory of monitoring to avoid underpinning was shot.

As we have quite a high water table and a lot of clay they reckoned piling out be the most suitable rather than needing excessively deep trench or strip foundations. And because we'd be piling the extension it meant we'd have a rock-solid new part of the house and the old part move at a different pace; leaving us with a real risk of differential movement. 

 

Whilst doing the above we also replaced the drains around the house; installing two new soakaways to the front and rear of the property (appropriately sized for our new roof area too.

When digging out the old drains this is what we found (no wonder we had some soil washout around the foundations)

 

We decided at this point that we may as well go ahead with the SE's suggestion and pile the underpinning too. Yes, this cost a lot more than we had planned but if I'm honest I'm pretty happy now knowing that the house is fully supported.

 

What was piling like?

SE's came back and said pile everything in sight. You can see below the piles for the new extension plus the cantilever underpinning to the front and side of the property.

 

So that's what we did. However trying to find a piling company who would work from someone else's plans AND do their own excavating was a nightmare. We had one company say they wouldn't excavate the ringbeams and would only pile, but also wouldn't tell us what level they wanted the ground at. They then agreed to excavate, and about a month later when someone else visited the site they changed their mind again and said they'd only do the piling. They were swiftly told where to go and we had to go back to RFQ stage to try find another contractor.

 

As it turns out this was good fortune as the company we ended up using was brilliant. They did all the works end to end and also supplied an engineer to mark out the beams and the block profiles so it was ready to go for our builders. They even cleared the oversite down to the required level for the air gap under the block & beam flooring.

 

Piling starts:

Very grateful at this point for the large carriageway drive - made it much easier for getting the kit off the lorries. Was rather strange to see so much gear for one house; I definitely wish they'd let me have a go with it.

 

We actually went through two rigs; one had the winch fail but we had a new rig onsite the following day to continue. The weight on them is about 750KG and is dropped from a pre-determined height. For us it was about 1.5 metres. On each drop of the weight they measure how many millimetres the pile slips and using calculations from the SE they can work out when the ground has enough bite to hold the pile. We thought we'd really feel this and we were also concerned for our neighbours, but it turns out aside from a little bit of vibration there wasn't actually that much disruption and we got used to it after a few days.

They were on site for about 2-3 weeks (2-3 piles a day depending on the soil conditions) and we had no complaints. Once the weight went below a certain depth the shockwaves from the impact just sailed right under the neighbour's houses. Our nearest neighbours who were about 30 metres away felt very little, but we did have some neighbours about 200 metres away who said they could feel it; but thankfully no damage to their property.

 

 

The contractors estimated about 6 metres for the pile depth for when we'd get the right bite, based on nearby BGS surveys and boreholes. Things were looking pretty good when we found a solid crust at about 3 metres and we thought it looked like we were in for a saving on the job! In typical fashion though what would have been the last drop forced the pile through the crust and it dropped about 13 millimetres. Once that happened they had to keep going and we ended up at 8 metres for the first pile. Because we found this solid layer at 3 metres but knew underneath was weak we HAD to go through it on every other pile. It got real solid in places but because they knew it wasn't solid underneath it couldn't be used for a fix.

 

The other 35 ended up at between 8-10 metres and we found some very different ground depending on which side of the house they were on.

When you got some good depth on the piles you could get a good echo out of them when shouting down though. Was really weird to just see it going down and down into the earth though.

 

Once all the piles were in the excavation team were back to do the ringbeams.

 

Some lovely Lincolnshire clay was waiting for them along with plenty of water. We had pumps running overnight to keep it clear as one trench just kept filling up. As a bonus in the above picture you can see the absolutely top-notch definitely not too thin or too sandy foundation that the original house was sat on.

 

 

To add to that, the foundation in the house also wasn't level. This one really confused us and the excavation squad. At one point it was 5 bricks between DPC and foundation; at the other it was 8 bricks. You can see in the above where we had to step the beams down to get it to fit; which required some custom Z bars to bind the ringbeam together. You'll also spot the gap this left in the high points where we then had to get a more specialist non-shrinking concrete to meet the foundation. Another nice surprise and request to open the wallet.

We still have no idea why they did this; either they decided after digging a certain depth for original foundations that they didn't want to did so deep (or didn't have to), but when you're doing a full set of foundations at once why wouldn't it be level?!

 

Anyway, we persevered (or they did) and at the end we had a nice base to start our extension on:

 

In the end we were in for 35 piles at about 8-10 metres, with 600mm ringbeam all round complete with hefty 16mm rebar sat in it but BCO have signed it off as good and we were on our way.

The drone also makes for some interesting photos... footprint of the house is just a bit larger than it was before!

 

Probably missed a few things here and there but hopefully provides a bit of insight into the joys of piling, subsidence, underpinning and just general mess. Absolutely loved spending £££££ for it to be covered in mud and never seen again; should have just filled it with £20 notes, definitely not being sarcastic. 🙄

tl;dr

Whatever you are quoted by your contractors, double it. 😂

 

[Mike]

A very good write-up! You certainly came across some interesting finds in the excavation :)

[TheMitchells]

Wow!  Looking forward to hearing about the rest.  Are you going to tell us what it cost???  😄