Subsidence and Foundation concerns 9

[Crank90]

I come from an electrical engineering background so structural engineering is not a strong subject for me. I kindly ask and would appreciate if there is someone that can offer their proffesional advice.

My property has suffered from subsidence, which happened back in the very hot and dry summer of 2022. Cracks have appeared through mortar lines and brick internally and externally, mainly around the front two window bays of the property. It also affects the opening and closing of the front living room door upon the clay soil swelling and shrinking through the seasons. Soil samples have been taken which show high VCP and very high plasticity levels. The vegetation has been mitigated and should no longer cause issues. Monitoring has been setup to see how the ground responds over the next 6 months.

I need help understanding what has / is happening to the front window bay foundations. There is a large crack / seperation that has been uncovered when a trial pit was dug. The property is a 1930's semi-detached chalet so the foundations are not deep and sits at the top of a sloping drive. Is there cause for concern here? Does it require underpinning? Should the part of the foundation supporting the window bays be seperate from the house foundation slab? Or should they be connected together?

I have tried searching the internet for answers but I can not find anything that describes how these foundations should sit.

Thank you in advance for investing your time in reading this.

 

[milkshakelake]

I'm not an expert in this kind of thing, but seeing that nobody else responded, I'll give it a shot. This is usually because of soil settlement. That's when you have uneven soil under the foundations. After 90 years or so, I'm guessing that a lot of it happened in the first 5 years. The rest of it is usually negligible.

If this separation happened recently, I'm thinking that the condition of the soil changed. That might be due to improper drainage. Meaning a lot of water enters one area. This conveys the soil around and loosens it in a certain area.

Underpinning would be one way to solve it. You'd need a geotech engineer on board to evaluate if that's the right solution, because the soil condition might not necessarily be better as you go further down.

The soil swelling and shrinking through the seasons is either because of the improper drainage thing I mentioned above, or because this foundation isn't deep enough. That's something that underpinning would solve, assuming that the lower soils are good. If the foundation isn't deep enough, water gets under the footings and freezes in the winter, which pushes it up. In the summer, this melts and it goes back down. The required frost depth would depend on your climate. In my area, it's 4' deep minimum.

Another possibility is that the foundations are on a compressible soil like clay. I really don't know much about clay, so I can't advise on that. The only thing I know about it is that your footings would generally be a bit wider if it's clay, to spread the load a bit more and minimize the settlement. I don't know what high VCP or plasticity means. I only have experience with sand and gravels and rock, which don't have this issue, so we never even look at plasticity.

If you want to close your eyes and not think too much about the solution, you can just support these foundations with helical piles. That's a tried and true approach. It will need a geotechnical engineer to design it. It might cost more than other options, but sometimes we engineers recommend surefire solutions when we don't know the exact cheaper solution, so that's what I'm doing now. Or hire a local geotech that knows the soils in your area pretty well. They would have encountered this same issue a million times.

 

[EngDM]

The house foundation looks like a footing. Not sure if underpinning would be appropriate unless you did the whole foundation to avoid further differential settlement. Basically, if you underpin this area it'll become much less susceptible to settlement, but the other side of your house will still be the same, so you might just end up cracking other areas.

If you're in a frost zone, adding rigid insulation outwards from the foundation to avoid frost below the footing, and insulating the outside of the foundation wall would be a start. Adding or reparing any existing weeping tile to keep the water from getting under the footing in the first place would likely help.

If you do decide to underpin, you also risk damaging the bearing surface of the adjacent footings, which will weaken them and if it's an old building there might not be any rebar in those footings so they will just crack since they aren't bearing on a stiff surface anymore.

 

[JoelTXCive]

I think a zoomed out photo would help paint a better picture. Maybe a shot or two of the exterior so we can conceptualize better.

Also, do you think the bay window is original to the home, or an add-on?

 

[MTNClimber]

Hire a local geotechnical engineer to help and talk to several foundation contractors that specialize in underpinning. You don’t have enough information to share that would give us a slight chance of giving solid advice to fix your problem.

 

[hokie66]

I think you have discovered the main reason for the movement in the building. Clay soil which shrinks and swells with changes in moisture is a common problem in many parts of the world. Your soil shrank during a drought, and the footings had not been designed to accommodate the movement. Whether or not underpinning will help is a judgment call. In the long term, the objective should be to raise the moisture content of the clay and keep it constant. That sometimes is achieved by flooding, or persistent water injection. That may or may not be practical in your case.

 

[StrucPatholgst]

You're going to get absolutely no where trying to get an opinion on this from the other side of the internet. This is not how the practice of structural engineering or structural assessments are conducted. You have to have an experienced structural engineer looking at everything: the building, the grounds, the slopes, the soil, the crack patterns, the locations, stormwater flow, the building history, etc. Posting a few paragraphs and a photo, and expecting a valid opinion, is quite frankly ridiculous. My advice is to hire a local structural engineer with experience in evaluating this type of issue.

 

[Crank90]

To provide some context, I've been grappling with this issue since September 2022. I am currently embroiled in a dispute with my insurance company. They have repeatedly shifted their reasons for denying coverage for repairs, which I have been advised would involve underpinning. Their latest argument is that the window bay should be separate and is in a satisfactory condition, but they do not provide any clear reasoning as to why this is the case.

I have consulted another structural engineer and a geotechnical engineer, and shared photos along with a soil sampling report. They believe underpinning is necessary but are hesitant to provide more advice without charge. My next step is to commission these experts to conduct a detailed assessment, hoping it will persuade the insurers to reconsider their stance. Although it's a long shot, I feel compelled to pursue this course of action.

Before proceeding, I'm seeking opinions on this forum regarding the damage observed, to better inform my decision to potentially invest in contesting the insurer's current denial.

The insurers have attributed the cause to vegetation, a stance they took even before any formal investigation. However, the soil samples I mentioned earlier suggest that soil conditions, particularly during adverse weather, play a significant role, as the geotechnical engineer noted due to the soil's high Volumetric Change Potential (VCP) and very high plasticity.

My primary concern is the soil condition and what I suspect is foundational damage. I worry that regardless of mitigating the vegetation, the soil will continue to shrink and swell, particularly during extreme weather conditions. This, combined with the already damaged foundation, could exacerbate the situation. The insurers assure me that removing the vegetation will resolve all potential movement issues, but I remain unconvinced.

Regarding points raised in your responses:

• Drainage as far as we are aware was ok. We have recently done renovations and dug up a lot of the ground around the back and side of the house and found it all in good working order. It has been renewed so it should all be ok. The subsidenced occured before any renovations took place.
• The foundations are shallow, typical of 1930s construction standards.
• There was no history of subsidence on the property until 2022. We moved in during 2021, and initial surveys did not indicate any problems.
• The window bays are original to the house and were constructed simultaneously with the main building.

I'm happy to share the soil sampling data if there's interest.

You might be right StrucPathologst. I’m aware it may well be ridiculous to find answers with a brief description and pictures, it is wishful thinking, but I hoped to find someone who might recognize this issue from their own experiences. Insurance companies often seem overly confident in their assessments based on mere photographs, which prompted me to seek additional insights here.

I have taken some additional photos that hopefully make things a bit clearer for those that are interested. The trial pit is dug between the two window bays. The pictures taken are of the right hand side of the left window bay. I appreciate you all taking the time to respond. Thank you.

 

[hokie66]

Insurance policies have lots of clauses. Most have clauses denying liability for ground movement. Just saying...

 

[SWComposites]

Where is the issue? On the view showing the windows there doesn’t appear to be any cracks or displacements.

If the cracks were only found after digging out the dirt, why not just shoot them full of grout and put the dirt back?

Though the presence of moss on the bricks may indicate drainage issues.

And vegetation issues? LOL! What is the insurance company claiming it to be? A couple of dandelions?

 

[Tomfh]

The window bays look like add ons. Different bricks. Different footings. You sure it was all built at once?

If you have two different footings sitting on different soil, this is going to happen.

 

[Crank90]

@Hokie66 It does seem to be the case; they are brushing it off as betterment works if underpinning took place. Betterment and preventative works are not something that is covered by insurers. However, they have not themselves investigated the state of the foundation but conclude that it is betterment works.

@SWComposites The external damage is now concealed behind new render. These photos are recent. Due to ongoing delays with our 18-month insurance claim, we proceeded with repairs, replacing cracked bricks and reinforcing the mortar with helibars. Although it carries a risk of further damage, we couldn't postpone our renovation indefinitely while waiting on the insurance.

The internal damage still requires repair. The internal brickwork, where cracked, needs to be replaced and reinforced with helibars, but we have not yet reached that stage of repair. The most severe cracking was around the window bays. Recently, these cracks have appeared less severe, likely due to several months of heavy rainfall which has helped the ground recover. This pattern was also observed last winter, following the initial subsidence, when the cracks reappeared over the subsequent summer. With the mitigation measures now in place, it will be interesting to see how the structure responds this year.

Filling the cracks alone seems unreliable, and in retrospect, excavating a trial pit may have complicated matters further. However, seeing the current state, I'm compelled to ensure the property is secure before moving forward.

Drainage is located on the sides and back of the property, away from the front window bays.

The blame was initially placed on trees in our front yard—specifically an apple tree and a small conker tree, both of which we removed. The primary concern was a Hornbeam tree in the adjacent yard, now also removed, situated about 14 meters from our home. I'm unsure of the extent to which tree roots impacted our ground conditions, but they can contribute to subsidence by drawing moisture from the soil.

@Tomfh The window bays are original and have been part of the property since it was built in the 1930’s. If they are different footings and that is the intended design then does it not bring into question as to how far they should sit apart? Should the window bay footings be resting on the house footings? Or should they not be touching at all?

I’ll include a photo of the garden wall, which is also part of the insurance claim. You can clearly see how the foundation has cracked, occurring simultaneously with the subsidence on the property. This raises serious concerns about the stability of the house foundations. Given that the garden wall foundation is now separate, I wouldn't be surprised if the insurers dismiss this by suggesting the foundations were originally poured this way, using it as a justification to limit their liability.

Thank you all for responding, I appreciate the feed back.

 

[Tomfh]

How do you know the bay windows were built together with the rest of the house?

I suspect the plans changed at some point (maybe during original build) and the additional bay windows were tacked on.

In any case, yes they should have been connected together more strongly, and founded upon the same material.


Given the issue appears to be related to ground movement, you may struggle to get an insurer to foot the bill for underpinning and other associated repairs.

 

[XR250]

Home warranties might cover something like this but not insurance companies.

 

[hokie66]

I agree, XR250. That is certainly the case in the US and Australia. But the OP is in the UK, so maybe it is different there.

 

[Crank90]

@Tomfh After researching, it appears that all the chalets in our area feature cantilevered window bays. Interestingly, not all of these window bays have foundational support; they are essentially hanging, which supports your theory that the window bay foundations might have been constructed separately from the main house structure. It remains unclear when these foundations were originally laid—whether they were part of the initial build or added later. The earliest records I can find, going back to 1997, already indicate that the current window bays were in place. Regardless of when they were built, the design now seems to be inadequately supporting the structure, showing visible signs of failure. Given this situation, underpinning might be the necessary remedy, especially since it's not only the window bays that are affected by movement, but they are bearing the majority of the subsidence impact.


@Tomfh, @XR250, @hokie66. Hokie66 is correct—I'm based in the UK. The insurers are covering other repairs, and they would be responsible for addressing the foundations under certain conditions. One valid reason for them to intervene would be if subsidence directly damaged the foundations, which is a point I've confirmed with them. However, they currently do not acknowledge any damage to the foundations, believing instead that natural ground recovery will resolve all issues.


Thank you for your replies, it's greatly appreciated.