General structural advice needed please

[saveasteading]

It is expanding foam. It expands under the slab, and forces its way into gaps and cracks, and also forces the subsided slab upwards.

 

It does not resolve an underlying problem. 

 

Underpinning extends the foundations downwards, into deeper ground that is stronger and less influenced by weather and trees.

 

Two different things. 

 

I would consider the foam if there was an obvious reason for subsidy other than foundation failure...eg in  a warehouse if 3 tonnes of steel had been dropped.

A guarantee from the installer would be interesting to see, if they have offered it.

[Jilly]

I can see why you have been seduced/sold by the description of the polymer lifting your house back up the required spot in a matter of days. Sounds easy. Can it overshoot and effectively cause heave? 

 

Underpinning is tried and trusted. I've bought a house which has had it done and we had to take over the same insurance policy.

 

If they've had their insurers out, find out why aren't they making a claim? The excess is usually only about £1K. I have a friend who bought a house with an expensive subsiding conservatory and they were able to take over the policy, continue with claim process and got a nice new conservatory eventually. It took a while tho' as I believe they had to reduce the adjacent trees to see if the (ground) settlement resolved. Is something like this an option?

 

 

 

 

[Alan Ambrose]

Hmm, another vote here for standard underpinning. Not much bother with the right crew.

 

Personally, I think 'in a subsidence area but has been underpinned and hasn't moved since' wouldn't sound so bad to future buyers'.

 

Alan

[saveasteading]

I have in my head £350/m for underpinning, but that was a while ago. 

A horrible and slow job, done in small sections, using shovels, tunneling under the foundations.

[markc]

7 minutes ago, saveasteading said:

I have in my head £350/m for underpinning, but that was a while ago. 

A horrible and slow job, done in small sections, using shovels, tunneling under the foundations.

Yes, not the nicest job, mini digger with bucket on backwards helps 

[saveasteading]

1 minute ago, markc said:

digger with bucket on backwards helps 

You have been doing it more recently than me then. Never done it (ie supervising from above) other than manually.

What else has improved? How do you pack up the last few mm to prevent a tiny gap and further settlement?

How long is each section and do you prop it or cross fingers?

In a tight space revert to hand dig?

[markc]

2 minutes ago, saveasteading said:

You have been doing it more recently than me then. Never done it (ie supervising from above) other than manually.

What else has improved? How do you pack up the last few mm to prevent a tiny gap and further settlement?

How long is each section and do you prop it or cross fingers?

In a tight space revert to hand dig?

Corbelled brick and stone (no strip concrete underneath) are the worst and have to be done in short sections, less than 1m to cast a strip footing to support the individual bricks/stones and then excavate again under those if extra depth is needed.

With deeper existing strip footings, 2m sections are possible with props for peace of mind.

cast the new section approx 10mm low and then hand pack with firm expanding grout to prevent settlement.

 

 

[saveasteading]

Cost per m for silvershadow's benefit?

Or if that is commercially sensitive, a range.

I assume that an insurance company or mortgager would want it professionally supervised.

[SilverShadow]

Thanks again guys for your insightful advice

 

To summise - as the house is a 1930's semi detached with shallow foundations on a hill in a clay-soil area, it is likely to be suffering from seasonal swell/shrinkage of the soil. The main issues of concern are adjacent to broken waste/rainwater drains running along the side of the house (perhaps this is causing 'heave' in the clay and actually lifting the house rather than erosion?)

 

The SE report indicated the house is structurally sound, and that the foundation cracking/concerns are likely to be a mixture of thermal movement, settlement, and the broken drains leaking water in the surrounding ground

 

Last evening, i read an article with concerns about the effectiveness of using resin injection on clay with shallow foundations, so it does sound like it is an unsuitable remedy. Likewise the BBA published a paper outlining how resin injection can be suitably used on reactive soils like clay (so i'd expect their opinion to have some gravitas). So it seems there are contradicting verdicts

 

So it seems like resin injection may work on clay soils or shallow foundations, but the question i have is 'will it work on both clay AND shallow foundations?'

 

I guess i'll need a SE to analyse the problem, so we can glean advice on the method of underpinning suitable. If we owned the house it'd be different, but as we're buying one i not sure we have the time/money/inclination to do this

 

 

Thanks again for all your help, and do please add more if you can find any insightful info around resin injection (pros & cons, relating to shallow foundations / reactive soil)

 

Mark

industry-voice-treatment-of-clay-shrinkage-via-non-disruptive-expansive-geopolymer-injection-final2.pdf

Edited November 16, 2022 by SilverShadow

[markc]

27 minutes ago, saveasteading said:

Cost per m for silvershadow's benefit?

Or if that is commercially sensitive, a range.

I assume that an insurance company or mortgager would want it professionally supervised.

Difficult to say without length of run, trial pit and site specific but I would work on £1k-£2K /m with a minimum £6k to cover enabling works. (Haven’t done any commercially, only F&F in last 6 or so years).

 

[SilverShadow]

I have seen this approximate guide - not entirely sure, but i think the sidewall is probably 7m long, so likely to be around £15k for 1 wall....

 

Although saying that, at £1200/m2, kinda implies they will go 1-2m further than the wall itself? (it's the only way i can fathom £10.8k for a 5m wall 🤔)

 

 

Edited November 16, 2022 by SilverShadow

[Dave Jones]

those mini piling figures are tosh.

 

Seen screw piles used before to underpin, clean and quick.

 

https://www.abcanchors.co.uk/access-all-areas-underpinning-with-helical-piles/

[SilverShadow]

Hey Dave - what do you think the mini piling figures should be?

 

And do you think the others seem fairly accurate?

[SilverShadow]

Hey again,

 

Sorry for another post - I don't seem able to keep off this thread!

 

May I ask: in order to ensure the  foundation method is suitable & all work compliant, what would the recommended process be for selection & any sign off of work? (I'm thinking in terms of having the right paperwork & assurances to reduce insurance snags or resale worries)

 

Can we just go to a reputable structural foundation company and say 'fix it' & use their surveyors/engineers, or does it really need additional SE to dictate the approach, oversee/review the work for full assurances?

 

 

Such a huge minefield....🤔

Edited November 16, 2022 by SilverShadow

[SteamyTea]

23 minutes ago, SilverShadow said:

Such a huge minefield

Hill, clay, subsidence and now buried bombs.

I will take the property off their hands for 10k.

 

You have actually raised a very important point.

Without being 'in the trade' it is very hard to to find out what companies liabilities are, and how they can be enforced.

Probably why we need solicitors to act on our behalf, then if the builder (expletive deleted)s up, we can claim against the solicitors insurance.

[saveasteading]

32 minutes ago, SilverShadow said:

go to a reputable structural foundation company and say 'fix it' & use their surveyors/engineers

Yes you can. If their Engineer is fully qualified, and has PI, then going straight to ultimate experts can save you a lot of money.

 

I am just concerned that you are still talking to Sales, and not Engineering.

Imagine buying this house, then the injection people turn up and shake their corporate head.

 

So you must get a written quotation, and written assurances from their Chartered Engineer that this is the correct proposal for this house, and confirmation  of their insurance cover.(which should be in Millions, not thousands.)

 

Part of my concern, and others here I believe, is that this process may close the cracks but not resolve an underlying problem, 

Underpinning takes foundations deeper, where the ground is more stable. I understand that the resin just fills gaps between the foundation and the wall,  but please advise.

[SilverShadow]

Thanks for the great insights

 

 

As I understand (from theirs and other websites) they drill boreholes several metres down, at regular intervals (1 metre apart is the norm). Then with a long nozzle they pump the resin at high pressure, which seeps through the soil filling gaps. Once inside the resin permeates the soil, expands and sets. The resin itself (I think I read) is hydrophobic (if that's the correct term?), so all treated ground isn't liable to heave or shrinkage.

 

I share your concern about what happens to the soil underneath, but the company we found have approval by the BBA regarding treating clay soil with resin injection

https://www.bbacerts.co.uk/

 

On an earlier post I uploaded their report, endorsing this company for such work. So I'm hoping it's fairly legit & above board 🤷‍♂️

 

[saveasteading]

2 minutes ago, SilverShadow said:

fairly legit & above board

No reason to doubt it. But I still say you do what I suggest above.

I have just about done with the discussion I think....now up to you, and do keep us informed.

[SilverShadow]

Absolutely understand and many thanks.

 

Tbh, a chat with their technical team will probably be a good start. Depending on their answer will see where we go. But sales team alone will not cut it for us

 

 

[George]

On 15/11/2022 at 15:21, saveasteading said:

You have been doing it more recently than me then. Never done it (ie supervising from above) other than manually.

What else has improved? How do you pack up the last few mm to prevent a tiny gap and further settlement?

How long is each section and do you prop it or cross fingers?

In a tight space revert to hand dig?

 

Classic method is to leave 75mm gap from the underside of exg foundation and once the concrete has cured dry pack (to minimise shrinkage) to fill the final gap

In good masonry, 1000mm width pins. Shore the sides of the excavation to prevent collapse making the excavation wider. In poor masonry or stone, down to 600mm or less width. May need additional shims to support very poor stone (although very poor walls should be strengthened or aren't suitable for underpinning) but most walls can easily arch over 600mm. 

Hand dig / access only ever will a fully shored excavation and with measure to mitigate against masonry falling in.

 

ASUC members are who to go to.

[BeccaB]

Hi,

I was just wondering if there was an update on your project? Did you go ahead and use the resin for underpinning? I am in a similar situation to yours and am also consider resin.

Thanks