Underpinning - how to do it.

marsaday · 15 Apr 2014

I am buying a house and the structural report from 1991 talks about a crack above the rear door and up to the bed window (diagonal in direction). I can see the crack today (repointed) and the house also has a report done in 2007 which mentions the same fault.

The movement does not look ongoing, but the report in 2007 covers itself by recommending 2m (minimum) of underpinning and to strap the floor and ceiling joists in this area to the wall.

So the strapping i can do and seeing as the whole house is going to be ripped out and rejigged all is not a problem.

The underpinning doesn't faze me too much and i wondered if anyone has any tips on this bit.

I haven't done any research yet, but thought i would post on here to get some advise. I know i cannot excavate too large an area at a time. Is 1m ok to do in one go?

Can i make my own mix on site and does it have to be a specific strength?

Will building control oversee this task and give me the necessary pass cert?

Alastairreid · 15 Apr 2014

//www.diynot.com/forums/viewtopic.php?t=248084

Static · 17 Apr 2014

You can get a structural engineer to do calcs, drawings and method statements for you..

You need to determine the depth to go down to and extent of foundations to underpin. It may determine if piling is require etc..

But that said, firstly who did these reports.. was it a structural report?
If the movement is not ongoing then why underpin?

To answer your questions, 1m width of dig at a time should be fine for domestic works.. maybe stretch it a little more.. its more about planning which sections to do at a time, will need to cure between castings so can take weeks to complete.

Normal foundation mix will be fine (C20 / GEN3)

BRegs will be needed.. and will provide a completion certificate.

fmck · 17 Apr 2014

If you are going to undermine the foundation then it would normally be carried out by excavator. The foundation could be in a dangerous condition as it has failed. The side of the excavation will be vertical as well not a good place to be in there.

wessex101 · 17 Apr 2014

Assuming it does need underpinning probably the first thing you need to do is dig a trial pit to find how deep the stable ground is and the make-up of the sub-soil.

The last one I did had wet soft sand down to 2.2 metres. Obviously that was a piling job.

Pretty much everything Static said.

One snippet of advice if you do go down the mass fill hit and miss route. Leave a good gap between the top of the new concrete and the underside of the existing footing, about 50mm. It is much easier to ram the dry mix in good and tight rather than trying to squeeze it into a tight 25mm gap.

IJWS15 · 17 Apr 2014

You may also want a warranty on the work when you come to sell on. You won't get that by doing it yourself.

tony1851 · 17 Apr 2014

IJWS15 said:
You may also want a warranty on the work when you come to sell on. You won't get that by doing it yourself.

Wouldn't a Certificate of Completetion from the council's Building Control dept. do for that?

freddiemercurystwin · 17 Apr 2014

tony1851 said:
IJWS15 said:

You may also want a warranty on the work when you come to sell on. You won't get that by doing it yourself.

Click to expand...

Wouldn't a Certificate of Completion from the council's Building Control dept. do for that?

I expect mortgage lender's and insurance companies will expect a warranty. A certificate from BC is no warranty. I believe that there are also checks to be carried out following the works to prove the subsidence has ceased or has been fixed by the works Which I imagine need to be carried out by a professional rather than the owner. Bear in mind that even when every possible box is ticked few lenders and insurance companies will touch it. The OP needs to get professional advice.

tony1851 · 17 Apr 2014

freddymercurystwin said:
tony1851 said:

IJWS15 said:

You may also want a warranty on the work when you come to sell on. You won't get that by doing it yourself.

Click to expand...

Wouldn't a Certificate of Completion from the council's Building Control dept. do for that?

Click to expand...

I expect mortgage lender's and insurance companies will expect a warranty. A certificate from BC is no warranty. I believe that there are also checks to be carried out following the works to prove the subsidence has ceased or has been fixed by the works Which I imagine need to be carried out by a professional rather than the owner. Bear in mind that even when every possible box is ticked few lenders and insurance companies will touch it. The OP needs to get professional advice.

Assume someone builds a single-storey rear extension, and then takes out the whole of the rear wall of the house and inserts a beam to open it up. Calcs/details etc are sent to Building Control, who approve them and in due course issue the Certificate for the completed work.

In the absence of some waranty from somebody, does that then mean that the house becomes un-mortgageable/uninsurable,? Or is there a difference between a foundation repair (which admittedly might not work) or inserting a large beam (which also might not work through overloading the foundations)?

freddiemercurystwin · 17 Apr 2014

tony1851 said:
is there a difference between a foundation repair (which admittedly might not work) or inserting a large beam (which also might not work through overloading the foundations)?

In terms of being able to get a lender to lend on the property I don't know and neither do you. But it may be advisable for the OP to check don't you think? Or can he say to the next buyer who's lender refuses to lend on it that some bloke on a forum thought it was ok so please be nice and give the buyer his mortgage?

tony1851 · 17 Apr 2014

I don't know either. Could be that if we do any work of a structural nature on our property, and which doesn't have some sort of warranty (in addition to a b/c certificate) we might never be able to sell on??

KenGMac · 18 Apr 2014

marsaday, hi.

A quick question? that is has the crack that was flagged in the two reports noted opened up again, if after this amount of time it could be regarded as being stable, if however the crack has opened then it is probable that there is on-going movement of the property.

As an aside the position of the crack, door on ground to first floor window all too common, several non Subsidence caused spring to mind, Lintel failure, Thermal and moisture movement of the wall being two.

If you are going to do a major make over then you could consider, provided the crack has NOT re-opened re-pointing the entire wall, this will have the effect of blending in the old crack, the insertion of Helibar across the crack will assist if a general re-point is considered.

As for traditional underpinning that you are contemplating, I have not seen that done for a long. long time, the last one failed because the inexperienced Developer / Builder removed a two to three meter length of fill under a massive wall and left it for a day prior to infill with Concrete.

When I go to a Subsidence insurance claim there are several protocols that happen.
1/. Instruct a Soils Investigation to a depth of about five meters in at least two locations, the results of this will give information not only on the various strata but also the bearing capacity of the soils at differing depths.
2/. instruct a drain survey to check for leaking drains, repair as required.
3/. in a few cases, now considered by insurers and Loss Adjusters as a waste of time, you can monitor the crack using studs or proprietary crack monitoring gauges.
4/. Depending on the type of Property instruct either Uretek or Shire piles to visit and design then get a cost for installation of either system, they will undertake the design work, obtain a building Control approval. The type of preferred method is dependant on a number of factors, for example, Uretek does not like Peat.
5/. Work starts and completes, building Control Certificate produced, Installers documentation forthcoming.
6/. Finally the insurer or more likely the loss Adjuster issues a Certificate of Structural Adequacy.

The above is an approximation of what tends to happen in an Insurance claim, as an aside Scotland is slightly different, well why not?

Now for the kicker, when you come to sell !

You will have to declare that there is Underpinning [of whatever type] to the prospective purchaser.

The prospective purchaser may be scared off instantly? or start to go through the process of purchase, the prospective purchaser Solicitor should pick up on the underpinning and may ask questions if the Solicitor for the purchased is on the ball.

Lets say the purchase is well advanced the prospective buyer starts to cast around looking for insurance.

If the Underpinning has been completed by an Insurance claim then there is a possibility, no greater than that, the next Insurer will consider the risk, but, there may be several exclusions as regards Subsidence, or the premiums will be higher than normal for this property in this area.

If you proceed and undertake a DIY underpinning, I think everyone will simply run and keep on going. what is your track record in underpinning? is the one question the underwriter will ask.

At one time if an insurer issued a Certificate of Structural Adequacy to the seller, if the purchaser applies to the same Insurer, and is deemed a good risk by that insurer then cover will [probably] be given, but not in all cases.

Insurance and such things are exceptionally complex issues, there are some rules, but and this is a big, big but no two Insurers will make the same decision as regards an event.

Ken

indus · 19 Apr 2014

KenGMac said:
marsaday, hi.

A quick question? that is has the crack that was flagged in the two reports noted opened up again, if after this amount of time it could be regarded as being stable, if however the crack has opened then it is probable that there is on-going movement of the property.

As an aside the position of the crack, door on ground to first floor window all too common, several non Subsidence caused spring to mind, Lintel failure, Thermal and moisture movement of the wall being two.

If you are going to do a major make over then you could consider, provided the crack has NOT re-opened re-pointing the entire wall, this will have the effect of blending in the old crack, the insertion of Helibar across the crack will assist if a general re-point is considered.

As for traditional underpinning that you are contemplating, I have not seen that done for a long. long time, the last one failed because the inexperienced Developer / Builder removed a two to three meter length of fill under a massive wall and left it for a day prior to infill with Concrete.

When I go to a Subsidence insurance claim there are several protocols that happen.
1/. Instruct a Soils Investigation to a depth of about five meters in at least two locations, the results of this will give information not only on the various strata but also the bearing capacity of the soils at differing depths.
2/. instruct a drain survey to check for leaking drains, repair as required.
3/. in a few cases, now considered by insurers and Loss Adjusters as a waste of time, you can monitor the crack using studs or proprietary crack monitoring gauges.
4/. Depending on the type of Property instruct either Uretek or Shire piles to visit and design then get a cost for installation of either system, they will undertake the design work, obtain a building Control approval. The type of preferred method is dependant on a number of factors, for example, Uretek does not like Peat.
5/. Work starts and completes, building Control Certificate produced, Installers documentation forthcoming.
6/. Finally the insurer or more likely the loss Adjuster issues a Certificate of Structural Adequacy.

The above is an approximation of what tends to happen in an Insurance claim, as an aside Scotland is slightly different, well why not?

Now for the kicker, when you come to sell !

You will have to declare that there is Underpinning [of whatever type] to the prospective purchaser.

The prospective purchaser may be scared off instantly? or start to go through the process of purchase, the prospective purchaser Solicitor should pick up on the underpinning and may ask questions if the Solicitor for the purchased is on the ball.

Lets say the purchase is well advanced the prospective buyer starts to cast around looking for insurance.

If the Underpinning has been completed by an Insurance claim then there is a possibility, no greater than that, the next Insurer will consider the risk, but, there may be several exclusions as regards Subsidence, or the premiums will be higher than normal for this property in this area.

If you proceed and undertake a DIY underpinning, I think everyone will simply run and keep on going. what is your track record in underpinning? is the one question the underwriter will ask.

At one time if an insurer issued a Certificate of Structural Adequacy to the seller, if the purchaser applies to the same Insurer, and is deemed a good risk by that insurer then cover will [probably] be given, but not in all cases.

Insurance and such things are exceptionally complex issues, there are some rules, but and this is a big, big but no two Insurers will make the same decision as regards an event.

Ken

Hi Ken

May I ask a question? You mentioned that the last traditional under pin you saw failed because the builder left a day between excavating and filling with concrete.

Why should this cause the under pinning to fail?

Thanks

KenGMac · 19 Apr 2014

indus hi.

It was a complete catalogue of errors from start to finish.

The Developer and his builder had shoe horned a load of what on the externals appeared to be at best mediocre detached, so called executive villas.

The information I was able to obtain came from two sources not connected to the Developer or the Builder.

My remit. acting for the insurer was to stabilise the wall and start what are referred to as recovery proceedings

To one side of the development there was a large, 5.0.M high retaining wall that was in effect retaining anything from 0.5.M to 3.0.M of earth on the opposite side of the wall from the Developers site. the wall itself was on average 600.mm wide made from solid masonry, predominantly Basalt.

This retaining wall was some 1.5.M from a timber framed property, hence the need to underpin the wall so as to get the house constructed after removing about 1.5.M of top soil from the Developers side of the wall.

Builder starts, using a Mini-Digger he under the direction of a Structural Engineer the Builder excavates out a trench but in place of a trench no more than 1.0.M the builder excavated out a trench about 3.0.M long exposing the Foundations of the retaining wall and completely undermining them. the Foundations were in fact very large stones, no concrete anywhere, after all the wall was well over at least 100 years old, probably more?

So to start to compound the problems for the Builder, the Concrete did not arrive to infill this Yawning gap below the wall.

There were no sacrificial props inserted over the full 3.0.M trench.

The builders went home, then it rained all night very hard, another compounding influence.

The next day the Builder pitches up and has to use a water pump to get last nights rain water out of the excavation, this dome he calls in the concrete, as an aside there was no blinding concrete placed, the concrete was placed into the trench, job done or was it?

What then [Obviously] occurred was that the wall subsided and rotated, that is the wall subsided and the entire wall moved outwards towards the Developers property.

Then it all kicked off, everyone headed for the hills in that, the Structural Engineer would not answer letters, or phone calls, the builder would co-operate but would not divulge what instruction he had received from the Structural Engineer, the Developer would not respond to anything at all, letters phone calls and was not available if you pitched up at his offices.

As for what I instructed, there was an exhaustive once a month visit by a specialist firm of monitoring Engineers who set pins in the wall and took the readings, this firm also undertook a form of level monitoring using at that time a theodolite to monitor the bulging of the wall.

Repair? I instructed a very reputable firm of Structural Engineers to inspect the wall, gave them all the Monitoring evidence that had been gathered Etc.

It was then decided by the Engineer that I appointed, to use a system of Ground Anchors to try to stabilise the wall, this was done and I think it worked.

Running parallel to all this the recovery Solicitor was being kept fully up to speed with all Info, photos, videos and explanations from the Engineers appointed, I had a two hour session giving a deposition about the entire event.

At about this point I lost contact with this event, I was TUPE transferred to another firm.

The Icing on the cake? the Builder who caused the problem attempted to prop the bulging wall with a bit of 100.MM by 50.MM to the brickwork corner of the timber framed property nearest to the damaged wall.

Its things like this that working for insurers makes life that wee bit more interesting.

Ken

tony1851 · 19 Apr 2014

Incredible stupidity!

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Underpinning - how to do it.

marsaday

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