Underpin internal wall on ex-outbuilding?

[nips]

I've recently had some underpinning design work done, and it's far more extensive than I'd hoped. I'm hoping to get advice here on how thorough I need to be. Thanks in advance to anyone who reads this!

We have a semi-detached 60s ex-council house, which had a flat roofed outbuilding joined onto the rear as part of the original build. When we moved in, the survey noted a diagonal crack in the rear wall of the outbuilding structure, likely indicating structural movement, and the previous owner said that there used to be a problem with overflowing guttering at the corner. The crack is ~10mm wide at the top of the single storey, and the slab had no visible damage.

The whole outbuilding structure (i.e. our half and the neighbour's half) is approximately a 5.5m by 5.5m square - so our half is just under 3m wide, and extends ~5.5m into the back garden compared to the rest of the original house. The walls are all single-skin brickwork, resting on a 250-300mm thick concrete slab. There are no foundations supporting the slab of the outbuilding. The outbuilding has a flat roof, previously containing asbestos although we've had that removed during building work.

The neighbouring property had a two storey extension to the main body of the house a few years ago, and we had an almost identical one built last year. The extensions bring the rear of the houses 3+ metres further into the garden, and included underpinning the outbuilding slab where there is now a shared wall, since the wall now helps supports two storeys of actual house rather than just the outbuilding roof. The foundations for the extension are ~1.5m deep. The outbuilding was made into a habitable space with addition of wall/ceiling/floor insulation, heating etc.

We didn't get the crack investigated at the time (which now feels foolish). We are planning on having a patio laid, and thought we should get the crack investigated before we pay money to dress the ground's surface. A structural engineer took a look and suggested we have a test pit at the corner in question. I dug to the level of the foundations (250-300mm slab) and the engineers then took soil samples down to ~1.5m below ground level. They recommended extending the foundations to bear within some stiff clay 1.2m below ground level.

We asked them to provide design drawings. The drawings detail mass concrete underpinnings along the rear wall and remaining side wall of the outbuilding, _and_ the whole way back along the _internal_ party wall, and then to replace the entire outbuilding slab with a beam and block floor.

Our insurance isn't handling this, so we are extremely reluctant to have the entire outbuilding (now an internal, lived-in space) floor dug up. Given the fact the subsidence seems to be historic, and so much of the outbuilding slab is supported now by the extensions, we'd be very happy (for ourselves) just having our remaining edges of the slab supported. However, we're worried about getting a half-job done and then finding out when we someday sell the house that it's considered insufficient by future surveyors.

What would you do? Do we need to underpin the party wall and replace the entire slab, or is that just an overkill, money-is-no-object approach? Is it worth doing the full, disruptive version? Am I setting myself up for a structural or house-selling disaster in the future?

Diagrams attached of:
- Original build
- After neighbour extension
- after our extension
- engineer proposal
- my "just underpin rear proposal"

 

[KenGMac]

Deep breath, stand back?

Is the crack still moving? Has it been monitored in any way?

Is the cracked wall brickwork or roughcast render? Externally? is the external crack replicated internally? or has it been hidden by [say] plasterboard.

Your Structural Engineer is not hanging about [with your money] and as for underpinning the party wall? that could??? end up in tears??

Did your S/Eng. tell you what the material was between the underside of the slab and the "firm" clay?

Sorry more questions than possible answers,

As for mass fill, underpinning, that [could] be considered an "old fashioned method" of repair, suggest you look @ Shire piling which in the Insurance Industry is the way to go BUT !! ONLY IF NEEDED??

Ken.

 

[nips]

More questions is good!

We haven't noticed any movement in the last three years (i.e. since moving in), but it hasn't been monitored properly - neither we nor an engineer have measured it over time for changes.

The cracked wall was peviously single-skin brick, bare on both sides (the section of outbuilding on the end was essentially just shed space - there was a second brick wall inside to make a small room and a single, non-close-fitting wooden door, going to the outside. No access to the rest of the "outbuilding"). The crack went the whole way through the brick work to the indoor of the shed space. It's now concealed on the inside by plasterboard.

Between the slab and firm clay is.... more clay. Moist, sandy clay with medium sized stones for the first 600mm below the slab, then chalky clay for 180, then the stiff clay.

Why is shire piling better than mass fill?

 

[KenGMac]

Right, understood.

Several things, in no particular order.

I can see no rational in ripping out a concrete slab and replacing it [at no little expense?] if there is nothing wrong, no major cracks in the slab or the other walls?

Given the cracked wall has in effect been "reinforced" by a second internal skin, did you see any cracking in this internal [secondary] wall? any plasterboard cracks? I very much doubt it??

It is of interest that you have not noticed any changes in the crack during your tenure, having said that because you are living with it ant increase of crack width will be very small.

As for Shire V Traditional Mass fill U/Pin??
Shire are far, far faster, less intrusive.

For Fill, you need to excavate a one meter by one meter hole [however] deep,
The excavated material has to be removed.
Concrete has to be imported and placed
The excavations are generally done on a [so called] hit and miss system.
All the above, very time consuming, messy and labour intensive + associated costs.

With Shire ?
Two guys turn up and can install several piles per day, nothing like as much excavated material to dispose of? Minimal concreting needed?

if it were me?? I would install [say] three one meter long "Helibars"
Roughcast render the external wall of the remaining exposed original building Introduce external Insulation at the same time??? just a consideration??

My background here is that I work in the Insurance industry, including Subsidence.
If this were an Insurance claim?
1/. Monitor the crack.
2/. If the crack is "stable" -- not moving, the Insurer walks away.
3/. if the crack is "Active"?
A/. Install helibar
B/. Install a few Shire piles --- Job done.

The Insurer would not undertake repairs to any part of your property not affected by a possible Subsidence event.

Sorry for the ramble, hope it assists??

Ken.

 

[nips]

That's a brilliant response, thank you Ken

It's been 16 months since the inside of the wall was covered (by stud wall plus wool insulation, plasterboard, then plaster) and there are no cracks visible internally.

On first visit (pre inspection hole), the structural engineer suggested there was a reasonable chance the only thing needed was helibar. I think it was the (lack of serious) foundations and poor soil that pushed them to suggest underpinning. That doesn't seem unreasonable to me, but it did feel like a massive jump to "replace everything", even if the block and beam replacement is being recommended as a "well you're already digging up half the floor", rather than being necessary in of itself.

I'll look into shire piling, but if cost and time is one of the main advantages, I'm not too worried - the builder who handled our extension estimated that hit and miss foundations should be approx a 2 day job. We're only looking at around 4 linear metres to have done the whole of our remaining outside wall sections. That said, since we have been unsure which direction to go in, we haven't got numbers from him yet on concrete plus soil removal cost.

I think my next step is to have a decent conversation with the engineer, just to get their steer on "need" vs "would be nice if insurance were paying", then decide what to do. It's been very useful to get the input from someone not as closely involved.

Thanks again,
Pete

 

[KenGMac]

Out of interest?

If you can could you post an outcome?? please.

Thanks.

Ken.

 

[gasbanni]

I would suggest you also get a second opinion from a different structural enginer, without giving him the recommendations of the first one.
A pair of fresh eyes on the subject may result in a different view and solution.

 

[nips]

Sorry for the silence so far - finally managing to hold a meeting with our engineer this PM, and will be able to provide an update.

 

[^woody^]

Ive not read all the saga, but if the crack is on the corner, implying the corner has dropped, why can't just the corner be pinned?

Or reinforce the wall with helibars to create beams to support the corner?

 

[RonnyRaygun]

Bit late to the party here, but I carry out structural surveys on damage such as this all the time.

We would never recommend underpinning as a first resort, regardless of the depth of the existing foundations.

As others have said, the critical factor is whether or not the movement is ongoing. Typically we would recommend the following:

• Begin monitoring the crack monthly using monitoring studs and digital calipers
• Trial pits dug to, say, 1.2m and if possible hand auger to 2m
• Determine depth of foundation
• Check nearby drains to see if they are intact - in gravelly soils leaking drains may wash out the fines from the soil
• Take soil samples to check soil type, level of water content, and whether or not the soil is desiccated
• Check for tree roots below the foundations. Take samples of any tree roots and have them analysed to determine their species
• If the soil is clay and affected by nearby trees, arrange to have the trees cut back or removed completely
• Continue to monitor the crack to determine if the movement is ongoing or has ceased

There is nothing to stop you monitoring the crack for, say, six months prior to carrying out any other work, to see if it remains stable.
It would still be prudent though to check what is going on with the foundation, as movement could begin again during a dry summer (if clay).
Only when movement has ceased for an extended period should Helibar repairs be carried out.

 

[KenGMac]

monitoring studs and digital calipers

Great method, but?

I have veered away from this and tend to instruct a Level Monitoring Survey.

Why?

2/. Reasons.

1/. A simple crack can / will open and close depending on seasonal changes especially in direct sunshine.
2/. The Insurance industry "standard" for either accepting or declining a claim for Subsidence is "the Vertical Downward Movement of the Foundations"

What can happen is that you end up chasing your tail using studs and calipers, what you are proving is that the crack is moving, the basic question remains Why is the crack moving ? [opening up or closing down]

Level Monitoring gives a clear indication that the Founds are moving [down] or indeed not, and the crack is simply Thermal and moisture related movement.

Ken.

 

[RonnyRaygun]

Great method, but?

I have veered away from this and tend to instruct a Level Monitoring Survey. Do you have your own equipment to do this or do you employ another company to take the readings?

Ken.

Interesting. Will have to look into this method in more detail. Do you have the equipment to carry out this type of monitoring, or do you employ another company to take readings for you?

With regard to the opening and closing of the crack, I tend to graphically represent the monitoring and if the graph indicates cyclical movement of relatively minor values, then the conclusion is that the movement is not subsidence related.

 

[KenGMac]

do you employ another company to take readings for you?

Yes, it saves my time, OK it costs, I cannot afford the time to drive XXX miles at regular intervals to obtain the readings, and the time needed to actually undertake the readings, and all the kit to actually take the readings, pins in walls, Set up a Datum, a device to take the readings Etc. Etc. as far as the Insurer is concerned it is another "professional" adding intellectual value to the claim.

What the Insurers are in effect doing [using level monitoring] is kicking the can down the road, and hopefully getting a negative result for a Subsidence issue.

The level monitoring is in the eyes of the Insurer an easy way for their handlers to either accept or decline, it is going down [Subsidence] or it is not, must be Thermal / Moisture movement --- claim declined --- Exactly what the insurer is looking for.

Sorry, a wee bit cynical, but unfortunately true.

Ken.