Previously unknown basement rears it's ugly head

[kandgmitchell]

We own a flat in the north-west in which our daughter lives. It's in an old red brick school built in the early 1900's and converted to flats about 25 years ago.

 

Late last year a ground floor flat owner replaced some flooring and discovered a crack in the concrete which upon further investigation turned out to be a suspended slab over a basement no-one knew about. The developer had removed most of the external access stairs, levelled the ground and put a manhole cover over them. Roll on 25 years and when access is finally gained it is found the steel beams supporting the concrete slab have seriously corroded. So bad that emergency propping was required and the flat owner advised to move out. It appears the basement is entirely under that one flat.

 

This has now blighted the whole building. Two sales have fallen through and no-one is able to move or re-mortgage. The managing agents bailed out as they felt the building was "too much of a risk". The insurers have wiped their hands of it saying it's a maintenance issue. That has been disputed with the ombudsman since the situation was hidden and no maintenance could therefore take place. A decision is awaited.

 

The developer who was also the head landlord has gone into administration. That leaves us 25 leaseholders on the hook for repairs. I haven't been into the basement as yet but my first thought was to cut the beams out, drop the old concrete floor into the basement and then mass fill it with say foamed concrete topped with a slab. However, that suggestion has been poo pooed by the flat owner involved as they say they have been advised by surveyors and engineers not to do this as it would create "maintenance problems" (?) and cause damp in the affected flat, indeed no professional would be associated with any such proposal apparently.........

 

As yet we await a formal suggestion as to how to deal with this but to be honest it has to be cost effective because we will all have to pay and some leaseholders are just young couples in their first home and we haven't got loads of spare cash lying about.

 

Am I barking up the wrong tree with my idea (the volume is probably 4.5m x 5m x 3m) or are we looking at replacement steel beams with say block and beam and screed which involves much more labour?

[Nickfromwales]

56 minutes ago, kandgmitchell said:

We own a flat in the north-west in which our daughter lives. It's in an old red brick school built in the early 1900's and converted to flats about 25 years ago.

 

Late last year a ground floor flat owner replaced some flooring and discovered a crack in the concrete which upon further investigation turned out to be a suspended slab over a basement no-one knew about. The developer had removed most of the external access stairs, levelled the ground and put a manhole cover over them. Roll on 25 years and when access is finally gained it is found the steel beams supporting the concrete slab have seriously corroded. So bad that emergency propping was required and the flat owner advised to move out. It appears the basement is entirely under that one flat.

 

This has now blighted the whole building. Two sales have fallen through and no-one is able to move or re-mortgage. The managing agents bailed out as they felt the building was "too much of a risk". The insurers have wiped their hands of it saying it's a maintenance issue. That has been disputed with the ombudsman since the situation was hidden and no maintenance could therefore take place. A decision is awaited.

 

The developer who was also the head landlord has gone into administration. That leaves us 25 leaseholders on the hook for repairs. I haven't been into the basement as yet but my first thought was to cut the beams out, drop the old concrete floor into the basement and then mass fill it with say foamed concrete topped with a slab. However, that suggestion has been poo pooed by the flat owner involved as they say they have been advised by surveyors and engineers not to do this as it would create "maintenance problems" (?) and cause damp in the affected flat, indeed no professional would be associated with any such proposal apparently.........

 

As yet we await a formal suggestion as to how to deal with this but to be honest it has to be cost effective because we will all have to pay and some leaseholders are just young couples in their first home and we haven't got loads of spare cash lying about.

 

Am I barking up the wrong tree with my idea (the volume is probably 4.5m x 5m x 3m) or are we looking at replacement steel beams with say block and beam and screed which involves much more labour?

Does this affect the structural integrity of the building as a whole, or is it entirely isolated to this one ‘flat’?

[Russell griffiths]

I like your idea, pumped in from outside, leave it 300 mm below the floor level then break out the floor and remove then insulation and a concrete slab on top. 

[saveasteading]

I don't like the idea. An SE is the best bet.

 

The steel has to be examined. Rust isn't always as bad as first impressions.

But there are usually other options such as multiple small beams, additional supports or a wall underneath.

 

It's going to be expensive. Some insurers reject on any excuse. All you can do when getting a quote is answer to the best of your knowledge, so reread what you told them then try again.

 

You are all in this together. Have neighbours been below? Pictures would help.

 

[Nickfromwales]

It would all need to be excavated out, prove and prep the sub-base, then lay a major reinforced fundamental slab. Then build up off that.
 

Settlement of new works will be the crux, so this will need to be done a-la adding a new basement methinks (hard packing etc).

 

Will need a contractor with experience, and very good commercial insurance. 
 

Look into collateral warranties also, as a doomsday parachute, in case the contractor falls over mid / post project. 

[saveasteading]

@Nickfromwales 

 

it's a very big deal as liability extends to the whole block of flats. And might there be other basements?

Who owns the basement/ void?

[kandgmitchell]

Only one basement - the building was mapped using radar. Yes the liability extends to the whole building under the lease so all leaseholders will need to contribute. The initial advice from a firm of consulting engineers was very brief, virtually along the lines of "the beams are severely corroded and should be provided with temporary support immediately". They obviously weren't asked to suggest remedial works. 

 

The building and presumably now the basement is owned by a management company which in turn is owned by all of us 25 leaseholders in equal shares. It's down to us. The previous managing agent (part of a national property consultancy) basically told us all at a meeting that the building was worthless. When a couple asked what that meant for their mortgage and investment in their home the advice was to "hand the mortgage company the keys and default. You'll only be credit blacklisted for about five years...." The company then announced it didn't want anymore to do with the building and resigned their position. 

 

As far as we are aware only this particular flat is affected as it sits entirely over the basement. Personally I'm not entirely convinced that mass fill isn't an option. The live loads from a domestic floor are small, the dead load from the concrete could be reduced by using foamed concrete and I would have thought the total distributed load across the existing basement floor and the ground below would be well within the ground bearing capacity.

 

I'm not wedded to that approach, I was just gaining other opinions to test my theory. I just want to be sure we are not being led in a particular direction to suit a single beneficiary with the additional costs of that being carried by the majority.

[Redbeard]

To take a few points from posts above, firstly you (the leaseholder 'collective') need to appoint a SE with the remit to clearly establish the issue and to make recommendations for remediation. As @saveasteading suggests, rust is not (necessarily) equal to disaster. This needs to be an invasive inspection by someone with the whole picture. We have the Professionals' opinion (Doomsday scenario) but no apparent assessment of the immediate issue. For example, if an inspection of the beams found that rust was bad enough to weaken, but not catastrophic, then that (a) supports to temporary propping suggestion and (b) may suggest that breaking the span - permanent props on footings/mini-piles, maybe - might be a feasible 'treatment'.

 

All of this from someone who is most definitely not a SE!

[Mr Punter]

I know someone who used expanded clay fill, then concrete slab with UFH.  It was a considerable depth.  You will need a SE input obvs.

[saveasteading]

2 hours ago, kandgmitchell said:

Personally I'm not entirely convinced that mass fill isn't an option

Simple can be elegant.

The trouble is the method.

Imported stone ( a lorryload) has to be placed in the basement and thoroughly compacted.

Where would it be stored?

That mean breaking out the floor. Would the stone all be barrowed in?

Then a new floor. So that does have the advantage of simple support to your floor and added lateral stability.

You would have to move out for a month minimum.

 

Another easy solution might be to support the existing beam. A central prop means that the beam only gets 1/4 of the current bending. Or a masonry wall all the width, or a new beam. All needing foundations.

Not quite as disruptive but still a month out of the house.

Gut feeling £50k including fees and unknowns. Lawyers will take more than the Engineers.

25 leases, £2k each.

 

PS.   On second thoughts there will be redecorating, accommodation and likely more that we don't know about. 

For discussion say £100k = £4k each.

Would they all contribute or some not be able to or want to?

 

 

Edited 7 hours ago by saveasteading

[Redbeard]

@saveasteading and OP,

 

I took it that rudimentary access exists from outside, (hidden under a manhole cover) so my musings assumed that full access could be re-instated temporarily. If this is the case the residents moving (everything) out may not be completely necessary - just a few days (weeks?) in a hotel for the 'worst of it'. Dunno. OP, any more details?

[saveasteading]

2 minutes ago, Redbeard said:

hidden under a manhole cover

A coal hole?

It would probably be difficult to fill and compact after the first few feet.

[Redbeard]

15 hours ago, kandgmitchell said:

The developer had removed most of the external access stairs, levelled the ground and put a manhole cover over them.

 

This is the bit I had picked up on. Of course they may have filled in the stairwell, but the manhole cover suggests maybe not? 

[Russell griffiths]

I thought there are a few void fillers that don’t need compacting and are very light and can be pumped. 
the days of barrows and compacting in layers has gone for these large fills. 

Edited 7 hours ago by Russell griffiths

[saveasteading]

1 minute ago, Russell griffiths said:

are very light and can be pumped

Not something I'm familiar with.

[Russell griffiths]

1 minute ago, saveasteading said:

Not something I'm familiar with.

Imagine concrete but the aggregate is like pumice or volcanic stone, very hard but lightweight. 
they use it a lot in void filling after pipeline work. 
 

it can be cheaper and easier to muck away everything that comes out of a trench and import in all the backfill because it doesn’t need compacting, it just pours and forms into all the irregular shapes of the hole. 

[Russell griffiths]

 

[saveasteading]

2 hours ago, Russell griffiths said:

just pours and forms into all the irregular shapes of the hole. 

I like the word 'just' there. Would it also creep into every corner under the floor and expand to put an upward pressure onto the floor, as in underpinning?

 

But its certainly going to be easier than barrowing in the first 80% So that is a decent shout for the bulk fill.

 

[Redbeard]

36 minutes ago, saveasteading said:

But its certainly going to be easier than barrowing in the first 80% So that is a decent shout for the bulk fill.

 

Yes, but is there not a possibility, albeit small, that if the rust is not seriously serious, the work - in the form of 'under-pinning' via piers - could be done from below if the external access can be re-instated? OP and 'Leaseholders' Collective' - do you have any more info?

[saveasteading]

1 hour ago, Redbeard said:

could be done from below if the external access can be re-instated?

Yes it might be moderately simple but this is SE work. If we are to help further we need photos.

[Iceverge]

 Sorry to hear about this predicament.  

 

Filling is a nice easy idea. Make a hole and back in concrete trucks and pour until you see it coming out the top. £10k of concrete and call it a day........

 

However 160 odd tonnes of extra mass over the basement floor might have unforseen consequences. 

 

Also pouring that much concrete might be ok in small bites in one go and I'd be worried about the walls of the basement buckling out. Then comes drainage pipes etc. 

 

Gut feeling is to do as little large scale disturbance as possible and retain access to the basement for future maintenance and inspection. 

 

I don't love the idea of not addressing the existing steels as they sound beyond salvaging, but we can't tell from this distance. 

 

Also it would be good to know if the existing floor is carrying loads that it shouldn't. I have a suspicion that if the external area around the apartment was backfilled the floor may be resisting the pressure of the ground pressing in from outside.

 

Could you perhaps make a semi accurate cross sectional drawing and maybe some pics. 

 

A good SE, a diligent local builder and a few £1000 in men and materials might be enough to solve this yet.

 

 

 

 

 

 

 

 

 

 

[-rick-]

3 minutes ago, Iceverge said:

However 160 odd tonnes of extra mass over the basement floor might have unforseen consequences. 

 

Depends what fill you use. I thought I'd seen people talk about fill that was a mix of concrete and other lightweight products, so the overall weight might be relatively limited. Especially if you are replacing the steels with a solid floor.