Previously unknown basement rears it's ugly head

[kandgmitchell]

I'm sure I've seen some photo's but can't find them. They tend to show a forest of brand new screwjacks which I believe we have purchased.

 

It's useful to have this sort of feedback as it demonstrates there are alternatives that need assessment before a design decision is made. Just to qualify my approach I'd say using foamed concrete would put an extra 3kN/m2 load on the existing basement floor/sub grade. I'd say that was pretty nominal in the great scheme of things and likewise, horizontal loading on the existing basement walls would be resisted by the existing external ground. Access to pump could be via one of the ground floor windows of the affected flat. 

 

Mind you if the costs come in too high it might be just as economic for leaseholders to club together and buy that flat leaseholder out. Remove the floor and put a simple steel stair behind the front door to give access into the basement. It's always worth thinking outside the box!

[saveasteading]

28 minutes ago, kandgmitchell said:

worth thinking outside the box!

Quite so. I've just reread the original post. 

Concrete cracks, steels corrode. Newish walls can be removed again.

The managing agents were not technical people, so their comments are probably irrelevant.

 

So the residents' trust perhaps need to get a fund together and start from scratch. A good local SE will be a starting point. A free first visit for a quote.

Then a solicitor (there are construction specialists)  to formalise the process, get insurance etc.

Then sort it. The value of all properties will then increase by more than the cost, esp for those who can't sell.

 

OR. Is there a local big developer/ landlord who might buy the lot and sort it out... takes the risk and makes money?

[-rick-]

Maybe I'm an optimist but to me the hardest problem you have to solve here is to get everyone to agree how to move forward. In a way, winning the appeal for insurance would help a lot with that as it will take the details somewhat out of your hands and into the insurance companies.

 

Still, while you wait for determination there well worth getting the SE in to do a proper assessment (which should be easy to agree).

[Gus Potter]

On 22/07/2026 at 10:42, kandgmitchell said:

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.

For me this is a interesting challenge to design a solution. I appreciate that you find it far from interesting and it's a worry. 

 

On 22/07/2026 at 12:51, saveasteading said:

Another easy solution might be to support the existing beam.

Stage 1:

 

One option I would be inclined to look at first is to install another beam or beams below the ones you have if it turns out they are so heavily corroded they are well below capacity. Remember we would look initially to design these for the latest loadings prescribed in the design codes. The following is based on using a trick we deploy when we are needing to support say a large opening in a historic building or repair steels, more applicable to you. 

 

The existing beams have a point of support on say the main walls. Thus in the round we know these are the strong and proven points. It's a reasonable starting assumption the walls locally will have pretty much finished settling  in and around the existing supports, thus we want to put our new point loads at these positions. 

 

Let's say, as we don't have much information that the beams are supporting the ground floor flat but also an internal spine wall that runs up the building and it turns out there is a fair bit of load. It make the challenge more interesting. This is in a bit of reverse order.

 

How do we calculate the loads and do some other thinking. Well the loads are easily calculated once we know the building layout, floor and roof span directions. From that we can calculate the forces on the existing beam/s. We would want to split these between permanent loads ( dead loads from the materials) and imposed ( live loads) from people, furniture, snow roof load etc. Next we would want to have a look at the existing corroded beams, try and identify their section properties when new and as they are now, corroded. We can then make a reasoned guess as to what their original design strength was and what it is now. 

 

The extra thinking comes for example as one obvious question is why has it not fallen down to date? Have the beams deflected a bit too much and shed load elsewhere. Remember a building does not often fall down until it has exhausted every alternative load path. This is what SE's often call redundancy, which helps stop what we also cause disproportionate collapse, like the domino effect. We would want to get a handle on this before we access any confined space. 

 

Any Engineer is going to want to do a pretty comprehensive dilapidations survey. This heads off any "adventure’s seeking to take advantage, but it also protects the honest residents if some further cracking occurs during the works. 

 

In summary stage one is doing some fag packet calculations, a survey and from that we can get a feel for how the building is "working".

 

Stage 2:

 

Again not quite in the right order but it easier to hold the narrative. Lets say we can install beams under the existing, then do some bolting etc to support the loads, maybe by stiffening the webs of the existing beams or by using other simple plates / angle sections. This means the new beam/s load the supporting walls in the same place, proven to have worked so far. The big thing is that there is a good chance you won't be able to get the new beans into the basement in one complete length. Thus splices will be required, which tend to flex a bit. Also critically the new beams won't take up the load unless we pre deflect them. For them to carry load the building would need to settle onto them.. high probability of cracking as we move up the building.

 

There is a technique we use on historic buildings where we get the new beams in with a bit of clearance and then jack between the old beams and say the top of a new beam to pre bend the new beam so it takes up the load. We have calculated the loads so have a bench mark as to how much we need to jack / pre bend. We are only talking mm here so we would use say dial gauges. In practice what we do is jack it a bit then run up stairs and see if cracking is starting or if existing cracks are opening up, doors starting to jamb an so on. This takes time, it's not a one day job!

 

Stage 3:

 

As above and in reverse order. The new beams often need to rest on a new padstone below the existing one spreading the load from the existing corroded beams or we need a spreading arrangement. This is partly a detailing and practical thing. But once access is gained with a fair wind usually there is a way of applying the load to the supporting walls in the same place as the existing beams are. Again simplistically we just trick the building, it's sub structure into "thinking" nothing has changed above. 

 

But this is not an exact science and 99% of the time some minor movement will occur and that has to be communicated to all parties. 

 

Comment:

 

Infilling the basement or sifting the loads to a different place invites lots of potential settlement problems and so on. You would need to investigate the basement floor and the soil under which is going to be less consolidated than the soil under the main load bearing wall for example. 

 

Construction safety is going to be the driver here in terms of a structural solution.  I would look to have at least head height clearance for folk to work. Two means of escape from the basement. Make sure we have a good ventilation strategy during and after the works, avoid any welding, use of volatile paints etc and make sure there is compliance with all the regs relating to working in confined spaces. 

 

In summary it may turn out that the SE solution is not complex, it will likely need a diligent / experienced and well supervised Contractor. The hard part is herding the rest of the cats, project administration and agreement of where the liability lies etc. This goes along way towards explaining  why many designers don't want the hassle of a project like this, the design fee has to be attractive not least.