Shallow foundations for loft - underpin?

[JS80]

Hi there,

Longtime lurker could do with some opinions. I have a 1930s semi on a sloping site which has brick corbel foundations which are shallow. There is no current subsidence but after trial holes were dug, SE has recommended underpinning prior to loft extension.

I am reluctant - especially as their opinion was not given immediately while showing me the problem but after 3 other holes were dug, the boss was conferred with in private, and 2 weeks later.

They believe if the house was directly on clay, it would be fine as expected loads won’t reach the 100kn that london clay can take, but it is sitting on what is either clinker concrete (IMO and in builders opinion) or ‘black granular material’ in SE assessment. I have now opened 4 holes, two of them have very firm material under the corbels, which SE is happy is sufficient, but at the front corner of the house the material under the corbels is a bit looser. It can be flaked away if attacked from the side - looks like it is either spalling or has always been just ash and aggregate. It also looks like it may have been semi disturbed during laying of utility gas pipe years ago which runs parallel to wall. There is no indication that wall has been undermined.

SE has offered to design underpinning for a further fee. I have concerns about differential movement after partially underpinning then putting lots more weight on structure. I am also not confident that it needs to be done. The SE doesn’t seem massively experienced, the material under front corner is most definitely concrete - it is grey not black - a bit flakey but it’s been there for 100 years. I guess it was just filled with a slightly different mix.

SE feels like they are digging in behind the liability exclusion this gives them and say it is not reasonable to wait and see what happens after extension goes on. This leaves me in a bit of a conundrum -

Is spalling clinker concrete in trenches under brick corbels unusual? Does the compressive strength get compromised in this case? Should I pay a new SE for a second opinion? Or should I just take the advice I have already paid a lot for?

Any thoughts gratefully received

 

[tony1851]

Has the SE actually calculated the increase in ground bearing pressure due to the loft?

Lofts are generally not that heavy, usually being mostly timber and cladding. You are also allowed a reduction in live load on the floor of around 10% for an additional storey; additionally, it is almost impossible to load a loft floor up to the normal requirement of 1.5 kN/m² due to the increasingly low headroom as you go down towards the eaves.

From a recent experience of checking engineers in one of the London boroughs, they wanted to see if the increase in load on the footings due to a loft conversion was under 15%, which seems reasonable; in this particular case it came in at around 13%.

Underpinning should really be a last resort, and then only if there has been a proper investigation of the increase inload, not just a 'hunch' that the ground might not be OK.

 

[^woody^]

Ive never had the need to underpin foundations for a loft conversion, even on 2ft brick foundations in Victorian terraces.

 

[JS80]

Thanks very much for your responses.

They have calculated the increase in soil bearing pressure - they say it is 28%! I’m not sure why it is so much given there is already a roof on the top of the house. But I guess the flank wall needs to take more load than the back and front because of window openings, it is a large space and a hip to gable/dormer extension. But the gable end will be raised in timber

many of our neighbours have had lofts done, and none of them were even advised to dig trial holes.

Anyone know if it is acceptable or unusual for the side of clinkerconcrete/ trench fill to be a bit friable when poked at with a trowel? Ie The clinker concrete that the bricks sit on is a bit flakey if you dig at it

 

[^woody^]

You may have a numpty engineer. Lots are not increased that much, neither on all the foundations.

Yes old clinker blocks are friable, and that's not a problem with their performance.

 

[JS80]

Thank you very much for you thoughts. I guess in this situation I need to seek a second engineers opinion.

 

[tony1851]

Anyone know if it is acceptable or unusual for the side of clinkerconcrete/ trench fill to be a bit friable when poked at with a trowel?

It's like compact sand - it falls away if you hack at it, but if it is well-contained and undisturbed, it lasts indefinitely.

If it's a flat-roof dormer on the back, I find it hard to believe that building up the gable, and spanning the roof beam onto it, would increase the load by 28%?

 

[JS80]

That is exactly what it is like. It is sitting at a shallow depth under a tarmac driveway so is very well contained. It does seem looser at the front of the flank wall than at the rear, but I suspect it was disturbed when the gas pipe was run nearby and so is a bit more crumbly in this area. It still completely meets the corbels along the length, it just slopes away from there.

I will need to run the calculations past another engineer too then I guess. Seems hard to have to do so when it will cost but I do not want to underpin if it can be avoided without taking unnecessary risk with the building.

thanks again