Strengthening concrete foundations - A question for the experts

Discussion in 'Builders' Talk' started by BuildMore, Apr 20, 2006.

  1. BuildMore

    BuildMore New Member

    My footings go down 900mm, and sit on a 200mm strip foundation. I live in a shrinkable clay area and have minor cracks in my brickwork and strip foundations that are not moving and don't warrant underpinning. I want to build a small single-storey extension on one side of my house.

    I have been advised to build the foundations for the three other walls as trench fill, to the same depth as the main house, but not to tie them in. I understand the reasons for this in terms of not affecting the main house foundations.

    If I dig down and expose the main house foundation that I am extending from, is there a way to stabilize that foundation? It is sagging at one corner. I don't want to underpin, but had considered running a 400mm deep strip foundation alongside and above the current foundation, using steel pins in the lower footings courses to tie them and the old foundation into the new one. Does anyone who understands this area have any advice to give?

    Are there any methods of jointing the extension foundations to the main house foundations that still allow for some movement, in the same way as the wall starter bar I plan to use to join the new extension brickwork to the existing brickwork?
     
  2. SDSMax

    SDSMax New Member

    why don't you want to underpin? looks like you're gonna have to!
     
  3. ­

    ­ New Member


    minor cracks in my brickwork and strip foundations
    that are not moving and don't warrant underpinning.



    What makes you think this doesn't need underpinning?


    > If I dig down and expose the main house foundation
    that I am extending from, is there a way to stabilize
    that foundation? It is sagging at one corner. I don't
    want to underpin


    Underpinning is the only logical and guaranteed solution. Your method is not going to work! Just underpin the footing properly and be done.

    The fact that it is 'sagging in one corner' shouts UNDERPIN.
     
  4. wood butcher

    wood butcher New Member

    if you have minor cracks at the moment then when you start your new build then these will get worse, have a word with your buildings insurance people they might cover the cost of underpinning.
    WB
     
  5. BuildMore

    BuildMore New Member

    Interesting feedback, but I don't agree with what has been said:

    Underpinning may sound like the ideal solution, however if I underpin the corner and effectively lower the foundation there, then the next time the clay shrinks or expands the area underpinned is likely to move differently to before. If this is hard to picture, imagine that I dug down another metre to solid ground. The next time the clay shrinks, the rest of the foundations will move, but not the part underpinned, causing differential stress and new cracking.

    As I said, the current cracks are not moving. They foundations and the soil they bear on are in equilibrium; something I don't want to disturb.

    I agree of course that I could underpin the whole house and avoid any differential movement, but that is not necessary. All I want to do is to stabilize the foundation along one wall.

    One option is to use helical bar reinforcement along the mortar joints, which is a proven technique, however I am still wondering if anyone has experience of sideways beam reinforcement?

    If anyone still doubts my reluctance to partially underpin something which is not moving, take a look at http://www.buildingconservation.com/articles/underp/underp.htm
     
  6. ­

    ­ New Member

    > As I said, the current cracks are not moving. They
    foundations and the soil they bear on are in
    equilibrium; something I don't want to disturb.



    If you're happy that the cracks are not moving, why do anything at all? Why not just repair any cracks and forget about it?


    I agree of course that I could underpin the whole
    house and avoid any differential movement, but that
    is not necessary. All I want to do is to stabilize
    the foundation along one wall.



    I don't get it, you firstly say 'the cracks are not moving' now you say you want to 'stabilise the foundation along one wall'. What is there to stablise if nothing is moving?
     
  7. BuildMore

    BuildMore New Member

    Why do anything at all? If ground conditions continue to get drier over the years, then potentially I may get some more movement in the future, at which point it will be a lot harder to do anything about it given my extension slab.

    By stablize I mean tie the foundations together, so that they are one structural unit again and the foundation's stability is maintained. You might compare this to repointing old brickwork.

    It is lateral support and ridgity that I am looking for. Partial underpinning would achieve this, but the risk of disturbing the equilibrium between the current ground condition and the foundations outweighs this which is why I am looking for an alternative.
     
  8. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    I don't see an alternative. I'm not an expert but, the alternatives you have are 1. underpin the whole house. 2. Do not join existing and new foundations.

    You know already that a new foundation is liable to settle more than an old one, so you cannot even think about propping the old with the new.

    Remember what caused the tsunami.(bit oblique reference I know, but)



    Mr. HandyAndy - really
     
  9. chappers

    chappers Member

    I would agree in the long term you would be far better off sorting out the existing foundations.
    If you really don't want to disturb your current foundations, then get your engineer to design a raft foundation for you.
     
  10. mudhut

    mudhut New Member

    i guess build more`s thinking is a new foundation AT THE SAME DEPTH as the old one will move in the same way (assuming subsoil conditions are consistent around site). this may be so but surely they are likely to settle a bit over a year or two first as has been pointed out. this may be why he has been advised to avoid tying foundations together. i am no expert (so i shouldn`t even be here!) but it seems to me the accepted way to deal with this would be untied foundations and wall starters which would allow for differential movement over the first couple of years or so.

    this all assumes that BC would accept such a shallow excavation in your clay, and that there are no old rubbish pits etc etc to spoil yer plan. it`s all change when you break the ground!!
     
  11. BuildMore

    BuildMore New Member

    Having dug the new footings trench now I have discovered that the house sits on a stepped foundation, which goes some way to explaining the minor cracking.

    I am going to stabilise the existing wall brick footings with helibars/helibond 3 courses apart. That will minimise any further cracking.

    I am going to build new untied foundations that correspond to the existing ones, smoothing out the step, and hope that ground conditions are comparable.

    I'll blind the trench bottom with sand to minimise any compression, and let things settle for at least a month. I'll then build the brickwork using wall starters.

    Thanks to all who replied. Very helpful!
     
  12. ajb1

    ajb1 New Member

    hi just searching and have a question. how does the helibond system work what area does it cover
    cheers tony
     
  13. BuildMore

    BuildMore New Member

    Hi Tony,

    The Helibond/Helifix system consists of helical s/steel reinforcing bars that are retrospectively embedded into the horizontal mortar course between bricks and bonded with a cementitious grout. This helps to spread tensile loads and stresses across the existing masonry. There are other types of application. Take a look at http://www.helifix.co.uk/uk.html.

    My experience of this, both from extensive research into the issue and practical application, is that it is a good remedial solution for brickwork that is suffering from localised stress where traditionally underpinning might have been specified.

    Just to be clear on this point, I am sure there are many instances where underpinning is required, depending on the situation. I have a made an argument previously as to why partial underpinning is not an ideal remedy, and this is why I used the approach I chose.

    David.
     

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