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Garden room heave shrinkage time?

Discussion in 'Building' started by Antony11, 9 Aug 2021.

  1. Antony11

    Antony11

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    Hello,

    I am in the process of building a 27m2 garden room, which is sitting on 15 reinforced concrete blocks, 3 rows of 5. The rubber roof has been on for about 6 weeks, however I didn't connect a gutter to a drain until yesterday.

    Upon putting the battens on for the cladding a few days ago I realised the whole building has been shifted out of level. It has been pushed forward 2.5 cm out of level over a 2.3m wall height (not including the roof). So I boiled this shift down to the rainwater run off expanding the soil at the back, pushing up the back 5 blocks. Truly frustrating and a major oversight on my behalf. Luckily though I haven't started any internal work or decorating yet, just the main structure is up with doors installed on the front wall.

    So to keep going forwards, in these current weather conditions (August 2021), does anybody know how long a clay soil heave will take to shrink back down? I am hoping the building will eventually sit back into level now a gutter is solidly connected and doesn't have a waterfall running off the back .

    Any ideas or advice would be very helpful

    Thanks,

    Antony
     
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  3. oldbutnotdead

    oldbutnotdead

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    No proper foundations I take it? Oops.
    Is this (timber) structure more than a metre from your boundary- if not then Building Control should have been involved...
    That aside, timber is fairly tolerant of ground movement but you will get seasonal problems with doors/window jamming and releasing. Whether it settles back down to level is anyone's guess-it might, it might not.
    If it was just a shed then it wouldn't matter, if you're plasterboarding etc. then you may get problems with surface cracks.
    If you have the space you might want to seriously consider jacking the thing off its blocks, rolling it out of the way and putting some concrete pads or strips in under your blocks (700mm depth is usual, it'll get you below the frost line).
     
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  4. Antony11

    Antony11

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    Thanks for your reply.

    The foundations are adequate for a garden room. They are concrete pads sitting on MOT type 1, running from 400 - 500mm deep. I poured them myself with high strength cement and installed ss rebar into them. The amount of rainwater run off is the issue, it would have lifted a slab, raft, strip etc... potententially cracking them to, so I am glad I didn't do that. The heave is not from seasonal movement but in effect what would be the same as a burst pipe as loads of water that would not naturally be in the area suddenly engulfing it due to no presence of a gutter.

    Rolling it out isn't a possibility as it is bolting down into the concrete pads themselves.

    Also it falls within the permitted development and is over a meter away from all boundaries..

    I also attached images of the foundations
     

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  5. ^woody^

    ^woody^

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    Heave is a phenomenon, not a certainty, and there is no guarantee that soils will move back to previous levels.

    For 25mm movement, that indicates that the foundations are inadequate, and further movement is always a possibility.

    That's the trouble with pads, they actually allow and encourage frequent and uneven movement.
     
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  6. Antony11

    Antony11

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    Hi woody,

    Thanks for your reply. I take your point on board.

    I understand there is no certainty but for something that expands goes up it must also eventually shrink and go down, if the expansion factor is taken away as the water should eventually evaporate.

    I guess only time will tell and am thinking to give it a month to see how it goes.
     
  7. ^woody^

    ^woody^

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    It might go back or it might not, but if some ground has shifted rather than just say being inflated like a balloon, then the soil may not be able to return to its precise previous position. Laying foundations on hardcore is generally a no-no too as it can (and often does) still move or consolidate however much you may have whacked it.

    Al those adjacent trees/shrubs wont help.

    If its a timber frame then it should be able to deal with any seasonal movement, as long as its not just all one way. But take this into account with any planned internal finishes such as rigid plastering, and any important external joints which should keep the weather out
     
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  9. Antony11

    Antony11

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    Hi woody I believe it inflated rather than shifted as the building still is sitting in the exact same location as where I laid it.

    To be honest I did think twice about laying them on hardcore, my idea was based upon a company called easy pads, I will attach a picture of what I mean.

    I was concerned about tree roots to, so I did install a root barrier around each block to prevent any roots interferring with the foundations.

    Thanks for your advice about the internal finishes. I hope to get some kind of flexible filler for internal board joints as I will ply line and paint the inside.
     

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  10. ^woody^

    ^woody^

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    It looks like a nice job and you have done your research, so hopefully it may be just as you suspect and excess localised ground wetting. It may be just a case of seeing what happens.

    BTW, the issue with roots is rarely the root itself, but the moisture that they extract from the ground.
     
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  11. Antony11

    Antony11

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    Thanks Woody.

    I think it will just be a matter of waiting to see what happens. Hopefully 1 month will be enough to give some answers.
     
  12. mrrusty

    mrrusty

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    Personally, I don't think your hypotheses is necessarily correct, because the same amount of water falls on the ground, building or not - all you have done is move it slightly sideways. It could be that the trees sucked excess water out of the ground during peak growing season in the spring, and now we are entering autumn, this is reduced and the ground has returned to "normal" - so spring shrinkage, rather than heave.

    I have seen this before - step-father was a horticulturist and arborist back in the day. 40 years ago a good friend bought a very nice big house very cheap because it was apparently subsiding and slipping down the hill towards the estuary it overlooked. Structural survey said it needed underpinning. SF said "chop all the trees down" - it was surrounded by tall poplars - this was done, trees stopped pulling water out the ground during the growing season, house came back up, cracks closed up and friend never did a single bit of underpinning and still lives in the same house today.
     
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  13. Antony11

    Antony11

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    Hi Rusty,

    Thanks for your reply.

    It is interesting to hear what happened with your step fathers house. And quite cool how the ground basically inflated and the cracks closed once the tree's were cut.

    In the case of my building I had thousands of litres of water that would normally be evenly spread out over 36m2 being channeled into a strip area at the back (rain water run off area) that measured less that 1m2 which was right behind the buildings back 5 blocks. The lean is completely level the whole way across the back of the building implying that something at the back is making it evenly rise. The rainwater run off was very even. If it were tree roots I don't think it would rise so evenly. Neverless I can't be 100% as I can't see under the ground but I feel that is most likely the case.

    Thank you for your point though it is another thing to consider.

    Besides seeing how things go, the main question I am still looking for some insight to is... Any ideas how long saturated ground that is shaded takes to dry, near tree's at this time of year (August). I could just wait but would be helpful to have a timeline so I can plan things for the buildings progress better. Any answers directly to that would be a great help.

    Thanks,
     
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