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Foundations for a retaining wall


 
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bobloes

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PostPosted: Thu Feb 11, 2010 3:11 pm Reply with quote

I am going to build a retaining wall 50cm high and 11 metres long using concrete blocks wide side down. I will put in one control joint in the middle. Can the control joint just be left 'open' ie no fibre board inserted? The soil is, at the foundation level, heavy clay. I was advised that I should put reinforcing steel in the foundation due to shrinkage in the clay. Seems a bit OTT. Any advice? Finally how important is hardcore. For example would not leaving out the hardcore and making the footing deeper have the same effect. Cheers.
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r896neo

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PostPosted: Thu Feb 11, 2010 5:17 pm Reply with quote

You Don't need to put anything in the expansion joint if you don't want to, it will make no difference. As for your footings it's up to you, a sheet of steel mesh is not exactly a great expense but for a wall 500mm high the loading isnt exactly astronomical, The further you go down the better and the less likely you need steel.

As for hardcore under the footings?? EH?? That sounds like a recipie for trouble.
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Chukka63

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PostPosted: Thu Feb 11, 2010 5:47 pm Reply with quote

just dig through the vegetation,concrete 150mm thick and 400mm wide will suffice.
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noseall

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PostPosted: Thu Feb 11, 2010 6:12 pm Reply with quote

You do not put h.c. in trench footings. Pour the conc' straight onto the trench bottom.

Make sure the foundation is 150mm wider than the wall each side as this will discourage any tipping and seek to stabilize the wall on a dodgy (clay) substrate.
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Blagard

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PostPosted: Thu Feb 11, 2010 11:01 pm Reply with quote

All agreed no harcore under the foundation!

Your wall is really quite low and am I right in thinking that 50cm is the difference in the ground levels ? Next thing is the gound behind the wall level or sloping down towards the wall ? If level then this is really a fairly simple job. I would suggest that you insert some tubes into the wall at around the lower ground level to allow any water build up to escape. Backfill the immediate area behind the wall with some aggregate to allow drainage to the weep holes. A few coats of RIW of similar on the back of the wall to help prevent the blocks becoming saturated with ground water

Normal retaining wall design will have the foundation wider on the back to reduce rotation and occasionally with a "Toe" on the front of the foundation to prevent it slipping. In your case I doubt a toe would be needed if your foundation is a reasonable depth below the lower ground level.

On the chance the ground is sloping down to the back of the wall or in the event you have any other foundations in the higher ground, please give more detail. A half metre retaining wall near your (or someone else's) house could give a serious problem. Also make sure the area you are presumably digging out is no danger to any services in it. (goes for all excavations!)
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Chukka63

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PostPosted: Fri Feb 12, 2010 7:14 am Reply with quote

i wouldnt bother inserting any draining tubes/pipes for a 500mm high wall,especially a 225mm conc block wall that low.
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Blagard

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PostPosted: Fri Feb 12, 2010 12:52 pm Reply with quote

Chukka63 wrote:
i wouldnt bother inserting any draining tubes/pipes for a 500mm high wall,especially a 225mm conc block wall that low.


Yes I agree you could get away with it - but why bodge the construction for a couple of quid. I walk past a slightly taller example each day, about 750mm. Nice Engineering bricks in a one brick thick wall. Shame the whole wall is slowly rotating. No drainage either. OK I am not saying it is due to that because it need investigation, but hydraulic forces do affect retaining walls so why not give them the proper relief and actually do the job properly when you have the chance. Its one thing less that could give you a problem later
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bobloes

from United Kingdom

Joined: 11 Feb 2010
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Location: Dorset,
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PostPosted: Fri Feb 12, 2010 4:10 pm Reply with quote

Thanks for all that. Maybe I should have said that it is part of landscaping the front grassed area. There are no structures between it and the pavement. It ground slopes slightly (less than 30cm drop) towards the proposed wall from the pavement (about 2 metres away). Now that you have pointed out my obvious ignorance I think I was basing it on the laying of a flagstone path last year where hardcore was advised as essential. There are no services in that part of the ground. I will take your advice re the drainage holes. I think you have answered all my concerns so thanks but if there is anything else please feel free. Cheers
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Chukka63

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PostPosted: Fri Feb 12, 2010 4:48 pm Reply with quote

Blagard wrote:
Chukka63 wrote:
i wouldnt bother inserting any draining tubes/pipes for a 500mm high wall,especially a 225mm conc block wall that low.


Yes I agree you could get away with it - but why bodge the construction for a couple of quid. I walk past a slightly taller example each day, about 750mm. Nice Engineering bricks in a one brick thick wall. Shame the whole wall is slowly rotating. No drainage either. OK I am not saying it is due to that because it need investigation, but hydraulic forces do affect retaining walls so why not give them the proper relief and actually do the job properly when you have the chance. Its one thing less that could give you a problem later


i wouldnt call it a bodge at 500mm,especially with plants soaking up some of the rainfall.a higher wall,yes i agree,drainage but not 500mm.
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