Lowering a retaining wall (from the bottom)

I've got a mangy old shed that is sitting on a small patio area. about 50cm above the garden. The garden rises up at the back of the shed, which was p art of an old railway embankment. Hopefully the photo makes it clear what's what: https://goo.gl/photos/4XchoR5U1FybKZgs7
My plan is to dispose of the old shed and lower the patio area where the sh ed currently sits so that my new shed can extend onto the garden a bit.
My question is, what's going to be the easiest way of retaining the bank at the back of the shed as I dig down? I don't want the existing retaining wa ll at the back of the shed to collapse as I lower the level and I don't wan t to have to shift more earth than necessary.
Does anyone have any bright ideas about what I could do or what I need to w atch out for when I'm digging away?
Thanks
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On Tue, 31 May 2016 08:00:50 -0700, matthelliwell wrote:

Looking at that pic, if it's really just the lowest tier that you're digging away, I don't think it's going to be too big a problem, so long as you're happy to keep a shortish step. If you want to go all the way back to that existing wall, and just extend the bottom of it, then you may well have to rebuild that whole wall, though.
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I could really do with going all the way back to maximise the space as the garden isn't that large so it looks like a rebuilding job if I go down this route.
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writes

Could you *underpin* it a bit at a time? Say a 500mm wide section. Dig out the soil to form a new foundation. Shutter the front and fill with concrete.
--
Tim Lamb

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On 31/05/2016 16:00, matthelliwell wrote:

Are you sure that there *is* a retaining wall at the back of the shed? Or is the patio simply at the level of the sloping surface at that point?
If you drop the level at the back of the shed to that of the garden in front of the patio, you'll almost certainly need to build a retaining wall there.
If I'm wrong, and there *is* one already, everything will depend on the depth of its foundations. If they go below the required new level, they'll be ok. If they don't, digging will undermine them, and the wall will probably collapse - so you'll have to take it down and build a new one.
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Yes, there's definitely a retaining wall at the back. Its about a metre high and block work (or the top part I can see if anyway). So it probably looks like I'll have to plan to rebuild it :-(
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On 31/05/2016 16:00, matthelliwell wrote:

n. The garden rises up at the back of the shed, which was part of an old railway embankment.
Hopefully the photo makes it clear what's what: https://goo.gl/photos/4XchoR5U1FybKZgs7

s so that my new shed can extend onto the garden a bit.

shed as I dig down? I don't want the existing retaining wall at the back of the shed to coll
apse as I lower the level and I don't want to have to shift more earth than necessary.

hen I'm digging away?

Can't you put the new shed at the same level as the old one? Extend the patio? Drainage is likely to be less of an issue.
Bill
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On Tuesday, 31 May 2016 16:00:53 UTC+1, matthelliwell wrote:

cm above the garden. The garden rises up at the back of the shed, which was part of an old railway embankment. Hopefully the photo makes it clear what 's what: https://goo.gl/photos/4XchoR5U1FybKZgs7

shed currently sits so that my new shed can extend onto the garden a bit.

at the back of the shed as I dig down? I don't want the existing retaining wall at the back of the shed to collapse as I lower the level and I don't w ant to have to shift more earth than necessary.

watch out for when I'm digging away?

You'd be digging out an essential part of its support. It might collapse, o r might not but be unsafe. In short, don't.
Underpinning might or might not cut it. Don't assume.
NT
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On Tuesday, 31 May 2016 19:27:55 UTC+1, snipped-for-privacy@gmail.com wrote:

or might not but be unsafe. In short, don't.

(I finally appeared to have worked out how to use this stupid google groups interface). Anyway, that's my conclusion too. If I go ahead I'll either so rt out the retaining wall properly or build the shed on two levels. Buildin g it on two levels might actually look better in the small garden as it'll help break up the roof line.
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On 31/05/2016 19:37, matthelliwell wrote:

+1 from me for not disturbing the retaining wall. Personally, I'd extend the patio thingy over the lawn, so there's then no arguing about the height of the shed, and it gives you a nice firm foundation.
I find the idea of a two level shed hard to envisage.
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1. can you reduce the weight by making the slope less acute? If not I saw a wall down my road that was built like a dam, ie curved toward the earth bank with deep posts at either end with girders inside them to retain the pressure,so, presumably as the earth slides and pushes the wall the wall gets its bricks pushed together pushing the end pillars out but they are too strong to move, at least they seems to have been there a good ten years so far.
Brian
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Buttress construction. Clywedog dam is built like that. The valley walls were thought insufficiently strong to hold back 200 foot of water.
--
Tim Lamb

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In case anyone is interested, the conclusion is that I won't be digging any thing away. Apart from rebuilding the retaining wall being a bit like hard work, I also end up undermining the fence posts so its more trouble than it s worth.
The current plan is to build a large a shed as possible on the existing pat io area (it can be about a foot wider and 3 feet deeper than the current sh ed) and put a separate narrow storage shed at the side of the garden so the new shed doesn't get full of crap.
Thanks for everyone's comments.
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