Shed on a steep slope project - retaining wall question

[Fitzroy]

Concrete H fence post and concrete gravel boards, been there over twenty years now, never moved.

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The corner post supporting the retaining wall and then having the section perpendicular to this will provide a huge resistance to the retaining wall toppling over. If you do go this way you would want to attach your corner post to the existing wall, providing the same resistance to the wall being pushed over.

Alternatively could you just build your retaining wall in the same way, but only 900mm high, say with 1500mm tall posts, buried 600mm in the ground, slot in your sections and back fill.

 

[ChaiLatte]

Are you suggesting replacing the timber fence with these?

I do not claim to speak for him, but I think he is suggesting making the retaining wall from concrete posts and gravel boards. It is a good idea as the structural thickness will be about 150mm maximum.

You could bore holes for the posts 900mm deep below the base of the wall and use 1800mm posts. It is a lot less digging.

If you thought the earth pressure was a lot, instead of one standard length gravel board with a post at each end, you could cut the gravel board in half and use three posts.

Drill some holes in the lowest gravel board and line up the back with geotextile so no water pressure can build up, then you do not need the pipe behind the wall. Even if you do not do this, the water will flow through the joints between the gravel boards, so could only build up about 150mm in height before it finds a way out.

 

[Mandrake]

The corner post supporting the retaining wall and then having the section perpendicular to this will provide a huge resistance to the retaining wall toppling over. If you do go this way you would want to attach your corner post to the existing wall, providing the same resistance to the wall being pushed over.

Alternatively could you just build your retaining wall in the same way, but only 900mm high, say with 1500mm tall posts, buried 600mm in the ground, slot in your sections and back fill.

This sounds like a pretty good approach to be honest, esp the fact it would strengthen the existing wall which doesn't see to have been built with the intention of it standing for a thousand years to say the least. On yours looks like you left a gap at the bottom of the wall for drainage, is that right? I guess it might not be as efficient drainage as gravel backfill, but your wall seems to be holding back a considerably higher level of earth than mine would, so I guess it was fine in your case and probably would be for me too.

 

[Mandrake]

I do not claim to speak for him, but I think he is suggesting making the retaining wall from concrete posts and gravel boards. It is a good idea as the structural thickness will be about 150mm maximum.

You could bore holes for the posts 900mm deep below the base of the wall and use 1800mm posts. It is a lot less digging.

If you thought the earth pressure was a lot, instead of one standard length gravel board with a post at each end, you could cut the gravel board in half and use three posts.

Drill some holes in the lowest gravel board and line up the back with geotextile so no water pressure can build up, then you do not need the pipe behind the wall. Even if you do not do this, the water will flow through the joints between the gravel boards, so could only build up about 150mm in height before it finds a way out.

Yeah I'm coming around to this idea, makes me wonder why retaining walls aren't usually built using this method. I suppose the trade off is you need to be able to drive the posts quite deep compared to the trench for a block wall. I don't know how far down my soil goes, we're on the side of a hill so I suspect we may hit rock sooner rather than later but no sign of it so far.

 

[Mandrake]

Before you go too far with your plans for 600mm between boundary and shed wall, have a look at building regs as applied to sheds. IIRC, if the shed wall is made of flammable material, it must be at least 1m from the boundary.

Hi the house is actually in Ireland, and I've had a good look through the planning regs and not found anything like this mentioned, I don't think it's a thing here.

 

[ChaiLatte]

I don't know how far down my soil goes...

You could do a trial by driving in a piece of 16mm rebar or similar. If it goes in one metre with reasonable blows, you could be OK.

Maybe some retaining walls need a small amount of visual niceness. A post and gravel board thing will look like the Berlin Wall, but that won't matter behind a shed.

 

[Mandrake]

You could do a trial by driving in a piece of 16mm rebar or similar. If it goes in one metre with reasonable blows, you could be OK.

Maybe some retaining walls need a small amount of visual niceness. A post and gravel board thing will look like the Berlin Wall, but that won't matter behind a shed.

ok makes sense I'll give that a try, thanks. I may end up replacing the whole fence rather than half as it's all 30 year old timber and not in great condition. So the look will be an issue to some extent, I guess it won't take too long for the ivy to cover the visible part so not the end of the world. Only slight annoyance here is local planning regs require concrete walls to be rendered or plastered, but I suppose it's not the biggest job in the context of the whole project

 

[Fitzroy]

This sounds like a pretty good approach to be honest, esp the fact it would strengthen the existing wall which doesn't see to have been built with the intention of it standing for a thousand years to say the least. On yours looks like you left a gap at the bottom of the wall for drainage, is that right? I guess it might not be as efficient drainage as gravel backfill, but your wall seems to be holding back a considerably higher level of earth than mine would, so I guess it was fine in your case and probably would be for me too.

Original post was not from me, it just caught my eye and I wanted to respond, so can't answer I'm afraid.

 

[TomGW]

When I read the initial post(s) my immediate thought was 'gabions'. Even more appropriate when access is difficult and limited to a wheelbarrow. If filled reasonably carefully the stone fill will act as a dry-stone wall so the integrity of the cage becomes less important. We have a stream at the bottom of our garden which was modified in the Victorian era to be a mill stream, with dry stone walls, only one stone thick, retaining banks of over 2m high. They are still doing the job perfectly. Your gabions will be at least twice as thick and retaining only half the height. Fill the stone roughly as you would build a dry-stone wall and the long term integrity of the cage shouldn't be a concern. Angle the base to lean the gabion back against the area to be retained. This will add to the stability and pressure and will be easier to excavate the face of the higher ground at an angle too.
Good luck with your project and just wondering whereabouts in Ireland you are??

 

[Mandrake]

When I read the initial post(s) my immediate thought was 'gabions'. Even more appropriate when access is difficult and limited to a wheelbarrow. If filled reasonably carefully the stone fill will act as a dry-stone wall so the integrity of the cage becomes less important. We have a stream at the bottom of our garden which was modified in the Victorian era to be a mill stream, with dry stone walls, only one stone thick, retaining banks of over 2m high. They are still doing the job perfectly. Your gabions will be at least twice as thick and retaining only half the height. Fill the stone roughly as you would build a dry-stone wall and the long term integrity of the cage shouldn't be a concern. Angle the base to lean the gabion back against the area to be retained. This will add to the stability and pressure and will be easier to excavate the face of the higher ground at an angle too.
Good luck with your project and just wondering whereabouts in Ireland you are??

Waterford, 'the sunny south east' though you wouldn't know it looking out the window 98% of the time

That's interesting re the cage integrity and makes a lot of sense, I'm from lancashire can't move for dry stone walls there many of which must be hundreds of years old. How do you foot a gabion just with aggregate like a wall? Or would you need to bury it a bit deeper?

 

[TomGW]

I'm at the other (north) end of the island so nowhere near and you definitely will get more sun than us.
I wouldn't be too concerned about a base for the gabions. When you have excavated about 900mm you should be into good firm clay which will be plenty. Just use decent sized stone to fill the cages, at least at the front. It may be easier to fill the back with smaller stone or even large gravel, which will fill through the cage and against the excavated face of the bank. A drainage pipe at the bottom will help get water away from lying behind your shed.
To be honest, in a situation like this I would be thinking about trying to keep as much water and debris out of the gap at the back of the shed as possible - certainly a gutter on the back of the shed and maybe even an extended overhang on the roof.

 

[Mandrake]

I'm at the other (north) end of the island so nowhere near and you definitely will get more sun than us.

Slainte! (hopefully that's not culturally insensitive )

Yeah the drainage is something that concerns me a bit, as you say the narrow gap, slope and an impermeable concrete base could be a problem unless managed. I was initially thinking of building the whole shed on a compacted gravel pad bounded by a 6x6 wooden frame, allowing water to run under the shed freely but with the wind we get here sometimes I feel it would be safer to have it anchored to concrete.

I suppose gravel could be used provided I found some other way to anchor the shed (maybe those ground anchors someone posted earlier in the thread.) That said at the stage you're buying ground anchors you may as well just have a concrete base and make sure it has proper drainage, probably easier and cheaper.

 

[MikeJhn]

Are you suggesting replacing the timber fence with these? It seems like a pretty good idea to be honest (assuming the neighbour doesn't object). Only issue would be (I probably should have mentioned this in my initial post) that thew access is really poor - narrow side path with 2 90deg corners, to even get to the path is up several steps and round a corner, so my concern is how to get these presumbly very heavy boards to the site, and whether the posts could be driven in with a micro digger. If I can figure out a way to overcome this could be the answer!

No leave the timber fence in place and build up against it, the concrete gravel boards and post where hand carried into position, the foundations for the posts where hand dug, the concrete fence being at right angles to the main run provides sufficient restraint that the foundations do not have to be that deep, don't overthink this, it's a simple small height retaining wall, I just put an impermeable sheet against the ground behind and continued it under the shed, if you must direct any moisture to one side.

 

[TomGW]

I would certainly be wanting a concrete base for the shed but also making sure that the gap behind was smooth conc, sloped and shaped to channel water away from the shed and out of the space. Wire mesh on a frame could reduce a build up of leaves and debris.

And, don't worry, no cultural sensitivities in our house. While I don't speak Gaelic at all, I'm still Irish.

 

[MikeJhn]

If the final retaining walls are a U shape in plan, then the support posts do not need a substantial foundation, I'm thinking post spikes driven into the ground and the concrete posts slotted into them, be wary of some of these post spikes, the internal face's have fins that would stop a concrete post from fitting into them, I also recently bought a IC post driver to help with hammering into the ground, great help, but I was using 100 x 100 timber posts for handrail and decking, just a thought.

https://www.amazon.com/NOLANTISI-32-6CC-2-Stroke-Portable-Cylinder/dp/B0CB7GBZS1?th=1

 

[steve66]

Two different sheds/summerhouse I’ve done, back/side of building's horizontal battening with small kerfs so water has escape route and clad with corrugated metal, detail of bottom of buildings to stop critters entering (was made myself from metal that came off old caravan)
The buildings were constructed with 4x2 timber clad with 9mm sterling board and covered with breather/ house membrane.
If the sides/back aren’t seen and you want to keep maintenance to a minimum have you thought of corrugated tin, on unseen sides?

 

[Sheptonphil]

Before you go too far with your plans for 600mm between boundary and shed wall, have a look at building regs as applied to sheds. IIRC, if the shed wall is made of flammable material, it must be at least 1m from the boundary.

The 1m requirement and flammable material reg is for structures over 15sqm internal. This one is only 12m external So not a consideration.

 

[MarkAW]

Have you considered avoiding all this groundwork by installing supports of various heights under the shed to bring it level, much like a deck is made.
Would then need some steps/ramp perhaps. I can't tell which way the levels are or where you intend to install the door.

Edit: ah I misunderstood. Looking more closely at the picture I see that the thing covered in IVY is timber, not brick. I have a similar situation in my garden shortly after I moved in the timber retaining posts were rotten and started falling. I contacted the housebuilder and they installed a new retaining wall (again out of timber, sleepers this time probably because it's cheaper, but I only expect it to last a several years) if it's holding back more than a foot or two I'd recommend building something properly engineered. Don't want it collapsing on your shed

 

[Mandrake]

Have you considered avoiding all this groundwork by installing supports of various heights under the shed to bring it level, much like a deck is made.
Would then need some steps/ramp perhaps. I can't tell which way the levels are or where you intend to install the door.

Yeah I did consider this, but the site is so much higher than the house already that going for pier and post type construction would make the shed almost level with the upper floor of the house. It's a big enough shed (12x10) in a not massive garden, so it would look too imposing, and the windows would have a view into the neighbours bedrooms. Not that I'd be looking but I feel like they might fairly complain and they'd have a point. I'd rather not get into that, so while it would be much easier to do what you suggest it's not suitable for the project unfortunately.

 

[Mandrake]

Two different sheds/summerhouse I’ve done, back/side of building's horizontal battening with small kerfs so water has escape route and clad with corrugated metal, detail of bottom of buildings to stop critters entering (was made myself from metal that came off old caravan)
The buildings were constructed with 4x2 timber clad with 9mm sterling board and covered with breather/ house membrane.

Interesting so the breather membrane keeps water out but allows damp air to escape from inside the walls is that the idea?

If the sides/back aren’t seen and you want to keep maintenance to a minimum have you thought of corrugated tin, on unseen sides?

This is the kind of idea that seems obvious once said, but I probably would never have considered, thanks! Back and sides will be completely hidden so a great idea to save me repainting in a narrow gap every few years.