What type of retaining wall?

[Milleniumaire]

As you can see from the photo's, our neighbours land is higher than ours and there is a stone retaining wall in place, which has so far lasted the 26 years since the house was built.

There was a low 4' fence on this retained land and some scaffolding planks had been used to attempt to keep the soil from leaning against the fence posts and panels, but ultimately they rotted and also the weight of soil behind them helped the posts and panels to fail in high winds. In 26 years, there have been 3 fences built along this length and we have now decided to plant a hedge instead. Before doing this, we need to build something more permanent to hold the soil back.

The following photos hopefully give an indication of the area that needs to be retained. The orange plumb line indicates the line of the new retaining wall, so as you can see it is set back by just under a metre from the front of the existing stone retaining wall.

The length of the wall is just short of 17 metres:

20240816_180443.jpg

• Milleniumaire
• 17 Aug 2024

The existing stone retaining wall is 80cm high:

20240816_180640.jpg

• Milleniumaire
• 17 Aug 2024

20240816_180627.jpg

• Milleniumaire
• 17 Aug 2024

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• Milleniumaire
• 17 Aug 2024

My estimates are that the retaining height will need to be between 15cm and 30cm along the 17 metre length. This is measured from the top of the existing stone wall to the orange plumb line you can just about see in the photos.

The garden next door slowly slopes down to the boundary. Over the years, the surface has been covered in plastic sheeting and stones spread on it, which has raised the height a little more, and this allowed cars to be parked on this area, but the current owners tend not to do that.

I'm looking for some ideas on the best/easiest/cheapest way of doing this, but using a method that will be more permanent that the scaffolding boards used by previous owners. I did wonder if concrete paving slabs buried in a trench and held in place with concrete might be a reasonable solution, but I'm not sure how strong this would be. Not sure if I want to take on a job like this myself, so I may get a landscaper or builder in to do the work.

I would appreciate any thoughts on the best way to build this small (secondary) retaining wall, the aim of which will be to protect the newly planted hedge from the soil and stones from the garden next door.

 

[Deluks]

Is the wall the boundary? If you plant a hedge won't that be on the neighbours land?

 

[Milleniumaire]

Is the wall the boundary? If you plant a hedge won't that be on the neighbours land?

Hi, no the wall isn't the boundary. The boundary line is around 1 metre in from the wall, which is where the fence has been position for the last 26 years, since our house was built.

We are planning to build the small retaining wall on the neighbours land, since it is THEIR problem we are paying to resolve. This should ensure we still have around 1 metre in from the wall, in which the hedge will be planted.

 

[cdbe]

Paving slabs will be fine. Easy (but physical) DIY job if you don't get reasonable prices.

 

[Milleniumaire]

So, we've decided to go ahead and build this retaining wall ourselves! We dug a trench yesterday, which is 17 metres long, 10cm wide. The depth varies but from the lower side (towards the wall) it is around 30cm deep and will probably end up being a little deeper as I still need to accommodate the slope of the soil area to be retained etc. The higher side (behind the slab in the neighbours garden, is much higher than 30cm and varies along the length of the trench.

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• Milleniumaire
• 15 Sep 2024

I will be ordering 24 600 x 600 x 50 concrete slabs. I already have 4 so will be fitting 28 slabs in total along the length of the trench.

I need to determine how much sand and cement to order! We will probably hire a cement mixer for a long weekend (3 days). I would appreciate some guidance on how to determine how much sand and cement is required for this job. For example, if I was to poor 10cm deep cement into the 10cm wide trench (but due to the 5cm wide slab, this would reduce to 5cm wide?), then lower a 5cm wide slab into it, would this be sufficient to hold it? If the cement is mixed with only sand and no ballast I'm assuming the weight of the slab was cause it to slip through the cement and sit on the ground, squeezing the cement up each side and causing the cement to displace around the front and back sides of the slab! If ballast was added, would the weight of the slab do the same, or would be ballast under the edge of the slab cause it to sit a little higher?

I'm also wondering if I should try to purchase some galvanised steaks; one per slab. Ideally, they would be 90cm long i.e. 30cm longer than the slab and we could then set each slab in the trench and hammer a steak in to support it, on the lower ground side, which would also provide additional support against the soil. The cement could then be simply poured in the front and the back of all the slabs that would be in position. This would avoid having to poor the cement into the trench, then lower the slab into it, allowing us to get the slabs "neatly" in position and lined up prior to mixing any cement. In this case there wouldn't be any cement under the slab, just around the front and back. Is this likely to be an issue?

This is the first time I've done this, so I'm probably over thinking it. The mixing and pouring of the cement is the thing that concerns me the most!

Appreciate your thoughts.

 

[cdbe]

You need to set the slabs on a bed of "semi-dry" concrete which will allow you to "mallet" them down to the line, and then back up each side with wet concrete, more on the lower side. Probably need a wider trench.

 

[phatboy]

Where are you going to position your mixer? Hoofing buckets of concrete or mortar up there is going to be exhausting. If you can have the mixer and materials above, on a large sheet of plastic, it will be a bit easier.

Whole job requires careful footwork all the time!

 

[Milleniumaire]

You need to set the slabs on a bed of "semi-dry" concrete which will allow you to "mallet" them down to the line, and then back up each side with wet concrete, more on the lower side. Probably need a wider trench.

Thanks for that feedback, it makes perfect sense. I've probably underestimated the width of the trench as we used a trench spade to dig it, so it is probably nearer to 20cm wide!

 

[Milleniumaire]

Where are you going to position your mixer? Hoofing buckets of concrete or mortar up there is going to be exhausting. If you can have the mixer and materials above, on a large sheet of plastic, it will be a bit easier.

Whole job requires careful footwork all the time!

Yes, I have been thinking about this and it would make sense to set up the mixer in our neighbours garden, if they would allow it, as we would then be walking "down" to the trench! As they are tenants, and we get on well with them, I suspect they wouldn't mind.

I agree about the careful footwork. We have struggled to place the soil from the trench alongside it and as it is now building up! Standing too close to the edge of the trench does result in the loose soil falling into it. I have already tried to remove any very loose soil around the trench while digging it.

 

[Milleniumaire]

The weather next weekend is looking okay, so I'm hoping to be able to be able to get the slabs in place during the week (after work) and then set them in concrete over the weekend.

Any thoughts on how much sand, cement and ballast (?) will be required? I guess it depends on how much is placed in the trench. I'm thinking that it should cover at least 10cm of the 60cm paving slabs. Assuming the trench is 20cm wide and the slab is 5cm thick, that would mean the area is 10cm x 5cm x 17m. Not sure how to convert that into bags of cement etc!

I guess it would be better to over-estimate and use more wet concrete than is necessary as this should result in a stronger structure.

 

[Milleniumaire]

For this job, I've calculated the volume of concrete required is 17m x 0.2m x 0.1m = 0.34m3

My local builders merchants have suggested the following will be needed to mix this amount of concrete:
8 x 25kg bags cement
1 x large jumbo ballast bag (I think this is around 800kg), which is probably more than I need, but it works out cheaper than buying many small bags.

I'm planning to mix it in a 1 cement to 6 ballast by volume.

Now I just need a cement mixer and some good weather!

 

[Milleniumaire]

It was hard work, but finally got the paving slabs in place, each one set on a bed of mortar:

20240922_173522[1].jpg

• Milleniumaire
• 23 Sep 2024

Next weekend, weather permitting, I hope to hire a cement mixer and mix enough concrete to fill some of the trench on either side of the slabs to ensure they are fully supported. Then the soil will be pushed back to fill the remainder of the trench and the new hedge planted.