Retaining wall details, is this ok?

[PXR5]

I need to build a retaining wall, would this be ok? On the left as is, the proposed works on the right.

 

The land already has a stone  retaining wall holding up my next doors garden, I intend to dig down about 800mm from this wall & lay compact hard core, then cast concrete with steel rebar, the rebar will go vertical 1.4m through the hollow concrete blocks. The land is compact garden dirt,there is some water movement, so gravel and 1000 grade poly between the blocks & gravel.

Thanks

 

Edited November 6, 2022 by PXR5

[markc]

Really depends on the blocks, I would go with holocore blocks then they can be placed over rebar starters and filled with concrete. Also you really need weap holes at the base of the wall or a French drain behind to relieve hydraulic pressure behind the wall

[PXR5]

You dont think the gravel will allow enough drainage through to the hardcore base ?, if not, use a perferated pipe at the bottom, what kind of gravel should I use? thanks

[Roger440]

Have you considered something like this: https://ag.uk.com/professionals/products/category/retaining-wall-system/

 

Ive used it twice to good effect. Plus its pretty much idiot proof to build.

 

And in the event of slight ground movement it wont just crack

[PXR5]

Yes i have, but Ive been given enough hollow blocks to do the job, money is a little tight so I'll be using these..thanks

[markc]

36 minutes ago, PXR5 said:

You dont think the gravel will allow enough drainage through to the hardcore base ?, if not, use a perferated pipe at the bottom, what kind of gravel should I use? thanks

Gravel will pass water but unless the ground is very well draining it will build up behind the wall unless you give it somewhere to go. 

[George]

I would recommend getting an engineer's advice.

The overall wall height is 2.4m and is holding up your neighbour's land. The construction of the new wall has the potential to undermine the existing retaining wall so consideration of temporary works or staged construction will be needed.

 

There's not enough detail on your plan but the reinforcement design will also need thought as there is the potential for some surcharge loading from the existing wall. Plus dealing hydrostatic pressure. Oh also, party wall agreement may be needed.

 

 

[PXR5]

Thanks for the comments, I have asked for advise on this, just getting more opnions on it.

From what Ive read, you do not have to have a Party Wall A'.

Which is just as well, as the neighbours are the most two faced trouble making c####s Ive ever met,& relations are as bad as they can be..

[Miek]

Given the height of the wall I would be wary of freestyling it. Better to get a structural engineers input IMO.

 

 

[Gus Potter]

23 hours ago, George said:

The overall wall height is 2.4m and is holding up your neighbour's land.

Plus one to that.

 

Much will depend on the type of soil you have. You'll often be surprised at how much load a wall like that needs to resist.

 

 

On 06/11/2022 at 22:43, PXR5 said:

I need to build a retaining wall,

What you are doing is interesting to us "soily / foundation" types on BH so if you want post more info folk will hopefully be able to help a bit.

 

Things I would like to know as a starting point

 

• Where is the boundary.
• What kind of soil do you have
• How far is the neighbour's house from the wall
• Where is your house in relation to the wall
• What are the ground levels in front of the wall.. do they carry on sloping down or flat?
• How long is the wall.
• How are you wanting to do the returns on the wall? Are these close to the boundary?

Good drawing by the way.

 

 

 

[PXR5]

Thanks, ok

1,The boundary is the 1m high stone wall.

2,Soil is compact undisturbed clayish.

3, Next doors house is 17m away at the closest point.

4,No house yet, its for a new build which will be built near to the retaining wall/boundary.The retaing wall is 14m in length, the new house I'll build will be 1m away at 9m long.

5,Ground levels are flat.

6,Returns at each end, coming away from the boundary onto my land

 

Thanks for your help!

[Kelvin]

On 07/11/2022 at 11:24, Roger440 said:

Have you considered something like this: https://ag.uk.com/professionals/products/category/retaining-wall-system/

 

Ive used it twice to good effect. Plus its pretty much idiot proof to build.

 

And in the event of slight ground movement it wont just crack

Thanks for posting. I have a retaining wall to build as part of our groundworks and these look ideal. My wall is 1m at the highest point and 21m long so these blocks look perfect. 

[Roger440]

1 hour ago, Kelvin said:

Thanks for posting. I have a retaining wall to build as part of our groundworks and these look ideal. My wall is 1m at the highest point and 21m long so these blocks look perfect. 

 

If my memory serves they say no structural calcs requitred for up to 1m.

 

Mine was 750 at the highest point. As it was near trees, that was my primary driver, so it could move a bit. Plus i cant lay bricks!

 

I cant really see many reasons why you would use brick/block/concrete. 

[Kelvin]

I got a design for ‘free’ as part of the structural calcs for the house and garage foundations. 
 

They also look pretty good too. Just have to pass the wife test. 

[Gus Potter]

8 minutes ago, Kelvin said:

I got a design for ‘free’ as part of the structural calcs for the house and garage foundations. 

Like that. Often I'll do the same as just like to do a good job in the round.

 

Look at this another way... last man on the job gets the blame. No point in doing a great SE job  on the house if the garden / retaining wall falls down.. even if you had no hand in it!

 

 

[saveasteading]

As always, read and absorb all Gus says.

Can I add?

Why build on hard-core? If the ground is virgin, far better to concrete in the ground.

1.4m is quite high. It looks less when shown as if 3 blocks, but it will be 6.

Just sticking a bar into the base will help  but what diameter, and will it be L shaped?

Base would be better all the way back in the excavation.

Does the existing wall have a footing? Don't disturb it.

Absolutely, insert drain holes, 25mm pipes,  at or near the bottom.

Line the excavated face with membrane to stop roots entering and blocking your gravel.

And assume the worst...what will you do if the existing wall seems precarious, or the excavation falls away? Be ready for a plan B.

Consider the kit walls as Kelvin suggests. You will save on excavation, concrete and risk. Then, with some, they are attractive and can be planted.

[Haylingbilly]

If I was doing it, I would do it in sections 1m long with a 2m break between them, so do the wall in three goes. You are digging down 1.9m 400mm away from a 1m retaining wall which may or may not be a problem. You could also do a test section to see how well the ground holds up. I had a similar issue and that approach is what the SE recommended - however in the end we piled it and then dug the earth away.

[PXR5]

Hardcore to allow drainage, there is some water movement acroos the land, though not like a swamp.

 

Yes, I intend to do a very small section at a time to allow me to guage the situation, thanks for the comments!

[PXR5]

As said, I do  have some water passing through this land, but not massive amounts, would people advise me to do this in the driest summer months? thanks

[George]

 

On 09/11/2022 at 15:26, Andy brown said:

If I was doing it, I would do it in sections 1m long with a 2m break between them, so do the wall in three goes. You are digging down 1.9m 400mm away from a 1m retaining wall which may or may not be a problem. You could also do a test section to see how well the ground holds up. I had a similar issue and that approach is what the SE recommended - however in the end we piled it and then dug the earth away.

 

Even with 1m strips, you'll need to put in trench sheets to support the excavation. Never rely on soil self supporting - just search YouTube for 'trench collapse' for why not (although be aware YouTube doesn't always vet videos very well so go for ones with a lot of views to avoid unpleasantness). 

[Gus Potter]

On 08/11/2022 at 20:22, PXR5 said:

6,Returns at each end, coming away from the boundary onto my land

Hiya @PXR5

 

For me I would have a look at this all over again.. a design review now you have got a bit of a heads up and starting to appreciate the costs / things involved.

 

In terms of appreance I would look at where the sun rises and sets, how you are going to use the space between the wall and the house. Would be helpful to have some more plans and so on.. do you like lots of plants or do you see this as a place for kids to play and a spot for your outside bar/ bbq ? let us know if you can. A bit of thought now can reallty make or break this.

 

May be worth you while posting? The next is in no particular order it's just my thoughts.

 

Here are a few SE / Geotechnical / risk thoughts.

 

To get started on this the last thing you need is for your neighbours to get upset. You don't want to do something that will cause the land to slip so it impacts on their house but in real life their garden may tolerate say some 25mm of movement top end without complaint.. especially if they have grass.at the rear.

 

A 2.4 m retaining wall can be a big ask if you are on a budget. My worry (not least) would be that when your neighbours house was built they maybe made up the gorund so you have a layer of made ground that will slip over the original ground. Lets assume the worst case for now.

 

Much has been said about the different types of retaining wall.. cantilever, gravity, gaboin walls and so on. Once you get over say 1500mm the cost grows exponentially.. if you have good chalk ground not so much but let's just assume you have some " dodgy" clay with a bit of made ground on top and water coming from above that makes it all unstable.

 

I'm wondering. My starting point would be.. that step back is about 800 mm.. now that could be a reinforced concrete beam ( slab) lying horizontally.. say 200 mm thick with 20mm rebar and a few lacing bars maybe.. at the bottom you could just have a small slab braced against the house. Then in between you could have a reinforced masonry wall all pretty striaght forward and easily buildable.. not figured out the sequencing of work yet but just thinking out loud.

 

if you can get the concept structurally then the drainage is the next part.. but it is the structure that will eat up the money.

 

Now rather than having a cantilever retaining wall or a gravity wall you now have a wall that spans on the top between the return walls?

 

My gut feeling is that this might be a good cost effective option worth exploring. Also I'm thinking with a bit of finesse you could make this a great feature, maybe extend the beam to create some kind of detail that looks good I don't know but there may be some mileage in nutting this out so it looks and is more useable than just a retaining wall at no extra cost.

 

In summary can you tell us about the space, where the sun is and what sort of finishes you think may appeal and what you want to use the space for?

 

 

 

 

 

 

 

 

 

 

[Gus Potter]

To add a bit and for clarity.

 

When retaining walls get to a certain height they can become expensive if the ground is not playing the game. An easy way to mitigate this to introduce restraint at the top.

 

For all.. sometimes you can brace the top of the retaining wall off the house.. and that can save thousands!

 

 

 

 

[PXR5]

Thanks for taking the time for such a thoughtful reply.

Next doors buildings are far away, about 15m, & are off at right angles to how the land would slip, so even if things did go bad & a collapse happened,  it would be just their garden,, the ground is compact & has not been dug for many many years. ( that's my land & next doors garden).

Having said that, they have large bushes planted on their side bang tight up to their retaining wall, for 5-6m mid way along, the roots have pushed out their R' wall, bulging  out over my land

I had offered to help push their wall back but they said I should do /pay for it given I'm doing the development work. Looks like Ill have to take legal action to get them to do some thing about it. It is their wall not mine.

Yes, capping the retaining wall with a slab sounds like a good plan.

The site is quite small, I don't have much of an option to maximise the space for sun light etc.

Ill get some pics up soon showing the site plans.

I might add, the neighbours are the most unpleasant people, they have recently moved there, & have done every thing possible to give me a kicking over my project, writing on my application the most stupid complaints, & reporting me to Building Control, trying to make out I had started works before getting the P'Permission.... there was a small chance I could have lost my CIL exception. ....

 

 

 

 

[saveasteading]

9 hours ago, PXR5 said:

 neighbours are the most unpleasant people

Some background to your situation is useful, but keep in mind that all our discussions on BH are public and come up on search engines.

 

Hedge roots will probably help hold the ground in place while excavated, but further growth could be a problem.

 

It is well known that you can cut back any growth overhanging your land, but I have no idea the legal position if your side of the hedge was damaged by, for example severe cutting or accidental weedkiller damage.

 

The fact that the fill has been there for several years msy not help much...it takes literally 'ages' for ground to become dense and stable.

[PXR5]

Yes I hear what your saying, I sure the ground both sides has not been disturbed for over 60 years.

The bushes have been planted recently say 15 years, their garden is quite narrow  the prevoius owners made the mistake  of planting the bushes on their side as close to their retaining wall, then the roots have grown down pushing  out the retaining wall.

 

The legal position is that their bushes have damaged their wall, & its down to them to repair the damage.