Hillside Retaining Walls

[Trw144]

Hi All, been lurking in the background having planning permission for the next build. We're now starting to move the soil ready to build a retaining wall, which brings me to the first question. I've had a SE design the retaining wall, and several people I have spoken to believe this to be overkill from experience. I know a few basements/hillside builds have been done on here so looking for a some inputs or maybe a suggestion for a good SE so I can get a second opinion (for reference the retaining wall will be separate to the house structure). My quick calculations put me at 2.5m3 if concrete per linear meter of wall, and circa 3.5m of wall heel that I would have to dig out and then backfill. It had been suggested to me that a multiple thickness of gabion baskets might be an alternative option (which would be both cost saving and far easier to install). 

 

 

Woodlands Retaining Wall Plan Markup_23.08.09.pdf

 

 

 

 

 

Edited September 13, 2023 by Trw144

[Conor]

No, I think they are reasonable for their heights. Our 2.4m high wall has a 1700mm toe and 1200mm heel, all 200mm thick with two layers of reinforcement.

 

If they were part of the building structure, where the ground slab and other corrected walls, your concrete would be reduced by about half. But building abasement comes with other Complications and costs.

[PNAmble]

Our groundwork is just underway and after moving earth they are building a 2.5m retaining wall, (24 meters long).   We are on sandy earth, we are using Gabions which our groundworker found to be the cheapest solution agreed with the SE.  They will sit on a bed of stone and concrete due to the ground conditions.   Base layer of gabions will be 1.5 (deep) x 1.0 x 1.0 second layer will be 1.0 x 1.0 x 2.0 and third layer 0.5 x 0.5 x 2.0.    

[Mr Punter]

It does not look like overkill.  A friend did some gabions in his garden.  They take up a lot more room than RC and need to be battered or stepped back.  They are best filled by hand, which is labour intensive.

[Trw144]

1 hour ago, PNAmble said:

Our groundwork is just underway and after moving earth they are building a 2.5m retaining wall, (24 meters long).   We are on sandy earth, we are using Gabions which our groundworker found to be the cheapest solution agreed with the SE.  They will sit on a bed of stone and concrete due to the ground conditions.   Base layer of gabions will be 1.5 (deep) x 1.0 x 1.0 second layer will be 1.0 x 1.0 x 2.0 and third layer 0.5 x 0.5 x 2.0.    

 

Yes, gabion is currently what I am looking to explore just to see if it is viable and more cost effective. I have looked into sheet piled steel but that was way way over the concrete/rebar route (even if much simpler to install) For both properties it's circa 100 linear meters of wall required so not insignificant. On the plus side, once this is sorted the build becomes more straightforward.

 

Space required for gabion isn't an issue on this site.

Edited September 13, 2023 by Trw144

[JohnMo]

We used wood strangers, basically 240mm dia by 4m long wooden posts. Dig trench circa 1.5m deep, concrete in posts.

 

[JohnMo]

During construction

 

[Conor]

22 minutes ago, Trw144 said:

 

Yes, gabion is currently what I am looking to explore just to see if it is viable and more cost effective. I have looked into sheet piled steel but that was way way over the concrete/rebar route (even if much simpler to install) For both properties it's circa 100 linear meters of wall required so not insignificant. On the plus side, once this is sorted the build becomes more straightforward.

 

Space required for gabion isn't an issue on this site.

You'll need to factor in the additional cost for excavation / muck away if using gabions. At the base they could be 3-4m deep. That's a lot of digging.

 

When I priced up our external retaining walls, concrete (ICF) came in cheaper than gabions. In the end we used bricks as the "shuttering" for the RC for aesthetic reasons. 

Edited September 13, 2023 by Conor

[PNAmble]

27 minutes ago, Trw144 said:

Space required for gabion isn't an issue on this site

 

There is a lot of excavation required as mentioned. Ours will be used to level of the upper area.  Once we have this sorted, we'll be into simpler times with a raft and timber frame.  We are reasonably close to a quarry so stone will be delivered direct.  

[Kelvin]

@JohnMoYou’ve posted a picture of your retaining wall before. It’s really nice and I’m likely to do something similar. My concern with wooden posts concreted in is the likelihood of them rotting just above ground level. Have you done anything to mitigate that?

[JohnMo]

1 hour ago, Kelvin said:

@JohnMoYou’ve posted a picture of your retaining wall before. It’s really nice and I’m likely to do something similar. My concern with wooden posts concreted in is the likelihood of them rotting just above ground level. Have you done anything to mitigate that?

They are pressure treated. They guy that did the work said he did one 20 years ago and it's still ok. The odd post has been replaced. But on our side the bank is free draining sand, so water doesn't sit for long if at all, with a layer of top soil on the top.

[Trw144]

3 hours ago, Conor said:

You'll need to factor in the additional cost for excavation / muck away if using gabions. At the base they could be 3-4m deep. That's a lot of digging.

 

When I priced up our external retaining walls, concrete (ICF) came in cheaper than gabions. In the end we used bricks as the "shuttering" for the RC for aesthetic reasons. 

 

So have spoken to a couple of people today, and gabions are looking a possibility with initial suggestions it will be a 2m base tapered down as it rises. The base only needs to be type 1 mot, rather 200mm of concrete/rebar as in the original design (and 3.5 meters deep for the heel). Once the design comes through I can cost it more fully.

[PNAmble]

41 minutes ago, Trw144 said:

The base only needs to be type 1 mot

Ours was only supposed to be type 1 mot, but due to the discovery of a spring / water , they decided to add a layer of concrete (2m base) to ours, which hopefully is going down today.  

[Mr Punter]

They are FO walls there.  Wall 1 is a full 2 storeys high!  

[Russell griffiths]

A thing to think about with any retaining structure is build in a bit of flexibility. 

If a solid block wall moves a mm you will get cracking, if a Gabion wall settles a bit, you will never know. 

 

Go the gabions. 

 

I priced to do some major retaking works alongside the river Thames. 

One engineer had it in timber and one in concrete and brick, I priced the timber, they went with the concrete and brick, two years later it got ripped out as the water rose and it all settled a bit. 

[Trw144]

3 hours ago, Mr Punter said:

They are FO walls there.  Wall 1 is a full 2 storeys high!  

Yes a small section needs to be 5m (depending on some other landscaping factors - long story), the vast majority is one story in height

[peter766]

hi there, totally get where you’re coming from with the whole planning permission saga - it’s a beast! 😅 so you're diving into the deep end with the retaining wall stuff, huh? been there, done that. you’ve got a SE to draw up the designs and it sounds like it’s giving you a bit of a headache. people always have opinions, right? especially when they've been around the block a few times with builds like this. 🏗️ i know you’re not alone in this. loads of folks on here have wrestled with basements and hillsides, so definitely, you’re in good company for some solid advice. getting a second opinion? smart move. 🧐 a good SE might not come cheap, but they could save you a bundle in the long run if they find a more efficient solution. those quick calcs of yours putting you at 2.5m3 of concrete per meter sounds hefty. that's a lot of concrete! and the whole digging out 3.5m for the wall heel? sounds like a back-breaking job. have you got the machinery for that, or are you doing it old school with shovels and sweat? 😓 someone suggested gabion baskets, right? they’re pretty trendy, plus they can blend in nicely with the landscape if that’s your vibe. cheaper and easier to install, you say? sounds like it could be a winner. 🏆 but hey, they’re not for every situation, so defo weigh up the pros and cons. might not want to gamble on something just cos it's easier on the wallet, y’know? anyway, hope this gives you a bit to chew on. good luck with the build! keep us posted on what you decide, yeah? 👍🏼