Retaining walls

[RogerM]

I am starting to build a low retaining wall around our new kitchen extension, which is a maximum of 120cms high. The ground behind is largely "shillet" - a very low grade shale - and as you may see from the photos below, has held it's shape unsupported for several months now.

Depending on what you read and where you read it, the thickness of the base of retaining walls should be between a quarter and a third of the difference in height between the land in front of, and behind, the wall. This means that the base of the wall should be between 275mm and 366mm thick. The foundations are 150mm thick concrete cast in a trench which is cut directly into the shillet.

For the walls down the side of the house, for reasons of both speed of construction and economy, I have elected to build in 100mm concrete block and then render and paint it. One layer is upright and the adjoining layer is on its side, giving a total thickness (inc mortar) of about 320mm. The first 2 layers of block are cemented with 3:1 mortar, and 4:1 above that. Behind the wall I'm putting a weed proof membrane and backfilling between the membrane and the wall with 20mm granite chippings, and the membrane will be tucked into the mortar joint for the top course of blocks to prevent the gravel drainage layer becoming clogged with soil over the years. Weep holes are installed every second block, so about 900mm apart. I'm happy with this and don't expect it to fail any time soon. So far so good.

Now for the bit which is causing me to ponder. For aesthetic reasons, the wall behind the house which will be in full view, I intend to face with Yennadon walling stone - a local hard rustic mudstone - which is uneven in size but normally between 100mm and 200mm "on the bed". Therefore logic dictates that the blockwall could be just one block thick, and the stone facing at 200mm thick would give a finished wall 300mm thick at the base. From half way up (about 600mm) I will dispense with the blockwork and rely on the 200mm stone. The block wall will be tied to the stone facing with stainless steel wall ties. Again I will put a membrane up the back, backfilled with gravel between the membrane and the wall, and with weepholes every 900mm.

Is this likely to be adequate? Will 200 mm of natural stone walling up the front of a single block wall, match 3 concrete blocks for strength? I have been getting mixed messages from what I read. The wall which it replaces was only a single block thick (although also only 900mm high), and with no allowance for drainage, and that stood fine for over 30 years until we demolished it, so my gut feel is that it should be OK. Anyone know any different?

This is the area that will be stone faced.

Down the side of the house the wall will be block and render.

Here's the block wall showing the membrane and gravel backfill, plus weep holes.

I'm hoping MikeG may be along in a minute! :eusa-whistle: :text-feedback:

 

[Rod]

If your shillet is as stable as you say them I don't think you will have a problem?
You've probably over done the weep pipes spacing especially as you have effectively built a rubble drain behind.
My back garden wall is built from Purbeck Stone blocks about 250mm wide and about 1m high supporting chalk and topsoil. Survived 40yrs so far.

Rod

 

[RogerS]

Weep pipes.

Has anyone EVER seen any water weeping out? I haven't. I put weep pipes in the same category as French drains.

Pointless!

 

[Mike G]

That 1:4 thickness: height rule of thumb is a good one.

The key issues with a retaining wall are the 1/foundation, which should extend back under the stuff being retained, 2/the join of the wall to the foundation (I always advocate casting some rebars into the founds so that the masonry isn't pushed off the founds horizontally), and 3/drainage.

You seem to have adequately dealt with 1/ and 3/, Roger, and the thickness should be fine (of course, the actual calculated thickness would decrease as the wall goes up, because, for instance, the top 200mm of the walls height only needs to be 50mm thick by the same rule of thumb..........careful not to push rules-of-thumb to an illogical conclusion!!).

That only leaves 2/.....the connection of blockwork to the foundation. I would certainly feel more comfortable if you could get something into the conc. which stuck up to hold the blocks against a horizontal force. There may be little force now, but imagine a storm/ flood situation, where that wall could be holding up hundreds of tons of water in the ground behind it.

It helps your cause that the wall doesn't appear to be straight. This increases its structural depth.

The usual caveat: I am an architect, not a structural engineer, so my advice is worth what you are paying for it. However, I would be comfortable with that wall if it is well built, and providing you can do something about 2/.

Mike

 

[RogerM]

My back garden wall is built from Purbeck Stone blocks about 250mm wide and about 1m high supporting chalk and topsoil. Survived 40yrs so far.

Rod

That's good to hear Rod. Mind you, in 40 years time I'll be more interested in whether my ar5e has fallen through the bottom of my box than if these walls are still plumb. :lol:

Weep pipes.

Has anyone EVER seen any water weeping out? I haven't. I put weep pipes in the same category as French drains.

Pointless!

Whilst i tend to agree, people expect to see them and are critical if they are not there. It could be that the reason nothing ever seems to come out is that they are working well and there has been no build up of ground water? Or they are blocked because either there is no drainage behind, or if there is, it has become blocked - hence the membrane and gravel. At this stage it's no effort to install it, so why not?

.....the connection of blockwork to the foundation. I would certainly feel more comfortable if you could get something into the conc. which stuck up to hold the blocks against a horizontal force. There may be little force now, but imagine a storm/ flood situation, where that wall could be holding up hundreds of tons of water in the ground behind it.

It helps your cause that the wall doesn't appear to be straight. This increases its structural depth.

The usual caveat: I am an architect, not a structural engineer, so my advice is worth what you are paying for it. However, I would be comfortable with that wall if it is well built, and providing you can do something about 2/.

Mike

Thanks for those words of wisdom Mike. I'll see if I can drill into the foundation to install some rebar, but I'm not hopeful. Flooding isn't an issue as we are very close to the brow of a hill - hence the shillet so near the surface. I left the surface of the foundation concrete very rough - still shows the rake grooves - so I should imagine that lateral movement across the foundation will already be resisted. Also, at its highest point, the cleavage of the shillet is at 90 deg to the wall and therefore very stable. If the wall was 2metres high and retaining a mix of clay and sand and was prone to flooding I'd be a little more concerned.

Hopefully your comments that you "would be comfortable with that wall if it is well built," also applies to the section that will be a single skin of conc block with a 200mm facing of stone, tied together with stainless steel walll ties?

Don't worry - I'll not be claiming on your prof indemnity if it falls over! :lol: