Soil retaining wall

[ademco]

Hello there, we are building a 75ft long x 4.5ft tall soil retaining wall around our extension. regarding drainage-on the first row we have left weep holes every 3ft and are also fitting a drainage pipe behind the wall-one with holes in it. I know that we need to put some kind of gravel ontop of the pipe, but what kind. Would pea gravel be okay?-our land is mostly clay.

 

[RedLeader]

Hi

We've recently had a conservatory built and the contractors had to install a new soakaway pipe. They put pea shingle on top of the pipe before backfilling but I understand this is not correct and it should be builder's or sharp sand.

Hope this helps.
Red

 

[noseall]

fitting a drainage pipe behind the wall-one with holes in it.................... Would pea gravel be okay?-our land is mostly clay.

Not only ok but ideal. Don't use sand of any kind to backfill.

 

[Thermo]

Hi

We've recently had a conservatory built and the contractors had to install a new soakaway pipe. They put pea shingle on top of the pipe before backfilling but I understand this is not correct and it should be builder's or sharp sand.

Hope this helps.
Red

someones told you wrong then. sand will not as a good filter medium and will simply be dragged down into th pipe to help clog it up. As noseall says, shingle is the way to go

 

[Shytalkz]

Pea shingle is entirely appropriate drainage medium. If it's surrounded by a geotextile, that will stop fine particles being drawn into and eventually clogging up, the perforated pipe.

Sand is most definitely not the medium to use!

 

[alittlerespect]

The second part of your discussion thread is known as a 'french drain', if put in correctly you will not need the weep holes, but you will need a soakaway or stormwater drain or somewhere to discharge the groundwater.

Take a look at 'flexibleliningproducts.co.uk' and you will find a outline detail on a 'wrapped french drain' this is where you should be heading, it uses a geotextile to stop the drain system becoming blocked with fine soil, which can otherwise happen over time.

Spec wise, lay the perforated pipe on say 50mm of pea beach washed shingle, but then use a coarser material for the backfill say 20mm washed gravel, finish off around 150mm below ground level and wrap over geotextile as shown on 'flp' website, then backfill with topsoil and lay your turf - job done.

Regards

 

[^woody^]

So, is more water going to flood through these weep holes than would otherwise soak into the retained ground naturally or into the wall material?

Is the perforated pipe at the correct level to actually do anything?

Most retaining walls don't need drains and/or weep holes. Many people thingthere should be one and just form some elaborate set up, which does nothing in practice

You are better off just putting a suitable membrane behind the wall to stop awful damp staining, salts and effloescence leaching through, and then the inevitable frost spalling

 

[ademco]

Here are some pics of our retaining wall so far. It is 78 blocks per row and the wall is 105 ft long not 75 as I said before. Still 3 more layers to go then in goes the drainage pipe then 20mm shingle as stated by building regs.

Untitled

• ademco
• 5 Sep 2009
• 1

Untitled

• ademco
• 5 Sep 2009
• 1

 

[freddiemercurystwin]

Has that wall been designed or guessed? Looks awfully thin to me. are you gonna fill the voids with concrete? What about reinforcement?

 

[ademco]

The blocks are 9 inch wide, the foundation has griddwork in it and iron bars to 1 block heigh-then when finished, full length bars are going in followed by concrete. All this is in our building regs plans.

 

[Shytalkz]

And are there any bars sticking up from the foundation? Is there actually any footing at all?

If not, you may as well not bother with wasting your time putting rebar in the blockwork.

Movement joints in the blockwork?

I look forward to seeing the pix after the winter, when the whole lot's fallen apart.

 

[Shytalkz]

Ok so you have some sort of footing and some rebar. But the amount the rebar sticks up is inadequate. Should be 450 ie two blocks. How much of it is in the footing? How big is this apparent footing?

 

[^woody^]

It looks like that bank is holding itself up. The crucial bit is the junction of the wall and foundation, so should be OK if the bars are in properly

If it helps, have done quite a few similar (although solid 9" blockwork) walls and they have been up for many years.

Hey, shy ...... is this wall going to contract much if it will be perpetually damp?

 

[Shytalkz]

Bank surface sloping = greater pressure on wall. Wall stands while soil effectively self-supports; wall ever called on to do what it is supposed to do, wall fails.

Same applies to those you built.

Back of wall might be damp, front exposed to sunlight, so differential expansion/contraction across the block width. Even more reason to form joints.

 

[freddiemercurystwin]

The blocks are 9 inch wide, the foundation has griddwork in it and iron bars to 1 block heigh-then when finished, full length bars are going in followed by concrete. All this is in our building regs plans.

So how do you join the foundation rebars to the wall rebars after the blocks are up?