Physical DPC needed for garden retaining wall?

[russellpeters]

Hello Forum Members,

My house is located on a slope. Because of this the rear yard has 2 levels, the lower level is level with the house and the higher level is about 1 meter above the lower level. The retaining wall for this higher level is made of breeze blocks. This wall was laid by a very capable bricklayer and has proper foundations etc.

My question is - does this retaining wall need to have physical DPC on the side where the garden soil is? If yes does it need DPC for the entire height of the wall (~1 meter) or only along the top 20 cm or so?

I am in the process of prepping the garden soil (it had been used a dumping ground by the previous owner for many decades and is full of construction debris, glass, large stones, metal pieces etc.) and have an opportunity to put in DPC. Putting in DPC just for the top 20-30 cm will be relatively easy. It will be more difficult if I have to put it for the entire height of the wall and therefore this question.

The other side of the retaining wall (i.e. the side facing the house) has a cladding of deckboards. These deckboards have not been directly fixed onto the wall. They have been fixed onto wooden posts that in turn have been screwed into the retaining wall, thus leaving a gap of about 2 cm breathing space between the deckboards and retaining wall.

What do you think?

Thanks.

 

[kbdiy]

I'm struggling to uderstand why you think a DPC at the top of this retaining wall is going to do any good at all?

 

[russellpeters]

@kbdiy - That was probably a stupid question and a sign of my laziness coming through.

So the question is now - do I need a DPC at all? Or can I just leave things as they are (i.e. no DPC) because the retaining wall is an independent structure in the garden area with no connection to the house?

My main worry, and I admit complete ignorance in this area, is whether wet or moist soil staying in constant contact with a wall made of breeze blocks will degrade the retaining wall. Is my fear unfounded? I would like the wall to last 10-15 years at least.

 

[russellpeters]

Apologies, I have got DPC (damp proof course) and DPM (damp proof membrane) membrane mixed up.

My question then becomes whether I should have DPM on the side of the retaining wall where it retains the soil.

I think the answer is yes.

This article (scroll down to "Retaining Walls") has good detailed guidance on how to build walls:
http://www.pavingexpert.com/featur03.htm

 

[r896neo]

yes you do need a dpm. you can use a paint on dpm if thats easier.

Remember though that in doing this you are creating a large damn and need to drain the water effectively with land drains or weep holes.

 

[russellpeters]

From reading online articles about retaining walls and from replies to this thread I have come to the conclusion that I do need some kind of a DPM.

This is what I plan to do:
(1) Excavate enough soil on the soil-side of the retaining wall, preferably all the way to ground level.
(2) Fix the DPM sheet to the wall (using screws, washers and metal plates?)
(3) Pour in gravel into this excavated area to aid in drainage.
(4) Drill weepholes from the other side. Insert tight-fitting plastic pipes into these holes. I am aiming for holes of 25-40 mm diameter spaced at 80-100 cm from each other.
(5) Put the soil back into the excavated area.

The weepholes will release the excess water under my wooden decking from where they will flow to the nearest perimeter drain (French drain) about 3 meters away.

Do you think this will suffice? Any glaring blunders or improvements?

 

[freddiemercurystwin]

Have a read here: //www.diynot.com/forums/viewtopic.php?t=278828&start=0

 

[russellpeters]

Hello Folks,

Thanks for all the useful advice. Just wanted to report back on what I finally did. Will try to post an update 6 months and 12 months from now too.

I used a double layer of DPC instead of DPM because I had a lot of it left over from a previous project. The sheets were held in place with thin wooden posts. I did not use a bitumen paint on the wall or another sheet in front of the DPC/DPM mainly because of time constraints. I used gravel/stones to fill up the bottom half of the trench with soil on top.

I have not yet drilled the weepholes but will do it from the other side afterwards. I know this is not ideal but I took this call because of shortage of time and also because I do not expect the retaining wall to hold up a lot of water because the soil drains quite well.

See the attached photo taken in the middle of the work.

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• russellpeters
• 6 Oct 2013
• 1