Dpm and mortar mix

[ChaserXV]

I'm building a 1.5m high retaining wall in my garden using concrete blocks. I want to stop moisture getting through so intend to place some dpm at the back of it. Do I place the dpm between the foundation and the first course of blocks?

Also, what mix should I use for concrete blocks, 3:1 or 4:1? What's the difference?

Paul

 

[ree]

You would be better off "painting" some liquid bitumen solution on the rear of the retaining blocks, and providing sufficient weep holes to relieve any water pressure build up. Forget the membrane.

Use a 3:1 sand to cement mix.

Blocks with hollow cores should have the cores filled with the above mix, and chippings.

If the wall is retaining higher ground or unstable ground then use re-bar thro the blockwork.

 

[ChaserXV]

.
Blocks with hollow cores should have the cores filled with the above mix, and chippings.

If the wall is retaining higher ground or unstable ground then use re-bar thro the blockwork.

I don't think bars and hollow blocks are all that common in the UK. I'm using the standard 440mmX215mmX100mm solid concrete blocks.

 

[ree]

They are available as "hollow core dense concrete blocks".

100mm seems too light for a retaining wall.

 

[ChaserXV]

They are available as "hollow core dense concrete blocks".

100mm seems too light for a retaining wall.

I know, that's why I intend to lay them face down.

 

[ChaserXV]

Anyone?...I thought they were pretty simple questions

 

[theoldun]

Anyone?...I thought they were pretty simple questions

They are if you know the answers.
The mix will depend on the strength and density of your blocks, how long wall is and expansion joints.
Neither a polythene DPM or a liquid DPM will stop damp coming through.
If it is a 1500 high full retaining wall, then a 215 block laid on its side is not strong enough.
Would be curious to know size and depth of foundation?
Regards oldun

 

[ban-all-sheds]

If it is a 1500 high full retaining wall, then a 215 block laid on its side is not strong enough.

What could go wrong?

In the dim and distant past I wrote a computer program to design retaining walls - IIRC the basic shape was an inverted t.

 

[stuart45]

Anyone?...I thought they were pretty simple questions

Retaining wall answers are not always simple. The design also depends on soil and water table conditions as well as the height. You also need a drainage system at the back of the wall.

 

[ban-all-sheds]

Don't remember anything about water table, but I know there were factors for how high, what the angle of the surface of the retained soil was, and something to do with the characteristics of the soil.

 

[ChaserXV]

Anyone?...I thought they were pretty simple questions

They are if you know the answers.
The mix will depend on the strength and density of your blocks, how long wall is and expansion joints.
Neither a polythene DPM or a liquid DPM will stop damp coming through.
If it is a 1500 high full retaining wall, then a 215 block laid on its side is not strong enough.
Would be curious to know size and depth of foundation?
Regards oldun

I guess they are simple straight forward questions to those in the know; hence why I posted the questions on this forum, well I presume the folks who frequent this forum are in the know?

Wall will be 3 meters wide, and about 1.5 meters high. Blocks are standard size 7n. They will be layed face down. Think I'm going to use a 4:1 mix

Foundations are 381mm x 457.20mm, they are supported by ground rock and a slight rock face. I used a mix of 3 crush and run, 2 sand and 1 cement.