Blockwork Retaining Wall - Advice Please

martindow

Hello

I have a retaining wall 3 blocks high and about 15m long which is cracked and leaning, presumably because it does not appear to have any drainage holes. I'm going to pull it down and rebuild.

Where and how many drainage holes should I put in and what size should they be?

Should I build buttresses into the wall. It is at the edge of a terrace which seems quite stable as it hasn't immediately fallen against the wall where it leans.

Any help would be much appreciated.

djohn2002uk

I'm assuming you mean 450mm 15" blocks.
First thing is to make sure you ha good footings. 4" deep should suffice and scrounge or buy a small amount of reinforcing from a building site and lay in it. Get 30 metres of 4"x1" timber from a builders merchants for shuttering (cheaper than hiring metal shuttering). And make sure it's all dead level.
Then just lay the blocks and either build buttresses every 2 metres OR build a second wall out of cheap blocks 2 courses high (so that the top can be covered with soil to hide it) and 2" apart and use metal ties to tie them together. Fill the gap with rubble.
You could knock the corner off a block every couple of metres and lay a piece of plastic pipe in there, the 2" plumbers use for the waste pipe if you wish. Not really necessary because I believe it is bad footings that is causing your problem.
I have a retaining wall 1 Metre high holding a bank back built as option 2 above. The bank rises to 3 Metres high over a distance of 6 metres with a footpath and a road at the top and it hasn't moved in 11 years.

I should have added, make footings out of concrete mixed at 4x gravel to 1 x cement and the cement mix for the blockwork 5 x sand to 1 x cement.

martindow

Thanks djohn2002uk. A friend has just suggested that it might be better to lay blocks on their sides. He suggested that this would obviously use more blocks but would be very strong and not need buttresses. What do you think?

rmb282

Martin,

For a wall of only 27 inches (686mm) high, you wouldn't really need to build in piers unless the wall is built as before (3 x 9" blocks high), your friend was correct in suggesting that building the wall with the blocks laid flat would give it more strength laterally as the thickness of the wall is greater, thus not requiring piers. I would however suggest that you make your foundation a little deeper, just in case you wish to increase the height of the wall at a later stage, and also not knowing what your ground conditions are like.

Reinforcing in the foundations for a wall of this height is not really required providing it is deep enough, reinforcing is mainly used to accomodate tensile stresses as the concrete is not good in tension, and your wall height will not be causing much tensile stress on the underside of the concrete, most houses are built on un-reinforced concrete foundations although deeper than 4".

Again, not knowing what your ground conditions are like, you can place pipe in as weepholes at the bottom 1/3 of the total height, every 3m would be ok, this would help relieve the ground pressure, although on a wall of that height there wouldn't be much, also it would be good to place some chippings or 40mm clean stone behind the wall to act as a filter drain.

Depending when you plan to do this, you could wait until your local council are surface dressing the roads, and ask the sweeper driver to drop a load of the swept chippings off for you, this normally happens between April - August time. Also they make a cheap aggregate for your concrete, councils otherwise have to tip this, as they have small amounts of tar on them (not noticable) and they can not be used by the councils for pipe bedding afterwards.


Hope this is of help. :beer:

coolagarry

We built a retaining wall at my son in laws but we used concrete fence posts, concreted into the ground at 6ft intervals and dropped concrete fencing panels into them. Great strong job and not too much like hard work

rmb282

Another source of info for any garden wall, whether it be retaining or otherwise would be the following link

http://www.odpm.gov.uk/pub/106/YourgardenwallsbettertobesafePDF273Kb_id1131106.pdf

It depends what you want, you could also concrete old steel columns in vertically at approx. 8ft centres and infill the columns with railway sleepers, more expensive in materials maybe than the concrete wall approach but really strong and something different, normal price for sleepers £ 9.50-£15 each and could be used in the 10" vertical position, quick way of building a retaining wall

djohn2002uk

martindow wrote:

Thanks djohn2002uk. A friend has just suggested that it might be better to lay blocks on their sides. He suggested that this would obviously use more blocks but would be very strong and not need buttresses. What do you think?

That's another way of doing it but would need the same number of blocks as my second sugestion. There is also the appearence to think about and I would be using facing blocks at the front and cheaper concrete blocks on the back out of sight. Get the footings in correctly, rmb282 sugests deeper footings then goes on to say reinforcing not needed, a bit of contradiction in my eyes. My wall is a metre high and is only on 4" footing with no reinforcing but is on solid stoney ground. I sugested some reinforcing to alleviate the cracking you've already experienced. Just the steel rods from the reinforcing would do, a couple laid 2" apart along the length and overlapped where they join.
For a wall just 3 courses high, this should be a permanent solution for you.