Retaining wall in garden

[emjay1711]

Hello,

Apologies if this has been asked and answered but I couldn't find a specific post with all the answers that allowed me to sleep well.

I'm putting up a retaining wall at the end of the garden. I've been told by the building control chap that, even though I'm on a slight slope, the land is unlikely to slip. Still, "unlikely" doesn't mean "won't" and I always err on the side of caution so I want to take the safest option possible.

I'm putting up a 10 metre hollow block wall that will be 5 courses tall (although the first course will be 50%-75% sunk below ground level). Plan is for it to sit on foundations 400mm wide and 300m deep using a 4:1 ratio of ballast to cement - and I'm thinking of putting 16mm rods of rebar into the foundation (but not solved how to lay it yet). The ground that the wall and foundation will be built is all chalk and flint (undisturbed), so quite solid.

Any opinions as to whether the footings are wide and deep enough? (Note that the building control chap I spoke to was visiting a house 3 doors along and didn't actually see my garden - but he would if I stumped up a "planning fee")

The hollow blocks will have the same 16mm vertical rebar in each pair of corner cells and every other cell in between - and will also have horizontal bars on the 1st, 3rd and 5th courses (did I say I err on the side of caution?) All block cells will be infilled. The thing that is stumping me now is control/expansion joints and "weep joints" for drainage on the 1st course (i.e. not putting mortar between the blocks every 3 blocks).

Do I need expansion/control joints in this? Would I put a single one bang in the middle? What would I "fill" this expansion joint with?

Finally, if it put weep joints in, do I need to install any "tubes" for drainage? Once the wall is cured I will backfill with shingle with the topmost 6 inches being soil so I can put down grass seed.

Any advice much appreciated.

 

[^woody^]

What slope? What's being retained?

A photo or sketch would be better

 

[emjay1711]

Thanks for your reply woody.

The wall is going across the width at the end of the garden, left boundary to right boundary.

It's retaining the lawn (and the earth under it). I dug most of it out last summer (still need to do the trench for the foundation) and, to be honest, it hasn't moved a bit even after all the rains and winter we had... but I'm ever the suspicious one.

The slope is such that over a 25 metre length, the difference in height, front to back is about 50cm.

Will try and post a sketch and attach it.

 

[emjay1711]

Hi,

Just a quick update. The building control chap was at the property a few doors away and he offered 15 mins of his time for free! Nice man!

This is what advice he gave:

• I don't actually need rebar anywhere as he said the ground doesn't slope enough and none of the neighbours have had slips - and mine is very unlikely to slip (result!!)
• He also said that a 450mm wide (not 400mm) by 300mm deep trench for the footing is fine as long as I don't place blocks bigger than 440mm x 215mm
• The footing depth of 300mm is fine as it is being dug out of a chalk base and it will not move
• I should back-fill the cavity between the wall and the earth with some form of shingle - and put a breathable barrier between the earth and shingle
• Doing the back-fill, I would not need to insert and drainage pipes into the wall (but my builder might think it a normal thing to do anyway) - but I should seriously consider weep joints on the 1st course
• He also suggested looking at a drainage pipe behind the wall, laid on sand, and then covered - but only if I was being extra-extra cautious
• He said it's a good idea to put in a 'cold joint' (and mentioned something about removing pressure) - but not necessary

Any more detail such as spacing of weep joints etc would require a more detailed discussion and lengthier visit - and based on what he saw and said I didn't get the feeling he was recommending this.

I also asked whether a 2-3 degree slope in the foundation to allow water to run-off as it exits the weep joints is a good thing. It would mean that the mortar under the 1st course is thicker (about 25/30mm instead of 10mm). He said that generally, the foundation would be flat but for a garden wall like mine I might choose to do this on.

So the only questions that remain are:

1. Is it ok to have a 2/3 degree incline on the foundation to allow a water escape path? (I'll still be infilling the hollow-blocks)
2. For the foundation I'm thinking of a 3:1 ratio of ballast to cement. Is this wrong?
3. For the mortar I'm thinking of of a 4:1 ratio of soft sand to cement. Is this wrong?

Really pleased with my 15 mins free time today! Once I've worked out the above 3 bits I think I'll get cracking.

Thanks.