Reinforcing boundary wall footings

[John_1919]

Some general guidance please. To the rear boundary of my garden is a 1.8m high concrete block wall which is on the other side of the boundary line i.e. it is my neighbor's property and he insists that it remains untouched. It's footings are circa 200mm deep. I don't know how wide the footings are because the wall is right on the boundary line.

On my side I wish to reduce ground level by circa 200mm in order to level the site for a large shed which is to be located as close to the boundary as possible [minimum 500mm] i.e. new ground level will be the same level as the bottom of the existing wall footings. Height restrictions prelude keeping the existing ground level.

Clearly there needs to be something substantial on my side of the boundary to act as a “retaining wall” and prevent future movement of the boundary wall. The whole thing will be hidden so appearance doesn't matter.

So before I get a contractor involved, could someone please advise what the reasonably pragmatic solution would be – are there rules of thumb which apply? I know that the whole job should be done in sections, but how close to the boundary is reasonable without making the job complex, costly or creating major safety issues. How deep and wide should the new “retaining wall” footings be to provide sufficient stability?

I know that in detail this is a complex subject, possibly a matter for a structural engineer, but at this point I'm just after some general guidance to see what is reasonably pragmatic and what's not.

The footings of other nearby properties and structures are all of standard construction, so the sub soil is obviously capable of supporting normal construction methods.

Many Thanks - John

 

[theprinceofdarkness]

You don't say how thick the wall is or if it has got piers, I presume the soil level is the same both sides of the wall. That 200mm is to the top of the foundations?, if its to the bottom of the concrete then the foundations are far too thin and I would not build a shed within 12' of it.
Frank

 

[John_1919]

My thanks for rapid response and sorry for omitting some of the most important information!

It was built about 35yrs ago, is about 9m long, single sided rock face concrete block [circa 120mm thick] with piers every 3m. Total width at piers 240mm.
The top of the concrete is at soil level and as far as I can tell, the concrete only goes down 200mm. My neighbor has a patio on his side where I suspect there may be more width but not depth of concrete. Unfortunately he has refused my offer to completely rebuild at my expense.

15 yrs ago [in my nativity], I reduced ground level by 200mm over 2.5m of it and put in, what I now realize was a totally insufficient, 4 courses of 7N blocks on flat adjacent to wall footings and then paving slabs at the reduced soil level. I was obviously very lucky because there has been no subsequent movement. However this time I want to get it done properly, but at the same time keeping the job simple.

Unfortunately space is very limited, so I've got to locate my new shed as close as possible – certainly within 1m.
John

 

[^woody^]

Drop a concrete gravel board in, or slabs on edge.

 

[John_1919]

I can see that a concrete gravel board or slabs on edge would be OK if 1m away from boundary, but what about closer - say 500mm?
John