Concrete foundation needed??

I'm about to rebuild a stone wall that has tumbled in the December rains. The wall is I'm guessing at least 100 year old and built with clay mortar. The reason the wall fell was water entering the wall through the top where there was a broken tile, expanding and washing out the clay inside..
As far as I can see there is no problem with the foundation (literally just dug down to subsoil and build there on). However people have advised to remove the whole wall, pour a concrete foundation and build th new wall on that. My worry is that it only one wall connecting other outbuilding that have been flexing with the moving ground and a slab of rigid concrete may disturb the equilibrium?🤔 Moreover if it is an expense I can do without paying all the better!
The wall btw is not huge around 9m long and not quite 2m high..

My worry is that it only one wall connecting other outbuilding that have been flexing with the moving ground and a slab of rigid concrete may disturb the equilibrium?🤔 Moreover if it is an expense I can do without paying all the better!

Did you read what you actually typed?
There seems to be something missing.
I think your self appointed advisors need to be grilled for their experience in wall building  and their depth of knowledge ton the subject.
I have experience, but I would need to see the wall and have some local experience on hand to help me.
The issue of water getting inside the wall, that is a key point.

Foundation work, especially repair, makes me a little queasy because you have a building on top of it!

I have a rubblestone foundation on my house and I am in the process of repointing it because a lot of the lime mortar is reaching its end of life. By stone foundation, is this the style you mean?

If you have pictures, that might be of help to give you better advice.

Hi Mike;
If I am reading this correctly, you are repairing a free-standing stone wall 2 M high and 9 M long.
Not the foundation of a building.

If so what purpose is the wall performing?
If it is a boundary wall, you would not need to pour a concrete footer.
Just repair it as it was originally built.

If I am wrong and it is a building foundation then I have to wonder how enough rain got on it to wash it out.
Perhaps a larger roof overhang would eliminate excessive rain on your mortar.

If the wall stood for 100 years without a concrete foundation, I see little need for one now. …not that a well laid foundation would be harmful.  It sounds like the damaged portions need to be rebuilt and a new cap put on.

I would rebuild the sections that need it with the absolute minimum of clay mortar. A well built drystone wall can last for centuries; adding clay means the need to keep water out.

What kind of stone do you have? Some rounder varieties may need some sort of mortar to stand up in the first place. I personally think cobbles are not suitable for building walls of any kind. In some places where that is the natural shape of loose rocks, I have seen them split in half or smaller to give reasonable structural properties.

Mike Schofield wrote:I'm about to rebuild a stone wall that has tumbled in the December rains. The wall is I'm guessing at least 100 year old and built with clay mortar.  
The wall btw is not huge around 9m long and not quite 2m high..

Clay mortar, exposed to elements? for the last 100 years?  I was under the assumption that clay mortar, was basically a DRY only material used for any blocks/stone and so on.

2 meters high, depending on the width, is a sizeable wall,  as stated by others, a photo would be a great help.

John C Daley wrote:

My worry is that it only one wall connecting other outbuilding that have been flexing with the moving ground and a slab of rigid concrete may disturb the equilibrium?🤔 Moreover if it is an expense I can do without paying all the better!

Did you read what you actually typed?
There seems to be something missing.
I think your self appointed advisors need to be grilled for their experience in wall building  and their depth of knowledge ton the subject.
I have experience, but I would need to see the wall and have some local experience on hand to help me.
The issue of water getting inside the wall, that is a key point.

To explain further. This is not a boundary wall it is the wall of an outbuilding. Not a particularly important one but one I would like to keep all the same. The site is on a slope and this is the highest building. Connected to it is a larger workshop of the same material. It is both higher and deeper it you get my drift. I will attach pictures tomorrow..
When I say th wall has been moving and flexing I'm assuming that is the case in a wall built of stone and clay. Not the issue that brought it down. The water got into the wall through broken roof tiles. My fault for not patching it up in time but I have only had the place 6 months and there are a lot of roofs to repair amongst a long list of other jobs.. thanks for replies

If it is the wall of a building, that explains the clay mortar. I would still advise to use no more clay than absolutely necessary, letting the stones themselves do the structural work and the clay just for weathertightness.

Hi Glenn. I was planning on using nhl3.5 for the rebuild

What is this please?  "I was planning on using nhl3.5 "
I wonder how a lime mortar for the repairs would work?
I assume the base stones are larger to spread the load?

There's nothing too huge no

It's going to be rebuilt not repaired or patched up.. so...  Given all that. Who thinks I need a poured concrete foundation?

I vote no concrete.
That building stood for 100 years, and with some repair maybe it can make another 100.
My home is 95 years old, no concrete here.

Thanks Thomas that's what I'm leaning towards. Maybe throw a couple of biggies under the wall if they seem to be missing when I take the thing down

Make sure the new base course is even and well spread so you don't get lopsided settling... How deep does the existing wall go? Drystone walls typically don't need a deep foundation as long as they are uniform. Joining with other building walls may be the point of stress. Work from undisturbed earth, and make any fill with gravel or small stones well tamped so that there will be no settling in the spot. The subsoil has been compacted by the weight of the wall for a century already.