footings depth? +French drain

[jungle_jim01]

I've finally started tackling the raised external ground level (which has been causing damp). I've removed the concrete surface and dug down about 18inches to just below the internal floor level. The house is victorian brick but sat on sandstone (flush with the brick) which starts 2 courses above floor level. My question is how far should i dig? and what kind of foundations should i expect to find? I'm curious as to where the original ground level would have been and where's best for the building. Obviously i don't want to go too far and risk causing any structural damage.
Thanks again

 

[88v8]

About 6" below floor level, or 6" below dpc if there is one, should be sufficient to deal with the damp.
If you're putting in a land drain - not saying you should, just saying if you are - then you're far enough down already I would think.

So the house is sitting on a sandstone plinth; that suggests a relatively high-quality building, or perhaps there was a bogof on sandstone. Footings, if you find a couple of spread courses of brick 18" down, that would already be pretty good for the period. Sounds as if it must have pretty decent foundations.

Original ground level, unless there's a dpc to give you a clue, I would look at doorstep levels, and delicately dig a small pit alongside the wall looking for an old tidemark or old paving fragments or a discrete soil type which might give a clue. Bit of an archaeological exercise really.
Pictures?

Ivor

 

[jungle_jim01]

Thanks Ivor, so i've been doing a bit of digging (tentatively) and not convinced the foundations are that good after all. It would appear that there is one course of sandstone below the floor level and that's it. no stepping it or any fancy features, just 1 line of stone on very clay like earth...hmmm
So...question about positioning the land drain. Presumably i shouldn't be digging below the foundations. I think i read somewhere to go as far as the bottom of the wall (foundation) and angle the trench 45 degrees. I've got to say i'm a bit worried that if i over do it i could cause bigger issues with subsidence. As usual, advice very much appreciated. Thanks

 

[Lime]

Do not dig below the foundations whatsoever.
You have no idea what ground movement may occur.