French drains (again)

[Ushy M]

We have damp on the inside of our gable end wall because the ground level is higher outside due to it being in the churchyard. I've posted with enquiries about constructing a french drain before and we are now planning the project out in more detail.

One thing that bothers me is that the recommended construction method I've seen (IHBC paper) emphasises the need to dig the trench walls at 45 degrees. We need to dig about 3 feet down and can only dig a very narrow trench because the nearest burial is only a very few feet away from the house wall, so constructing such a trench with walls at 45 degrees is impossible.

Any help or advice on how to overcome this or otherwise equalise the ground level and allow the wall to breathe in such a narrow gap would be gratefully received. I've seen a reference to creating a "dry area" - could this be an alternative approach to a french drain?

 

[Gervase]

The 45 degree sides are recommended, but if you line the walls of the drain with geotex or a similar textile you can get away with a steeper slope - what you're aiming for is a drain side that won't collapse in on itself over time, and 45 degrees is the ideal.
If you were to shore the sides with the sort of steel used for piling and coffering you could get away with vertical sides, but that may be extreme!
That would certainly be applicable in the case of a dry area, though. You could dig straight down and, provided you shore the side adequately, you could get away with a trench a couple of feet wide. There's always the danger, however, that it could actually trap water unless you had some sort of soakaway or sump connected to it.

 

[Ushy M]

Thanks Gervase,
We were actually considering shoring the sides of the trench with paving slabs and inserting a perforated land drain (wrapped in geotex to stop it filling with grit) in the bottom. The drain can run out onto the pavement - I'm assuming this is OK because that's what's been done on the house at the other end of the churchyard. Sadly that drain was dug before the present occupants bought the house so I can't ask them how it was done.