Damp garage

[Geevee]

A friend has a garage that he is starting to refurb., new roof, power,water etc.
Unfortunately the garage lies at the foot of a considerable slope with earth rising nearly to the top two courses of blocks- not surprisingly the back wall gets very wet.
We've dug out a metre wide trench with the intention of inserting a vertical dpm.Once this is done ,is it better to backfill or to build a retaining wall to keep the soil completely away from the wall?
Any suggestions most welcome.

Cheers

[Tony McC]

Backfill with clean gravel.

The best construction would be a fin drain laid against the outside of the wall, and then connected to a suitable outfall point, the face of the excavation lined with a permeable geo-textile, and all backfilled with clean gravel.