Looking at the topography behind the wall, above the road, the adit would have been around 5-10m further back and there would have been a short cutting leading up to it. I would guess that the adit is backfilled along with the cutting, then the wall which forms the boundary to the road built effectively across the end of the cutting. I think the water pressure will always tend to make the run-off bubble up through the backfill behind the wall. A massive blow out is unlikely.....but you never know :devil: Straight across the road and into the river!