The plaster on one wall of my basement has blown and come away from the wall. I've stripped it all off which leaves a layer of concrete painted with bitumen. What's the best method for sealing off more water coming in? When I started to strip back the bitumen layer I hit a pebble!! It appears that there is not brickwork down there instead perhaps the foundation? The wall also has a custom built stair case running up it which complicates things further.