We have water flowing under concrete stairs that run down the side of our house. The water enters under the stairs from the neighbour's garden bed adjoins our stairs on the boundary, so we don't have space to excavate along the side of the stairs. The water has eroded under the stairs and flows under the concrete path at the bottom of the stairs, which I think is the cause of some subsidence we've seen as cracking in our brick wall of our house that adjoins the path.