Plastics parts and moldings of other material include a drainage pipe (2) parallel to the tunnel wall and a subtended floorplate fixed to the tunnel floor. The wall part facing into the tunnel stands upright on the floorplate to reach above the pipe between wall part and tunnel wall proper. Wall part and pipe are spaced by more than filter gravel diameter and the respective plastics parts come joined or can be joined to one another. Wall part and floorplate present a prefabricated molding with holders to place the pipe which in turn has slots or openings of at least 4 mm width. The interval between wall part and tunnel wall is packed with gravel (1) to wall part level.
