 In fact, deep drainage trenches do not have bottom piping that is inserted in the end part of the trench, downhill, where the depth of the cut is reduced until the campaign level is reached.Drainage galleries constitute a rather expensive stabilisation provision for large, deep landslide movements, used where the ground is unsuitable for cutting trenches or drainage wells and where it is impossible to work on the surface owing to a lack of space for the work machinery.Their effectiveness is due to the extensiveness of the area to be drained.