Slab concrete on erodible soil is a poor construction practice, IMO. Besides flatheads do more earth removing on ramps in my area than powerloading will ever do. With that said, only use enough power to get the job done, no more than what is necessary. It also helps to have the trailer far enough into the water (way to often I see others that don't back their trailer all the way down, cause they don't want to get there feet wet).