I built a pad near an abandoned well building, put the pressure washer inside the insulated building.  Don't remember the size, but it kept getting smaller from dirt building up around it and creeping into the middle.  I crowned the pad, didn't clean it off but once a year, and that was not often enough.  It was strange how many acres of land I accumulated just removing dirt from equipment.

Plan for a place for the scraped off dirt to accumulate, and scrape off often.

My pad wasn't big enough, as someone else mentioned, when washing the combine I was usually standing in mud while washing the bottom, which was after washing off the top, of course.