What is this 所 here?
http://www.zdic.net/z/1a/xs/6240.htm


  ◎ 所 suǒ
  
  〈助〉
  
  (1) 表示结构 [used before a verb or a V-C construction together with 被 or
  为 to indicate the passive voice]


For translation, you could just ignore it:

In the end covered up by dirty things again.