OK, my two cents (sense?) worth:
     You should use coverings that allow the passing through of
moisture. The best, if it can be afforded, is geotextile (I don't know the
brand names). This is the material that is placed under the gravel of a
road bed and used for riverbank stablization. If you are covering a
previously sealed layer (clay covered) then I don't  know... One local
site used hay bales instead of sand bags to fill in excavation pits
between seasons.
 
Paul Morrissette
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Collections Technologist
Canadian Heritage
Professional and Technical Service Centre
Winnipeg, Manitoba