Cob: Rubble Trench Foundation

[Vivian Westfall]

We are building a log duck house with a RTF.  I was wondering if anyone 
can help us out.  When I went to look at perforated pipe it was pretty 
reasonable, but the cloth covering more than doubled the price.  The guy 
at home depot said it really doesn't matter if we use it or not since it 
is pretty dry here in Northern New Mexico (although we live at 7500 ft 
and the shed is on the north side where we have quite a bit of snow 
sitting all winter).  I was wondering if I could just cover it in a 
regular bed sheet?  Or is there something else that is cheap and easily 
obtained? Can we just skip the covering altogether?  
Thanks,
Vivian