asc,

according to this guy,who imo is quite good but i do disagree w/him on a few things, he says 'down'. paragraph 8

http://www.askthebuilder.com/NH029_-_Dual_Purpose_Drain_Tile_-_It_Can_do_More_Than_One_Thing.shtml

but many builders opt for 'flexible perforated D tile' which should Not be used at depths of more than 1.2 meters according to this manufacturer recommendation. And believe me, 'this is exactly' what many use! We know because we`ve dug up 'many' over the years and seen with our own eyes.

http://oxfordplasticsinc.com/perforatedpipe.htm scroll down to'selecting perf. pipe' gl!