The Brew Cabin

I'm a civil engineer in Pennsylvania, USA looking for some input. We have an ongoing debate about whether or not to use a geomembrane to separate a stone filled trench from the subgrade. I think it's fine on the top and sides, my experience has shown that if put on the bottom, clogging will result much more quickly. If a bottom separation is needed, I think an aggregate filter should be designed.

What do you think?

It's standard practice over here to use the geo-membrane all around the trench, but the filter medium (or stone backfill) used to fill the trench is specially selected to have a low or non-existent fines content, so clogging hasn't been an issue - at least, not on the jobs on which I've been involved.

If you have a well-draining sub-grade, then I can't see a problem with using a graded filter medium to line the bottom of the trench, and then just lining sides and top with the membrane, but I have to wonder where all this clogging material is originating: is this a leach field for a septic system?

Alternatively, if it's just a distributor drain, would a catch-pit structure at the head of the run be a valuable addition? I'm thinking of the extra labour element involved in placing the filter medium and then lining just the sides and top, as opposed to lining the whole trench.