Question
I'm building a retaining wall out of huge sandstone, stacked flat on each other. The wall is about 55' long and 4' tall.  Do I need to put some type of draining pipes or tubing along with gravel to ensure proper draining?

Answer
Hi Jay

Assuming there is a slight slope to the grading, it's never a bad idea to include some 4" drainage pipe just under the gravel sump.  This is simply common perforated-black-ridged pipe that gets installed 4-5" below the gravel.  Depending on the soil type, you may need to get the pipe that is wrapped in material (AKA a sock) to keep the holes in the pipe from clogging up.

Hope this helps - Good Luck.

Tony