Typically drain lines are installed at the base of new walls before the soil is backfilled.
Installing french drain behind retaining wall.
Mar 19 2014 when building a retaining wall lay perforated drainage tubing at the base of the wall slightly above ground level so runoff can drain out without undermining your wall.
Check out this video for some simple instructions on installing a french drain behind an 18 to 24 tall retaining wall before it is backfilled with dirt.
This is so that when water gets into the space it does not become bogged down in soil but instead can flow down the wall to the drains or weep holes.
Otherwise water moving down the hill will build up behind the wall and undermine it.
The pipe should rest on the same compacted gravel base or concrete footing that supports the wall.
If you re building a retaining wall add a french drain behind the first course of stones or blocks.
Fill the trench with topsoil and compact to completely cover the entire french drain system.
When filling the area around the inlet grate temporarily cover the grate to prevent any stones or soil from falling in after installing the french drain.
This video shows you how to properly install a drainpipe behind your wall for prop.