It should be located on the uphill side of the wall so it can catch water running down the hill and prevent it from undermining the wall.
French drain through retaining wall.
You want to use a perforated 4 plastic pipe below the footing not on top of the footing on the up hill side.
Drain pipes go behind the wall on the side with the surcharge the soil that that wall or foundation is holding back.
Some of these drains are vented through the front of the retaining wall while others may run the length of the wall and drain out to the sides.
This video shows you how to properly install a drainpipe behind your wall for prop.
A french drain is a virtual necessity when building a retaining wall on a hill.
Because of the danger of allowing water to accumulate most retaining walls have built in drainage mechanisms.
The purpose is to take water from wet areas and disperse it into dry areas.
If you re building a retaining wall add a french drain behind the first course of stones or blocks.
Otherwise water moving down the hill will build up behind the wall and undermine it.
Pipe drains also known as toe drains are perforated pipes that collect water along the length of the wall and drain it to the outside.
If you re building a retaining wall on a hillside.
The pipe should rest on the same compacted gravel base or concrete footing that supports the wall.
The loose gravel and perforated tubing help facilitate that water movement.
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.
Sometimes this means a pipe running through the wall from the soil to the other side.