After the retaining walls base is in, the wall blocks start getting stacked on top of one another. This is the fun part of a retaining wall build!
Each row is built, and gravel is placed behind and inside of the blocks.
This clear gravel is uniformed in size, and acts as drainage for your retaining wall.
Some contractors use the excavated soil from the trench as backfill because it saves them money on gravel, and the cost and labor for soil disposal.
If soil is used instead of gravel as backfill your wall will eventually fail due to the heavy, wet soil that will be pushing up against your wall (lack of drainage).
Lack of drainage is the number one reason a retaining wall fails!
Importing enough gravel for a retaining wall requires special equipment due to the weight and volume of the gravel required, which is why we move all material with our track dumpers.
Gravel behind the wall acts as drainage, and typically drainage zones underneath and behind an Allan Block wall is a minimum depth of 6” of ¾” clear gravel below the wall, and 12” behind the wall.
The typical, or minimum amount of gravel to use is a starting point, and can quickly change by specific site conditions.
If there is poor draining onsite, we will increase the drainage zone, and if there are any other weight surcharges, like a shed, slope above the wall, parked vehicle etc. the drainage zone will also be increased
If you are getting a Retaining wall done, do your due diligence in making sure that your contractor is putting enough gravel behind your wall!