At Back 40 Landscaping our expertise is in building Allan Block retaining walls. We are trained and Certified in building Allan Block retaining walls, and have a passion for building retaining walls that actually last.
Here are four questions that we frequently get asked about retaining walls.
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How far apart should retaining walls be if they are terraced?
Terraced retaining walls are a series of smaller walls that are often used when you don’t want the look of a large single wall. So, for instance instead of a single 8’ high wall that would look quite daunting. 2 – 4’ high walls are built with a space in between to help soften the look.
They are also most commonly used to avoid needing a permit or engineer as most cities allow retaining wall under 4’ to built without any oversight.
This space in between should be a minimum of double the lower wall height. In this example because the lower wall is 4’ high, the space in between these 2 walls would be 8’.
If the wall is designed by an engineer there may be additional support installed to lessen this distance, but without an engineer its best to stay at least double the height away from the lower wall.
Should I use filter fabric vertically behind my wall?
Filter fabric behind a wall is not recommended by Allan Block in most cases. The exception would be in a waterfront application where you would use the fabric to control erosion from the waves.
Filter fabric behind a normal wall in your yard is not recommended as it doesn’t provide any additional benefits, and in some cases can trap water which will create a weight surcharge behind your wall.
Should I use filter fabric horizontally on top of my wall?
Yes, you should use filter fabric at the 2nd row from the top. This helps prevent soil from working its way downwards into the clean free draining gravel behind your wall.
What is the best type of gravel to use for my retaining wall?
¾” clear gravel is the best gravel to use under, behind, and inside your Allan Block retaining wall.
¾” clear gravel is angular in nature, an average size of 3/4”, and contains no fine material. This is an excellent free draining material that will pass water through it instead of trapping it.
¾” clear gravel also achieves about 90% compaction simple by dumping it on the ground. This is very practical for using in areas where access is limited to hand compaction.