Two tier retaining wall

[coolbrze]

We're going to build a sm. two tier planter retaining wall using EP Henry Coventry Garden wall stone (link below) and was hoping for some help from you experts. The highest point will be ~ 18". How much 21A should we use as the base? What else will we need for the project?

http://www.ephenry.com/wall-systems/coventry-garden-wall.aspx

[DVS Hardscaper]

A few wheel-burr loads Otta-dew-er....
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[zedosix]

I figure you mean how much depth of granular you will need. Typically 8" is good, but soil conditions and ground disturbance are a couple of factors that could be important to consider.