There are a couple of important things to consider with retaining walls.
Foundations are critical. Weak foundations = weak wall. It's no good just compacting road base on top of the old soil. It needs to be sitting on a good base of clay, so by the time you've dug down to the clay, you're better off with a concrete footing, and TBH that is the only thing i would put under a retaining wall.
Second critical point is drainage. Hydrostatic pressure is one of the biggest reasons a wall will fail. A new aggie pipe with good coverage of rock to drain water away from the wall wouldn't go astray.
As others have said, the first course of these interlocking blocks is critical. Needs to be dead level. Nothing worse than seeing retaining wall with waves in it!
The interlocking blocks will do the job to a certain height (found in the manufacturers specs), above that they would need to have steel and concrete in them. Every retaining will I've ever built has always had N16 steel vertical and horizontal and corefilled with conc. It will last forever if built right!
Mate, if you want me have a squiz sometime, just shoot me a PM. I'd be more than happy to have a look and give you some ideas.
Matt
