Does a retaining wall need to be set on top of a base of crushed stone?

I am building a 2-1/2 ft high retaining wall of 64 lb Oldcastle Rampart concrete blocks. Do I need to set it on a base of crushed stone or can I set it directly on top of the earth? How many inches of crushed stone or gravel would I need directly behind the wall? Is gravel okay or does it have to be crushed stone?

Answers:
Straight on top of the earth mate.
Just make sure its level first.
You can do this with a string from one end of your wall to th other.

Other answers:
Well compacted of crushed stone will be better than weak earth. About 20 cm of gravel/crushed stone/any high permeability material will do enough drainage behind the wall. Just don't forget to insert several pipe to drain rain water from behind the wall.
Well compacted of crushed stone will be better than weak earth. About 20 cm of gravel/crushed stone/any high permeability material will do enough drainage behind the wall. Just don't forget to insert several pipe to drain rain water from behind the wall.
Use compacted crushed stone for your base. Don't forget to put in drainage pipe and back fill with 2B gravel, 24" deep and 2' back from the wall. this will ensure that you have proper drainage and your wall will not push out on you.
Yes, definately use a stone base!! 2-A modified works pretty good!! Generally you need AT LEAST 2 feet deep of 2-B gravel behind the wall!! You also need to go at least 2 foot high with the gravel allowing for 1/2 foot of coverage. REMEMBER TO PUT IN A DRAIN PIPE! I would use 4" corrugated pipe, works pretty good!!
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