Chris, this is the best advice you'll get, ever!! This is exactly how I would do it, and it's almost to Swedish building code!! I wish I remember where I saw (didn't save the link), but it was a profile of how it should be done...
As Steve wrote, dig up, check and possibly repair (you could have more than a small crack), paint with bitumen, and do that to the whole sole, get some of the perforated tubing and lay on a gravel bed, and fill with gravel. If you want to make it realy right, dig a trench out from the building, and end the tubs there, in a stone bed...
I wish I could help more... What kind of earth is it around the garage? Gravel (unlikely, drains well), sand, mud, normal earth?
Kjelle