another soil fix question

Hello all. I just joined up.
Our 6yo house/yard is sitting on top of a former truck depot. The soil, i mean dirt is black and nothing but gravel. The back fill is only 6" deep at most near the house and only 3" deep at most points in the yard. What I've been doing once a month or so since moving in in '06 is getting bagged Scotts brand top soil and spreading & raking it in. It's not the cheapest way to do it but i can do a bit at time as funds permit. Lawn size is approx 20'x80'. Will this practice help me build my depth of soil? I'm worried that if the roots should ever reach below the back fill it would be like poison. I'm also working on building up the quality of the soil.
I'm transioning from Maryland Certified Tall Fescue sod to Meyer Zoysia as can be seen here:  http://s791.photobucket.com/albums/yy192/KBzPTGT/My%206yo%20Lawn%20in%20Western%20MD%2021740/
I each time i spread the bagged soil, i only do it at the edge of the spreading Zoysia in hopes of making it stronger to help over take the Fescue. I always mulch mow leaving the clippings and until now have never fertilized until just recently when I spread some Soybean meal & Milorganite.
Just now learning all this lawn/garden stuff.  Sorry for the long first post.

Thanks for raeding.
KB

I have a new yard with about 6 inches of dirt and I have tried a few different things. One that seems to work is sugar, the same type you cook with. I mixed in 1/2 cup of peper with 1 cup of sugar. The reason this works, as I read on different forums, is that it attracts the organisms and bugs so they can leave thier waste and enrich the soil or dirt. It makes sense and does seem to be working in some problem areas of my lawn. The peper is to prevent ants, I am still observing to see if that holds true.
SOmething eles yo might want to try is buy soem top soil, about 12 inches worth and cover the grass with it, then re-seed that area. The ides is that you are starting new, but like I said earlier I have tried the sugar thing in a coupel of areas just a s a test run.
Hope this helps.

i've heard of a similar method using molassas. I add a few bags of Scotts brand top soil every two weeks or so. It's not the cheapest method but it seems to be working. I just hope it stays around. Covering the grass is a no go as it's way to beautiful at this time.

I'd have a couple loads of wood chips dumped and seed them oyster mushroom spawn, give it a year to clean up the soil a bit before putting on a thin layer of topsoil and grass seeds, oyster mushrooms literally gobble up hydrocarbons and the mycelium should persist in your dirt for years.

call all the tree maintenance companies in town and put your name on their list for free dumping of wood chips, nothing they bring you will be worse than the oil contamination you have now