Depth of Soil to allow a turf to root and grow

Afternoon folks.
I was reading through the discussion topics with great interest after coming accross a slight problem with my current outdoor project.
I just ripped up a concrete patio and have been preparing the area for laying turves.
I started digging up the soil and came accross a poured concrete foundation of a wall that was previously there which I have painstakingly hammered into and reduced the height of it so that it will be at least 20 - 30 cm below the grass level.
I did also come accross a large quarry of builders sand that must have been kindly left for me to discover. It's approx 2 to 2 1/2 ft down. Is that far enough not to degrade my turves growing capabilities or should I get down and just dig it all out ?

cheers in advance

 

How wide and how long is the foundation?

If this foundation is just a narrow strip then 20-30cm down will be fine. The only thing it may do will be to absorb moisture from the surrounding soil which would mean in times of drought you may see the grass above it die back at a faster rate than the rest of the lawn (if you don't water it.) Every year the outline of an old ice house appears in one of my customer's lawns and the tops of the walls are approx 50cm underground. Makes a nice talking point!

 

Axl - Now that is smart thinking - making a problem into an oportunity

 

Many thanks for the replies folks. Glad to see that the worst of the back breaking work has been done, although from what Axl is saying, I am wondering about burying that concrete that I removed below the surface of the soil to spell out the word "Hi" for my neighbours to read if we ever experience any drought conditions up here!