[mumofstig]

Part of my garden, a strip beside the greenhouse, is clay overlaid with a layer of pebbles, like a beach 
I'd like to build a raised bed on top, to grow mostly salad, and wonder what depth of extra soil/compost I could get away with adding? Obviously I'd have to buy in the soil/compost!
I don't know if the pebbles are an advantage with drainage, or if they'll stop any roots from penetrating 
Has anybody had similar experience?