If you are digging a new garden in a grassed area, you will save yourself a lot of trouble if you deal to the grassy cover from the start. You can spray the grass with glyphosate to kill it. Wait three days after spraying before you dig. Alternatively, you can skim the top layer of turf off to a depth of 5cm and stack the layers upside down to rot down. This will give you fertile topsoil to spread back on the garden in due course. Or, if you are digging deep enough, you can spread the top layer at the bottom of the trench, preferably upside down, and cover it with soil to stop the grass from regrowing.
Cicely: When I see a spade, I call it a spade.
Gwendoline: I am glad to say that I have never seen a spade. It is obvious that our social spheres have been widely different.
Oscar Wilde, The Importance of Being Earnest (1895)