I have heavy, clay, adobe brick soil. And I have a bit of experience
with forks. Even if you are able to sink it into the ground that 16",
you won't be able to turn the soil or lift it without Superman helping.
I've had luck working at a depth of a few inches, turning it, chopping
up the soil and then prying up a few more inches of soil. It helps if
you soften up the dirt with a good soak a day or two before. -And
warm-up exercises for you!
Spend your money on a nice sturdy fork with a handle at the end you can
really turn without breaking it off - or you could rent a tiller for
that.
Mix in lots of organic matter to keep the soil from turning back into a
brick. Good luck!