Late in the movie, Red gives us the most on-the-nose instance of symbolism we could reasonably hope for:
RED: Geology is the study of pressure and time. That's all it takes, really. Pressure…and time.
Obviously, he's not just talking about rocks here. Andy, who was a self-professed rock hound, took twenty years to build his tunnel, and was spurred on by the various pressures of Shawshank; the pressure of needing eyes in the back of his head to watch out for Bogs, the pressure of walking a tightrope with the guards, the pressure of wanting freedom so badly and not being able to grasp it, of having to be submissive to a warden he despises.
Pressure and time. If it's good enough for rocks, it's good enough for Andy.