I believe you are right about the heaving. Our frost has been down over 6 feet here the last few winters and I noticed there are two slight banks by my driveway that look like drop offs in the early spring, I believe from me plowing the driveway and driving the frost down, the drive lifts. I have never seen anything like this before moving here. My pad lifted and slanted about 6 inches, returned to level in the spring.