I have a similar situation, an approximate 30' long double driveway on a 15% slope. The driveway was poured about 40 years ago and over the next 10 years it slid toward the street such that the first expansion joint opened from about 3/8" to about 2". The only thing that seems to be stopping it from sliding further is the paved street. But if you look closely, the street adjacent to the curb is deforming. So far I've done nothing other than fill the expansion joint gap. I'm not really sure what would stop it. My Father always maintained that the earth is always moving. If you look at leaning retaining walls and out of level A/C unit pads, I think my Father was right.