James;

Frost heave occurs only when there is a change in volume of the underlying
soil when it freezes. As dry soil doesn't expand or contract when it
freezes the problem occurs in the presence of water. If you can keep the
supporting soil dry it won't heave in the winter. Thus you can prevent
frost heave by constructing a good under-drainage system for your steps.
Use a deep gravel base, 6 to 12 inches deep, to allow water on the slope to
drain downwards out of the soil. Also use a good method to drain water out
of the soil at the bottom of the steps. Since your frost penetration goes
down 2 ft you need to keep the top 2 ft of soil dry so it won't heave.

Frost penetrates a lot deeper in dry soil than in wet soil. Thus it can
become a vicious circle - the dryer you make it the deeper the frost goes,
the deeper you have to keep it dry, etc. You can address this by good
surface drainage. If water doesn't sit around near the steps the underlying
soils will be dry and won't heave. The shallow gravel addresses the short
term rainfalls and snowmelts and the surface drainage addresses the long-
term wetness of the area and thus the deeper soils.