Frost heave is not water expanding
The common explanation for frost heave is that water expands nine percent on freezing and pushes the ground up. That is real but far too small to account for observed heave, which routinely reaches tens of centimetres. The actual mechanism is more interesting.
In fine-grained soil, a freezing front does not simply advance. Unfrozen water films persist around soil particles even below zero, and those films transport water toward the front by cryosuction — a pressure gradient created by the freezing itself. Water arrives, freezes onto the growing ice body, and the result is a discrete ice lens that thickens over time, lifting everything above it.
The soil type matters enormously. Coarse sand and gravel drain and have too little surface area to sustain the films, so they barely heave. Clay holds water but conducts it too slowly. Silt is the troublemaker: fine enough for strong cryosuction, permeable enough to feed it. Frost-susceptible soils in engineering practice are essentially silts.
This is why the standard fix is not insulation but substitution — remove the silt, backfill with clean gravel, and the mechanism has nothing to work with.