Freeze and thaw
Why does a parking lot heave in the spring?
Water in the ground under a lot freezes into layers of ice that lift the pavement, and the lot drops unevenly when those layers melt in spring.
The numbers
- Frost drives down from the surface through the winter and lifts whatever is above it.
- The lift is uneven because the ground below is not uniformly wet or uniformly drained.
- The damage mostly appears on the way back down, during the thaw.
- Heaving concentrates where water collects: low spots, curb lines and around drains.
What is happening under the surface
Frost works its way down through the winter. Where the ground under a lot holds water, that water freezes into layers of ice, and each layer takes up more room than the water did. Everything above it goes up.
Nothing under a parking lot is uniform. One area drains, another sits wet from a downspout, a trench from a utility repair holds more water than the ground around it. So the lift is uneven, which is why a lot that was flat in November has ridges and dips in March.
Why the thaw does the real damage
Coming back down is worse than going up. The surface thaws first while the ground below is still frozen, so the meltwater has nowhere to drain to and the base turns soft. A soft base under a hard surface with traffic on it is how a spring pothole appears in a week.
That is also why a lot looks its worst in April and why the same lot is worth walking then. What shows up in the thaw is a map of where water is getting in.
What the fix has to deal with
Heaving is driven by water, so the four things worth checking are all about where the water goes: catch basins that have sunk or blocked, low areas that pond, downspouts running onto the lot, and open cracks feeding the base.
What the fix actually is depends on which of those it turns out to be, and on what is under the surface where it moved. That comes out of an inspection rather than off a photograph, so walk the lot with somebody during the thaw while the evidence is still on the ground.
Patching a spring dip without dealing with the water that made it buys you one season.