Freeze and thaw
Why do potholes form?
A pothole is what is left after water gets under the asphalt, freezes and lifts it, and traffic breaks the unsupported piece out.
The numbers
- A pothole starts as a crack. Water is the only way under a sound surface.
- Most of them appear in late winter and early spring, during the thaw.
- They grow at the edges, because every wheel hits an unsupported lip.
- Filling one without cutting out the failed base underneath is a repair that comes back.
How one gets started
Water finds a crack and reaches the compacted stone under the asphalt. It freezes there and lifts the asphalt above it, which is now a lid over an empty space. Then it thaws, the water drains or gets pumped out by traffic, and the lid has nothing holding it up.
The next few vehicles break it, and the pieces get thrown out by tires. That is the whole mechanism, and it is why potholes show up in a rush in the spring rather than gradually through the year.
Why it keeps growing
The edges of a pothole are asphalt with no support under them. Every wheel that drops in hits that lip and breaks a bit more off. A hole the size of a dinner plate in March is the size of a trash can lid by June if traffic keeps running over it.
Water also now has a direct route into the base, so the failed area underneath keeps spreading outward beyond the hole you can see.
Why a shovel of cold patch does not hold
Throwing material in fills the hole you can see and leaves the failed base and the broken edges exactly as they were. There is no clean face to bond to, so it is a plug rather than a repair, and the first freeze pushes it out.
A repair that holds means cutting a clean square through sound asphalt, taking out everything that failed, rebuilding and compacting the base, and sealing the seams so water cannot get back into the joint.