Grades 3–5 · Earth & Space
How does a pothole form?
A pothole is a place where pieces of a road’s surface have broken away. A road has a hard top layer supported by material below it. Water can enter small cracks in the top and spaces beneath. This starts one common pathway to a pothole during cold weather.
In the diagramWater and traffic can break a weakened road surfaceOne cold-weather pathway to a potholeWaterseeps inFreezingIceexpandsThawingSupportweakensTrafficPiecesloosenThawed, wet material may support the road less well.Other potholes form without freezing; timing is not predicted.
When water freezes, the ice takes up more space. Ice growing in cracks and wet ground can push surrounding material apart. When the ice thaws, the material beneath the road may be very wet and less able to support the surface. Repeated freezing and thawing can add to the damage.
Wheels press on the road each time a vehicle passes. A weakened surface can bend, crack and break into loose pieces. As traffic displaces those pieces, a hole appears and may grow. The process depends on water, the road materials, weather and traffic, so it has no single schedule. Potholes can also form without freezing when water and repeated loads weaken a road.
About this explanation
Starting knowledge: Know that water can freeze into ice and thaw back into liquid.
Sources
- Minnesota Department of Transportation — Potholes: A recipe for potholes
- Federal Highway Administration — Geotechnical aspects of pavements: Potholes
- U.S. Geological Survey — Water Density