Potholes are caused when moisture gets into the cracks in the road which expands when it freezes.
The holes get bigger as vehicles drive over them damaging the structure of the road below its surface layer.
Potholes are always a fairly round shape because they stem outwards from the area of the least resistance (the initial crack).
This means when damage continues and a pothole grows it acts like a hole where the ground falls into the area of the least resistance and shortest path from the weakest point (the existing crack) - which creates circle.
If it was a square then damage from the corners would have to travel further than the sides of a circle.
Adverse weather conditions and repeated freeze-thaw and wet-dry cycles make the pothole situation much worse.
However, it’s not just the weather that is to blame for the problem.
The RAC believes the rise in potholes across the UK has stemmed from many cash-strapped councils’ reactive ‘patch and dash’ approach which means rather than resurfacing roads properly, potholes are repaired individually in a hurry, and sometimes in wet weather, leading to them quickly breaking down and reappearing.
Planned preventative road maintenance would, in the longer-term, be a more efficient and cost-effective way of dealing with the issue.
Original Source - RAC https://www.rac.co.uk/drive/advice/road-safety/report-a-pothole/