It is currently raining off and on quite heavy and my area and when I’m out for a drive at the moment I am finding many potholes that weren’t there a couple of days ago. Wondering why.
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Water is the biggest threat to pavements for a few reasons. One of them is that crushed stone pavements get their strength from the angular faces on the stones locking together in a mesh. When you apply force to the top of the pavement, the stones locks tightly to the ones underneath spreading the load out. If you build it deep enough, then the force is spread wide enough not to damage the ground underneath.
The problem is that when you saturate the pavement, the force is applied to the water as well as the stones – the problem is that unlike the stone mesh which only loads down, water pressure acts in all directions. This means you have a pressure between each individual stone pushing them away from each other so they don’t lock anymore, instead they can slide freely against each other. This effect can also act between rigid layers pushing them away from each other causing them to separate from each other. Both are very bad for the pavement for hopefully obvious reasons.
This is why good road design almost always tries to prevent water getting into the pavement, both by waterproofing the top and with drains at the side. If it does get in, the aim is to get rid of the water as soon as possible. When it rains, especially if there is any kind of localised flooding or if it rains a lot without the ground drying out in between, it exposes any weaknesses in the waterproofing and drainage by quickly destroying the pavement. The other reasons people have given you about poor pavement construction or soft subgrade (the ground under the pavement) etc are all good reasons but water where it shouldn’t be is why it is particularly bad when it rains
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