Naked Science Forum

General Science => General Science => Topic started by: Karen W. on 29/08/2008 14:07:15

Title: What is the actual cause of potholes in a dry gravel Road?
Post by: Karen W. on 29/08/2008 14:07:15

I live off a main road unpaved Loose gravel with years of packed gravel loads added to fill pot holes etc..

We have this very nice young man who takes a small tractor and drags the road regularly to redistribute the gravel back in place so as to keep the road smooth as possible.

Now this morning at 4:30AM I drove Bob to work as I was doing so I 3was admiring the gravel job the young man had done.. I was tooling along at about 5 miles an hour .. Thats the neighborhood speed preference so people don't get dusted out since gravel is so dusty! Anyway I started thinking.. hmmmmm when I tulle along so slow the gravel seems to stay in place nicely.. when I start and drive say 10 miles an hour that is nice too, I noticed that it was when I increased speeds that, that was when my tires kicked out gravel.. and I was not gunning it.. but once I established that higher speed the car seemed to stop spitting gravel... so my question is:


1: Is it the constant rolling of tires which initially start a pot hole?

2: Or is it the actual starting up and stopping that causes the start of holes?

3: Or is it changes in speed which require more traction , that is starting the holes in the road....?

I am really curious, as when it is dry and not raining, and people maintain a speed and not fluctuate too much the road seems to stay nice for quite a while.. I know the rain makes this worse so Please do not consider the winter season.. only hot dry weather... for this question...

What says, YOU experts of gravely Knowledge and calculations?

Title: What is the actual cause of potholes in a dry gravel Road?
Post by: LeeE on 30/08/2008 14:19:08

As a tyre, supporting a load, rolls along the ground it flattens at the area of contact with the road surface.  This distorts the tyre, which being made out of rubber is elastic, and stores some energy in it, like a spring.  As that area of the tyre is then rolled off the road surface it has to release that energy when it returns to it's original shape and if it does so with enough force it can pull the ground beneath it apart a little bit, splitting it and breaking up the surface.  If the tyre is rolling fast enough, the energy released as the distorted region rolls off the road surface can be enough to actually kick bits of the surface that have split off up in to the air and redistribute them.

It is this same distortion process that cause most of the wear on the tyre, along with a lot of the noise, in addition to sapping power in the form of rolling resistance (the more a tyre is distorted while rolling, the more energy that is lost to rolling resistance).

Title: What is the actual cause of potholes in a dry gravel Road?
Post by: AllenG on 30/08/2008 18:46:21

I keep our dirt road scrapped.
I've noticed that the pot holes reappear in the exact same places, even if I fill them with fresh gravel.

Title: What is the actual cause of potholes in a dry gravel Road?
Post by: lyner on 30/08/2008 19:55:51

I understood it is to do with wave formation. Your tyre forms a wave on the surface. If you're fast enough the wave builds up and breaks up the surface. Slow enough and the wave dissipates. That's why fast trucks damage roads.

Title: What is the actual cause of potholes in a dry gravel Road?
Post by: LeeE on 31/08/2008 14:27:47

That probably is a factor too.