Does rain cause Tarmac / asphalt pot holes? If so, how?

After a period of rain in my city there are plenty of fresh new pot holes in the Tarmac / asphalt surfaces. The immediate cause is obviously axle overloading, due to appalling regulations, but these holes are fresh from a fourtnights heavy rain.

I can comprehend erosion in dirt or gravel roads as I know these substances are penetrable by water. But my impression is that Tarmac is impenetrable to water.

What process am I missing?

  • 3
    $\begingroup$ Just a hunch: look for asphalt concrete cracking and then a similar washing out effect as in other types of roads. $\endgroup$
    – gschenk
    Commented Oct 14, 2018 at 8:59
  • 1
    $\begingroup$ It's both crack potholes and circular "rip up" overweight potholes. $\endgroup$
    – Samuel Russell
    Commented Oct 14, 2018 at 9:01
  • $\begingroup$ I had assumed that pot holes in asphalt surfaces is caused by hydraulic pressure from tires rolling over small cracks in the surface that force water underneath the asphalt that then lifts said asphalt. The pressure generated by an automobile tire can be several times the ordinary pressure due to a force being applied to a small area, much like how a brake cylinder works. $\endgroup$
    – BillDOe
    Commented Oct 26, 2018 at 19:54
  • $\begingroup$ It should be noted that if the water in small cracks (and larger ones) freezes it will expand and split the asphalt apart. This is by far the most damaging weather effect in climates where winter occurs. $\endgroup$
    – Hot Licks
    Commented Oct 26, 2018 at 20:11

1 Answer 1


old tarmac has cracks in it which can admit water under the material, softening the earth underneath. when a vehicle rolls over the cracked material, it yields because the softened earth fails to support it. in yielding, the cracks open up progressively and the tarmac develops a bowl-shape which collects water, which then drains through the cracks, softening still more earth, etc. etc. and in no time you have a pothole.

  • $\begingroup$ Is this process similar for roads with concrete slab under-roads? $\endgroup$ Commented Oct 27, 2018 at 0:03
  • 1
    $\begingroup$ the dynamics are different: concrete roadway slabs are reinforced with steel and they do not sag; instead they crack and fracture. when water gets under them, the softened earth allows the fracture angles to increase and the fracturing spreads. Flexure at the crack joints caused by rolling tires shatters the concrete in the vicinity of the cracks and the tires tend to pluck the concrete shards out of the roadway. when the cracks have thus opened up to a width of an inch or more, the road has to be repaired. $\endgroup$ Commented Oct 27, 2018 at 5:49
  • $\begingroup$ Oh, sorry I mean asphalt over concrete slab under beds. $\endgroup$ Commented Oct 27, 2018 at 5:55
  • 1
    $\begingroup$ sorry, do not know! $\endgroup$ Commented Oct 27, 2018 at 6:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.