Those Cracks on the Edge of the Road…What Causes Them?

I'm sure you have seen them. Long, arching cracks near the edge of the pavement. First there is one,then later another one. And before too long the edge of the road begins to ravel and deteriorate.

Have you ever wondered why they form? Why do they curve outward, toward the edge of the pavement? Why do you see them more often on rural roads than on city streets? I have often thought about those questions. I'm not sure I have it right, but I will share my conjecture with you.

I believe a combination of factors leads to edge cracking, not just a single factor. Let’s look at a couple of examples to see if we can figure out what might be happening.

The first photo illustrates the sort of cracks I'm referring to, and it gives us our first clues. The cracking here is fairly extensive, covering the entire outer wheel path. The curved nature of the cracks is not so easy to see here because transverse cracks have formed between the parallel arcs.

Photo of a pick-up driving along a road the edge of which is severely cracked

Clue #1

Note the very dirty material on the shoulder. It will not let the water get out of the base beneath the pavement edge after it enters through the cracks. This softens the sub grade and base under the edge of the pavement.

Clue #2

Note the shallow rut in the inner wheelpath. This suggests that the surface and base are weak (i.e., too thin).

Clue #3

Note also that the road is not very wide. The vehicle that is traveling away from us in the photo would need to shift closer to the edge of pavement to pass a vehicle coming in the opposite direction. This can pose a safety hazard.

Several more clues are evident in the second photo. Water is ponded on the shoulder near the pavement edge. The edge of the pavement has broken off and raveled away.

A white edge line is visible in the foreground. The pavement in the distance has been in place for more than 30 years, while the closer pavement has been there for less than 4 years. Nevertheless, the cracks are just as extensive in the newer pavement and minor edge raveling is evident.

The yellow centerline that is visible in the upper right corner of the photo shows that the lane width is relatively narrow.

Clue #4

The main crack seems to be a short distance in from the edge of pavement (red arrow).

Clue #5

The asphalt surface is fairly thin,probably only 1-1/2 inches thick. Transverse cracks(yellow arrow) form small blocks that are easily dislodged by traffic. The easiest blocks to get loose are those at the very outside edge of the pavement, hence the edge raveling.

Looking at all of these clues, here is what I think is going on.

Photo of road cracking pointing out details of cracking

Diagram of how roadsides fail and crack

The drawing below illustrates these points. The edge deflection under the wheel is greatly exaggerated.

What can you do to stop the progression of edge cracking? Three things should be done. The goal is to re-establish the pavement edge for the safety of the traveling public.

  1. Promote good drainage along the edge of the road. Make sure that surface water will run to the nearby ditch, and not pond along the pavement edge.
  2. Remove the dirty, poor draining shoulder material and replace it with a more permeable material. "Daylight" the material to the ditch so water in the base can get out.
  3. If you have a lot of truck traffic, place a substantial structural overlay on the road surface. You need three or four inches of hot mix to provide adequate support for today's heavier vehicle loads.

It would also help to widen the road if you have enough right-of-way to permit doing that. Eleven or twelve foot lanes are commonly used these days for traffic safety. We think of wider lanes being done for this reason only. However, there is also some added benefit in helping the longevity of the pavement by getting the heavy loads away from the edge of the road.

Remember, the cracked surface along the roadway edge is not much better than a granular base as far as its load support is concerned. So to keep from having those edge cracks come right back (as seen in the foreground in the second photo) you need to put a substantial thickness of new surface over the cracked and raveling material.

If the damage is as extensive as in the first photo, the best solution may be to remove and replace the surface and the base. Remove the cracked section, then provide a new gravel base that is clean and will drain well, improve the shoulder drainage, and put back the pavement thickness that will support the traffic. We realize that in most cases such extensive reconstruction simply will not be done. But the three remedial measures listed above will re-establish the pavement edge, which will make the road safer and smoother to drive on for a period of time.

Spring 2008

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This work by the Cornell Local Roads Program (CLRP) is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.