Corrugated Roads

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CORRUGATED ROADS
Revisited in 3 parts: Karl S. Kruszelnicki

[This is a really excellent paper by an excellent Man. Ed.]

If you have ever driven the length of the Gibb River Road in the Kimberley, you would have had many hours to think about corrugations. Way back in 1994, I wrote a column in a 4WD magazine about road corrugations. I described the 1962 experiments of Keith B. Mather from the University of Melbourne, who did some clever experiments and came up with a theory about how corrugations formed.
Since then, many people have contacted me about their experiences and theories about corrugations. Some of them have spent several decades driving on corrugations every day. A few have spent years doing experiments on corrugations, and have even published booklets about them.
Over the years, one thing I have discovered in trying to understand natural phenomena is that there are usually a few explanations - not just one single explanation. When I wrote my story about corrugations, I thought that Mather had solved the problem of how they form. But now that so many people have corresponded with me, and I have also read more widely, I think that his explanation is a small part of the answer. In fact, I strongly suspect that we (ie, road engineers) don't fully understand corrugations!
Now the first thing to realise is that Outback dirt roads are not the only surfaces that get corrugations. You can see corrugations on bitumen and even concrete roads - usually after some kind of break in the road surface. You can also see corrugations on steel railroad lines. The train can make so much noise going over these corrugations, that railroad people call these sections of track "roaring rails".
There are corrugations on the overhead metal rails that feed power to electric trains and trams. These vehicles have a "pantograph" on top – to carry the electricity between the overhead rails and the train. These overhead rails have tiny corrugations with tiny valleys and hills. The little valleys are where the pantograph broke contact from the overhead rail, and set up an electric arc that vaporised away some of the metal.
Skiers in the snow country often find rough corrugated washboard patterns on a well-travelled ski trail. And if you ever have replaced noisy wheel bearings, you'll see regular wear patterns (= corrugations) on the rollers, balls or track.
Keith B. Mather did an experiment to try to understand why roads get corrugations. He put a small wheel on a metre-long arm, and then drove it with a variable-speed electric motor around a circular concrete track. He covered the track with different materials - fine-sifted sand, different coarse sands, and various dry products like gravel, rice grains, and even split peas and sugar! He found that provided that these materials were dry and not stuck together, he could always generate corrugations.
He found that corrugations formed rapidly with hard tyres, but slowly with soft tyres. The corrugations were produced very easily in dry materials, but very slowly, if at all, in wet sand. It made no difference to the corrugations if the wheel was a driving wheel, or a driven (or rolling or idling) wheel. In his experiments, the major factor in making corrugations was the road speed of the vehicle.
So here is Mather's theory of corrugations.
It's based on the fact that you can never make a road perfectly smooth. There will always be tiny little bumps. Once his wheel got up to about 6-7 kph, it would bounce up when it hit a tiny bump. As the wheel came down and hit the sand, it would spray sand both forwards and sideways off the track, leaving behind a little crater. This crater would then be the valley of a corrugation. As the wheel came up out of the valley, it would jump into the air again, and so the pattern of valley-and-mountain would repeat itself.
Making corrugations is a two-stage process - first the corrugations establish a stable pattern, and then they spread along the road.
Mather saw that the first few corrugations to appear on the "smooth" road were quite shallow, and very close to each other. But as the corrugations got deeper, they gradually moved away from each other, until their height and their distance apart had settled into a stable pattern. Once this stable pattern of corrugations was set up, then the entire pattern of corrugation would migrate down the road in the direction of travel of the wheel. In the Australian Outback, engineers have seen corrugations heading in opposite directions on each side of the road from (say) a cattle grid, with each set heading in the direction of travel of the cars.
How come all the corrugations look the same, if the cars are all different?
Most of the vehicles travel around the same speed. So, a bump on the road that makes one car's wheels bounce, will also make any other cars' wheels bounce. These bouncing wheels will all tend to land at the same point. And that's how the corrugations form.
Of course, the faster the road traffic, the further apart are the corrugations. French engineers working in the fast and flat desert roads of North Africa found corrugations that were about one metre apart. Mather found that Australian corrugations in Australia were about 0.75 metres apart.
And that seemed to be the perfect explanation - until the mail/phone calls/emails started flooding in!
The other explanations for corrugations involve engine resonance, wind (either natural or created by the moving vehicle), wheel hop, braking, acceleration, the road itself shrinking as it dries out, the wave of dirt pushed in front of the tyre, and the influence of shock absorbers.
So I will discuss these other theories of Road Corrugations next time...

© Karl S. Kruszelnicki Pty Ltd 2001. http://www.abc.net.au/science/k2/trek/s315126.htm

Corrugated Roads Revisited 2

Any time you hit a dirt road, you hit corrugations. And you ask, “How did they get here?” The answer to this question is a good example of Good Science. Science is not a Bunch of Facts.
Science is a process that brings you closer to the Truth.
Eight years ago, I wrote a story in a 4WD magazine which tried to explain how corrugations form. And, over the last 8 years, the readers of this column have confronted me with a whole bunch of different explanations. Most of them seem to have an element of truth in them.
Let me introduce you to a few of them.
One reader, CT, thought that I had not considered all the possible causes. He wrote, “I suspect that road corrugations are a product of not only suspension tension, but also wheel diameter, and most importantly vehicle weight and wheel base. My argument for this is. Whilst touring the Northern Territory, I found a dirt road that only large semi-trailer and road train (2 and 3 trailer trucks) were allowed to use. Being a law-abiding citizen, I only followed this road for a few kilometres, because it was utterly devoid of any corrugations. It was, and still is, the smoothest road I have ever been on. The Territory has no speed limits, so I tried this road up to 140 kph. The only noise was that of the engine. I was in a 1990 Toyota 4WD with stiff suspension, and the ride was amazingly smooth and quiet.
I believe this dirt road was ironed out by these 42-tonne trucks due to their un-even wheelbase, and vast weight. Whereas a light 1 tonne car will tend to bounce around on any bumps, a truck will tend to smooth out the road.”
I was impressed by his ultra-smooth road, and by his explanation involving vehicle weight. I read it out on my Triple J radio Science Talkback show, and received this reply from DQ. DQ concentrated more on the effects of the suspension.
“It is unlikely that trucks travelling on the road ‘flatten out’ corrugations - the odds of there being enough trucks to flatten out all the corrugations are simply too high. Rather, I suspect the explanation lies in the nature of the suspension of the vehicles travelling the road.
Because of their light weight, cars bounce about a fair bit. If a smooth ride is to be achieved, the suspension must push the wheels of the car back down onto the road. The force of the suspension pushing down (along with the weight of the car pushing down via gravity) causes dirt to fly out, and hence corrugations are caused (as you explained).
Further, if corrugations already exist on a road, cars driving over them cause the corrugations to deepen even further. Each time the car hits the peak of the corrugation, the shock absorbers compress. When the trough of the corrugation arrives the shockie can decompress into the gap created by the trough.
However, the force of the shockie decompressing causes more dirt to fly out, thereby deepening the trough.
That ‘corrugations-are-linked-to-the-shockies’ would also explain why corrugations are equidistant apart on any one stretch of a particular road, and on different roads. The ‘throw’ of car shock absorbers, time to compress and decompress, etc would be pretty similar on most cars. Cars also tend to travel at pretty similar speeds on country roads. Hence, each car’s shockies would be applying similar forces in similar places on roads, causing equidistant corrugations.
I suspect that trucks don’t move about on the road as much as light vehicles (compare rolling a tennis ball over a rod, and rolling a ten pin ball over the same rod - the lighter one bounces much higher). As a result truck suspensions don’t need to be very active, as the trucks simply don’t rise into the air like cars do. Hence the downward force of the shockies is negligible. Hence no corrugations.”
This explanation also sounded reasonable, apart from a minor quibble about “shock absorbers”. Shock absorbers don’t absorb shock - the springs do. Shock absorbers damp down the shock, so that the springs stop bouncing after a bounce or two. Even so, in most 4WDs, the combination of shockie-and-spring has roughly the same natural bounce frequency. If you replace the words “shock absorbers” by “springs”, his explanation sounded reasonable.
Peter Wood agreed that “natural resonances” were probably the cause, and suggested another resonance that I had not considered.
“The piston/conrod-to-crankshaft configuration of the internal combustion engine powering most road vehicles creates a pulsing force which is transmitted through the tyres to the unsealed road surface. The variation in tractive force varies the erosion of the surface, with the ‘spurt’ of force on each power stroke excavating a small amount of material and piling it up behind. Once the corrugation starts to form, subsequent vehicle passes reinforce the peaks and troughs of the corrugations, assisted by the characteristics of the response of the vehicle suspension components.”
Personally, I would agree that there is a vibration transmitted to the road from the engine, but that the effect of this vibration would be so small as to be insignificant.
But Peter Wood also threw in a very interesting comment.
“Corrugations are most noticeable where heavy breaking and acceleration occurs, subject of course to the degree of erodability of the pavement material.”
DT agreed. He wrote that “Road corrugations also seem to be worst in positions when cars are accelerating up a hill - eg, after a sharp bend. I assume this is from the wheels being more prone to skip in this situation.”
Well, these are all interesting theories, but to advance knowledge, you need experiments - and next time, I’ll describe how the Turnbull family did experiments on their 3-kilometre airstrip with a 4WD, a 2WD and a semi-trailer....

© Karl S. Kruszelnicki Pty Ltd 2001. http://www.abc.net.au/science/k2/trek/s315128.htm

Corrugated Roads Part 3

Over the last few columns, I’ve been working through various reasonable theories about what causes corrugations in the road. I’ve paid attention to these theories, because they have been devised by people who drive on corrugated roads every day, and have made keen observations that eventually led to a theory.
These theories include the tiny road bumps making the wheel bounce (with a nod to the effects of suspension, vehicle weight and wheelbase), the vibrations from the engine (didn’t like that theory much), and the effect of hard acceleration. But I was especially impressed by the Turnbull Family, who did experiments on their 3-kilometre airstrip with a 4WD, a 2WD and a semi-trailer (nope, they didn’t test a bulldozer).
They ran each vehicle 10 times along their airstrip. They put each vehicle through acceleration, cruise, and slow breaking.
Why did they do this? Well, they said, “... number one son Cory chose a science project to do on holidays (he boards at Toowoomba Grammar School)...”
Cory made some good scientific observations:
1: The 2WD had more corrugations
2: Most corrugation occurred at acceleration (all vehicles)
3: 2nd most corrugations occurred at braking (a) Corrugation increased with traffic; (b) Size of corrugations was in proportion to tyre diameter (semi, 4WD).
So young Cory came up with this conclusion:
Corrugations are not caused by small vehicles
2: Large vehicles do not smooth corrugations out
3: We believe wheel slip causes corrugations, and drive wheels do the damage.
Sam Blaxland basically agreed with Cory about the effects of vehicle braking. He emailed, “My parents live in a very hilly part of the US, with a sandy topsoil over a solid rock base (one of the few places where I have seen houses that come equipped with pumps to remove radon gas). I noticed that the corrugations on the road surfaces were in areas where traffic tends to brake for lights. Now if we imagine the road surface to be akin to a carpet, and the sandy topsoil like a highly polished wood floor, as the car brakes, the road surface will gather in front. Obviously this will happen over longer amounts of time, but the rate would also be accelerated (excuse the pun) by hot weather softening the tarmac.”
Mr. C. had a theory that completely ignored the effects of the wheels and suspension. He emailed, “I believe that corrugations along dirt roads are caused by drying and shrinking action which comes about due to poor pavement gravels. Have you ever noticed corrugations when it has been raining strongly? I haven’t and I live on a corrugated dirt road. Evidence to support this also comes from AUSTROADS, A Guide To The Visual Assessment Of Pavement Condition.” So his theory blamed the poor quality gravels, which led to a drying and shrinking action on the road.
Danny Burkitt also ignored the wheels and suspension, and the gravel. He blamed the wind. This wind could be from nature, or from a moving car.
“... think of the dirt on the roads, as similar to the sand on a beach. The sand dunes are formed by the wind blowing the smaller particles of the sand matrix across the beach, some of which are rolling, some of which are flying through the air.
The rolling particles keep rolling until they reach something that will stop them rolling, either: (a) The plants located at the top of the beach (b) Simply the slope of the beach has a gradient that exceeds the force of the wind on the particle, (c) As the particle is rolling across the sand surface, there are small valleys and hills between other sand particles that are resting on the beach.
A rolling sand particle may fall into a valley and not be able to roll up over the next hill.
When the wind initially starts blowing, there is a range of particle sizes that are moved. The heavier particles will “drop” out first for any of the reasons described above. This in turn causes a small “dune” to start to form. The next time the wind blows, some of the particles in the next blanket of sand will be caught in these new small dunes, which will in turn increase the size of the sand dune. This process will continue until a sand dune, as you would see on any beach now, is formed.
... corrugations on a dirt road are simply small sand dunes. The action of a car driving over the road at speed causes wind to blow across the dirt particles, just as on the beach, and relocation of the dirt particles occurs to form small indentations in the road. When the next car drives over the road, the dirt particles that are blown by the car generated wind may be subject to the action described in (c) above. The process will continue and form small “dirt dunes” ie. corrugations in the road.”
Lloyd Junor, who has spent many years travelling the Outback, agreed. He also pointed out that, “Corrugated roads have the corrugations road-wide, not precisely where the vehicles’ wheels traffic them. So traffic does not explain the corrugations.
The wind is the culprit for corrugated roads. (Admittedly helped by traffic, which may loosen some surface materials.) The wind moves the looser earth, piles it up, and lo and behold, an untrafficked road is corrugated without ever having seen a vehicle since being graded.
An item you may care to alert your listeners to: the wind builds the corrugations in the shape of an inverted saw-tooth pattern, the steep ramp being on the lee side. (Dunes are built the same way). Vehicle traffic can accentuate this shaping. That‘s why at times (when out in the scrub, and no-one around for ever, therefore safe to do so) an Outback driver will go to the opposite side of the road to experience a less jarring ride.”
I have been given a lot of theories. They all seem to have an element of truth in them. I reckon that the only real way to pick between them is to do experiments to test them.
There is a real benefit to doing this testing. Road corrugations cost councils lots of money to smooth out. Some Outback councils cover a very large area, and have a very small population and tax base. So any saving in money would be very significant.

© Karl S. Kruszelnicki Pty Ltd 2001. http://www.abc.net.au/science/k2/trek/s315129.htm

See also: http://www.abc.net.au/science/drkarl/default.htm
See also: http://perso.ens-lyon.fr/nicolas.taberlet/washboard/
See also: http://blog.thoughtwax.com/2008/01/the-enduring-mystery-of-corrugated-roads/
See also: http://www.scribd.com/doc/73083060/A-Guide-to-the-Visual-Assessement-of-Flexible-Pavement-Surface-Conditions-JKR-20709-2060-92

Submitted by Ian Johnson @) October 2014