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Should Heavy Trucks drive more slowly on weak (bitumen) roads to reduce potholes in the road surface?

I am discussing with a feasibility engineer what causes more potholes. He says it is simply the 'Weight' on the road (ie of the vehicle touching the road), and I argue that it is the acceleration AND weight combined...(Perhaps the Work to be a little more exact... Something that relates to acceleration/direction, mass and time). My thoughts are, that a truck travelling fast will bounce and produce more short term weight, and also accelerating back and forth will put more pressure on the road surface - more than the truck weighs in fact, all combining to create potholes or enlarge existing potholes.

(What 'he' means by Weight Only - is that they can predict how a road will wear down simply by a factor of the Weight of the trucks that will drive on it. They do not consider the speed (limit), and I also have no real idea of how accurate they are but it seems roads wear out a little faster than they think)

Can anyone explain how I am wrong? Or why the feasibility engineer is correct? (Or please let me know if this question is not in the correct forum, for which I apologise in advance)

NB: We have forestry trucks running on our small road, which 'seems' to be tearing it up - and they drive fast! Faster than the 30 km/h recommended limit (legal limit 50...People, dogs, kids, deer, moose, bikes around). So I would like to argue that the forestry trucks should drive super slowly to preserve our precious roads, which are ostensibly for local traffic only. Not that I mind the trucks on the roads every now and then, I just want them to reduce impact. The roads are probably only 'made' for local car traffic, and the odd tractor.

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migrated from mechanics.stackexchange.com Oct 25 '17 at 20:46

This question came from our site for mechanics and DIY enthusiast owners of cars, trucks, and motorcycles.

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    $\begingroup$ Although interesting, and I have a few thoughts on it, I do have to say that the question really has nothing to do with "motor vehicle maintenance". Maybe if you were talking about suspension wear based on vehicle speed vs. cargo weight... Physics.SE is the only place I can think to point you, but that might not be the best fit. $\endgroup$ – JPhi1618 Oct 25 '17 at 19:32
  • $\begingroup$ Hi, yes I know the question could be unrelated...But I guess we all encounter potholes - and they are an issue for every vehicle $\endgroup$ – Grantly Oct 25 '17 at 19:34
  • $\begingroup$ Does my edit make it more relevant? Trying to reduce maintenance on the roads that we all use? $\endgroup$ – Grantly Oct 25 '17 at 19:37
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    $\begingroup$ Well, it was a valiant attempt, but "driving technique" questions are also off topic. At any rate, it takes more than one vote to close a question, so the community will decide. I'm interested in the topic, even if its not a good fit here. $\endgroup$ – JPhi1618 Oct 25 '17 at 19:40
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    $\begingroup$ biggest cause : cheap construction practises - the Romans made better roads - some have lasted 2000 years... perhaps the thickness of the final layer is a major contributing factor. We have just had our shared drive done, and were offered 3 different grades of construction basically 5 yr life, 10 yr and 20 years - we opted for 20 years as the cost was not too much different. $\endgroup$ – Solar Mike Oct 25 '17 at 19:43
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Vertical loading does (almost) nothing to a properly designed and constructed bitumen road surface, it is horizontal loading that causes deterioration.

Horizontal loading comes from acceleration, braking and turning. For acceleration and braking speed is unimportant, it is only the rate of change of speed that matters.

For turning, speed can in theory make a difference. The faster a vehicle is travelling when turning a corner, the more horizontal force is required to turn it sideways. But the biggest deterioration on bitumen roads tends to come from low speed tight turns, such as at junctions. In fact the place I most often notice deterioration is at bus stops, where heavy vehicles frequently make tight turns in exactly the same place each time.

Once a pothole has started, then in terms of it worsening I suspect the other answers about speed mattering for dynamic loading on the edge of the pothole are correct. But I don't believe that speed matters in creating the pothole in the first place.

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  • $\begingroup$ Saying that vertical loading does almost nothing is perhaps a rather bold statement. Flexible pavements are by design going to rut if temperature and load conditions are right. Roads are sometimes used more frequently than what they were initially designed for and rutting is not necessarily uncommon by any means especially where extreme traffic tonnage is experienced. $\endgroup$ – Isa Apr 24 at 13:46
  • $\begingroup$ And speed matters in creating the pothole in the first place because as pavement ages, cracking is generally inevitable. When cracks form, and grow, that becomes a localized surface irregularity that will be magnified by impact loading (albeit smaller than a pothole most likely). Those cracks will likely eventually induce pothole formation when sufficient cracks converge within a small area. $\endgroup$ – Isa Apr 24 at 13:48
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If you have a 20 ton truck sitting stationary on the road, it's exerting 20 tons of force on the roadway. If that truck is traveling at 100km/hr, it is still exerting 20 tons of force on the road.

I'm sure this is why your friend says that weight is the only factor that matters. The truck stopping or starting isn't even a major concern because that happens infrequently and randomly on the road. Intersections and the roads around them might need to be stronger because of the constant braking and acceleration, but that's about it.

But that's only for straight roads.

On bumps and curves, speed is going to matter because speed + weight = momentum. The inflection point of any bumps or elevation changes in the road will have more force on them as speed increases, and curves in the road will have more lateral (or perpendicular if the road is banked) forces as speed increases.

If a road develops a slight depression, faster vehicles will hit it harder which could make it grow faster than slower traffic.

That said, trying to add variable speeds to the calculations might be too complex or error prone for the engineers. Using a static weight might be the best they can do accurately.

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  • $\begingroup$ Thanks for all your points and the clarity. I'm not convinced that the engineers are correct in just considering weight though. (To save our roads). We had Tanks - yes rather large Tanks - drive on our small roads (which was exhilarating to say the least), and I started imagining how much more damage they would do to the road if they zoomed around at high speed and high brakes...They would literally rip off the road. Its a long shot, but wouldn't the tyres on a Heavy truck be a milder example? $\endgroup$ – Grantly Oct 25 '17 at 20:06
  • $\begingroup$ In Physics terms, a 20 ton truck 'accelerating' on the road, will actually exert more force on the road albeit forwards or backwards force, as opposed to the normal downwards force. So it could actually exert more force on the road than it weighs...Just like if it was 'bouncing' $\endgroup$ – Grantly Oct 25 '17 at 20:10
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    $\begingroup$ That's true, but I was trying to say that any acceleration is a small part of road travel. Normally vehicles are traveling at a constant speed unless there is traffic. $\endgroup$ – JPhi1618 Oct 25 '17 at 20:12
  • $\begingroup$ Yeah you are right JPhi I think in normal traffic :) I just think the 'heavy speed' factor makes all the difference. (I think forestry trucks under load are probably 20 - 40 tonnes...does that sound right? And that is amazingly heavy really) $\endgroup$ – Grantly Oct 25 '17 at 20:14
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    $\begingroup$ Max weight for an "18 wheeler" is typically 40 tones (80,000 pounds!) without a special permit. $\endgroup$ – JPhi1618 Oct 25 '17 at 20:30
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To answer this question, you need to consider the failure modes you are protecting against. To understand the failure modes, you need to have the full details related to the materials of the road, thicknesses of the respective materials used to pave the road, and etc. A bitumen road running heavy trucks will most likely fail by excessive rutting in my opinion, in which the main factor of consideration would be related to the weight of the traffic running on the road, and less so about how fast the trucks are traveling. Therefore, without knowing the full details of the scenario, your feasibility engineer is correct in a sense that the weight is probably the more important factor in this case when the road is new, and speed may/may not be neglected.

So when will speed make a difference?

Speed generally makes a difference when you have localized irregularities on the road surface, such as a pothole. A presence of such an irregularity will deteriorate much quicker in the presence of higher speed axle loads - this is what people generally refer to as "impact loading" or "dynamic impact load factor" if you are working on doing some calculations. Dynamic impact load generally increases exponentially with speed (not necessarily exponentially in mathematical terms, but you get the idea).

I personally work in the rail industry but the underlying principal is the same. I am not familiar with the numbers for trucks, but for HAL axle loads in north America (higher axle loads than EU), the amount of impact that is generated by a wheel impact from a rail gap can be up to 400% +/- of the normal load for a train wheel traveling at 70 mph. I would imagine that trucks would experience a lesser increase due to the attenuation produced by the rubber tyres. But impact loads are certainly a significant force in accelerating deterioration of a road once potholes or localized failures on the surface has started.

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Speed makes it worse; big vehicles cause the great majority of road damage. The damage cause by cars is mostly limited to'polishing" ; the pavement is smoothed which reduces the friction. Many roads a scarified , those small grooves about 1/4 " deep to offset the polishing damage.

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I think JPhi is correct. if you have a slight unconformity in the road surface- say a small depression- then the leading surface of a fast-moving tire will strike the edge of the depression with a significant impact force, which is entirely different from the response of the road to a purely static or even a suddenly-applied load. In this way, fast-moving vehicles will "hammer" the unconformity in a way that a slow-moving vehicle will not. On this basis, then, fast-moving vehicles should do more damage to an "imperfect" road surface than slow-moving vehicles would.

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It seems some official documentation does indeed mention 'speed' as a determining factor in the amount of road damage, especially with respect to heavy vehicles...

Will post the link, as the document is very long, but they even mention a milestone speed of 40 kmh where speeds over this are known to cause more damage by heavy vehicles.

http://www.nvfnorden.org/lisalib/getfile.aspx?itemid=261

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