3

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.

Grantly
  • 131
  • 1
  • 7

7 Answers7

3

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.

JPhi1618
  • 133
  • 5
2

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.

Isa
  • 298
  • 2
  • 7
2

Bitumen is a viscoelastic material and the time/duration that force is exerted on it is directly proportional to the fatigue of the material. Simply put, low speed causes more damage to the bitumen roads specially. Please refer some scientific journals or astm standards for your ref.

Anshul
  • 21
  • 1
2

For what its worth our (Asutralian) asphalt test tracks fail by weathering because they don't get enough traffic. Water seeps in among the stones and spoils the bond between the bitumen and the stones, which then loosen. Regular traffic would squeeze the water back out, and recompress the roadway.

Greg Locock
  • 1,147
  • 1
  • 4
  • 10
1

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.

blacksmith37
  • 6,192
  • 1
  • 11
  • 16
1

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.

niels nielsen
  • 15,513
  • 1
  • 15
  • 33
0

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

Grantly
  • 131
  • 1
  • 7