Dirt roads posted — blame thixiotropy

Spring is coming, and with it comes the yearly ritual of the posting of dirt roads around town. While passenger cars may continue to travel them, heavier vehicles are banned between March 1st and May 15th. 

The limits on these are as follows: 

• 2-axle trucks 15,000 lbs
• 3-axle trucks 18,000 lbs
• Tractor-trailer 20,000 lbs

What this means in practical terms is that several types of service and delivery vehicles are prohibited, including:

• Residential rear-loading garbage trucks (three-axle). These weigh 20,000-35,000 lbs when empty. A fully loaded garbage truck weighs 50,000-60,000 lbs.
• Residential "bobtail" propane delivery trucks (two-axle) that commonly weigh 18,000-20,000 lbs when empty and 26,000-33,000 lbs when full.
• Standard local delivery UPS trucks (two-axle) are on the line. They weigh 10,000-12,000 lbs when empty but 16,000-24,000 when fully loaded.  
• Standard local delivery Fed Ex vans (two-axle) weigh 9,000-13,000 lbs when empty and 19,000-23,000 lbs when full.

It seems self-evident that heavy vehicles are damaging to dirt roads when they begin to thaw out.  And it doesn't help that many local soils contain clay and silt and were not good road base material to begin with. 

Most dirt roads began as trails, some made by the indigenous peoples of our region. After colonization, the settlers used them and added more trails, often connecting one farmstead to the next. Many such trails followed the courses of streams where the terrain was the least extreme. They were designed for walking and horse drawn carriages, not automobiles. Road beds, the foundation layer that provides support and stability, were made from whatever dirt and rocks were at hand rather than the engineered and layered construction of crushed rock and gravel typical of a modern road designed for cars.

As the temperature warms, dirt roads begin to thaw from the surface down. Meltwater cannot drain due to the still-frozen layer below; thus the thawed layer is saturated with water, and the whole road loses integrity and is weakened.

Heavy vehicles like trucks sink through the saturated layer, making large ruts that collect water and re-freeze at night, creating an alarmingly uneven and sometimes barely passable road surface.

And there's one more phenomenon at work, one that explains why a thawing dirt road seems perfectly walkable, yet turns to mush whenever a heavy vehicle passes over it. It is the phenomenon of thixiotropy, where viscous substances like gels, tomato sauce, or mud are semi-solid until they are vibrated, shaken, or stirred. This causes them to become temporarily liquid. Thus, the shear forces and agitation exerted by a truck's wheels cause mud to become, well, muddier. The mud reverts to its semi-solid state when allowed to rest.

Road crews may be called upon to fill particularly bad mud holes. That means heavy, gravel-laden trucks are asked to travel up fragile, muddy dirt roads to reach that location. In the process they risk doing additional damage to the rest of the road.

To live on a dirt road is a choice which includes accepting the times when it is hard to come and go, when garbage can't be picked up and packages can’t be delivered. If you must have a visit from a large vehicle (like a moving truck), please contact your road foreman. They will inspect the road and determine whether it's good to go. Emergency services are, of course, exempted from the "Road Posted" prohibition.