It’s possible that we owe this spring’s horrendous mud season to weather conditions back in December. An early winter storm put several inches of snow onto dirt roads that had not yet frozen. After the snow was dutifully plowed, the snow banks that lined the roads slowly melted and fed moisture into the depths of the road bed. Freezing weather followed. At the other end of winter, the start of the thaw consisted of many warm days followed by rain. Thus the partially thawed roads received a soaking, helping the soft surface to become even more liquid.
Thetford has about 49 miles of dirt roads, and many of them became impassable. The town’s heroic road crew worked for 21 days without a break to restore vital traffic connections to residents.
Roads freeze from the top down, to an average frost penetration of about 4 feet below the surface, in this area. Roads also thaw from the surface down, and it is common sense that if the lower layer is still frozen solid, the water cannot drain. Therefore it is retained in the top layer, turning it into a blanket of mud that deepens with the thaw, until all the frost in the ground is gone and the water drains away vertically and laterally. In the meantime, vehicle traffic carves ruts and holes that trap any rainwater, making the mud softer and wetter.
But there’s more going on than meets the eye, as Thetford Resident Bob Eaton explained. Bob enjoyed a 33-year career at CRREL, the US Army’s Cold Regions Research and Engineering Lab, in which he studied and experimented with roads and airport runways at the CRREL facility and all over the world. He then worked for the New Hampshire DOT Maintenance District 2 in Enfield for 8 years, where he was involved with pavement maintenance and operations.
For a start, our gravel — AKA dirt — roads were never designed as real roads. Their historic origins were as trails used by the indigenous people or by game animals. They were adopted by colonizers as horse trails that became wagon trails. To accommodate the increased weight and traffic, miscellaneous material was thrown down as necessary. Dirt roads have no base layer, just the native soil they happened to cross. And therein lies most of the problem. By contrast, a modern road is purpose-built on a foundation of well-draining crushed stone or gravel that can be several feet thick.
And as CRREL has discovered, there is more to frost action in soil than just the freezing of water. As water in soil freezes to “lens-shaped” pieces of ice, the soil around the ice is left dry. Capillary action — “the upward movement of water through a narrow tube against the force of gravity” — occurs in the dry soil and draws additional water up from unfrozen layers below. The more water there is in the lower layers, the more is available for capillary action. Thus, as shown in a CRREL experiment, soil with an original water content of 19% contained 55% water by the time freezing was complete.
The culprits here are fine soil particles with microscopic spaces, or pores, between them that are capable of producing capillary action. The spaces between coarse particles are too large to do this. Thus roads on a base of crushed rocks or gravel don’t suffer from this problem. Interestingly, extremely fine clay particles do not generate a lot of capillary action either. Clay swells when it gets wet and the small pores between particles become blocked.
Dirt roads exist in a Catch-22, because some amount of fine particles are necessary to solidify and bind together the coarse materials; otherwise there would be flying stones and ruts. Ideally, the top layer should contain a higher percent of fines and clay than the layers below. The optimal percent of fines, somewhere between 5-15%, is determined by local rainfall and weather and differs between wet and dry climates. Suppliers of road gravel generally provide an analysis of the percent of large and small stones, sand and fine particles. Unfortunately, Thetford has relied mostly on less expensive gravel with more than enough fines, obtained from a conveniently close source to minimize trucking expenses. (A typical dump truck gets 5 miles per gallon of diesel, which currently costs about $6.00 per gallon.)
There are yet more variables that confound easy management of our dirt roads. The native soils under the roads vary from place to place, sometimes within tens of feet. Some soils contain more fines than others, drawing more water into the road material and explaining why mud holes favor certain locations. Microclimates mean that mud season starts on sunny stretches of road that thaw first, then moves to shady areas beneath trees. Finally, roads on the north side of slopes turn to mud.
The million-dollar question is, can dirt roads be cured of mud season? As Bob explained, this is difficult and very expensive. There are several possible approaches. A frequently used one, that may or may not work, is adding a fresh layer of gravel or crushed stone with few fines, on top of mud. (There’s a saying “Add a bucket of gravel to a bucket of mud and you get two buckets of mud.”) However, $80,000 worth of crushed stone certainly allowed people living on Thetford dirt roads to get to their jobs and other activities this spring. Time will tell if the many truckloads will have a lasting effect on mud season conditions.
Other approaches include using geotextiles, which have received attention as a solution to weak, mud-prone roads with a low volume of traffic. The textile adds integrity to the road surface and keeps the top gravel separate from undesirable materials below. It is laid on a well-smoothed sub grade layer and covered with a full depth of surface gravel. Geotextile price varies, depending on the durability of the fabric, but could be $450 per 300 ft at 15 ft wide.
Placing foam board insulation below the top 12 inches of gravel prevents deep frost penetration and cuts off water rising by capillary action. It has the effect of allowing the road to thaw rapidly with less mud in spring. However at winter’s onset, it causes the road surface to freeze quickly, possibly icing up, because it is isolated from the underlying warmth lingering in the soil.
Stabilizing the road surface by mixing in cement powder or fly ash (coal ash) using a pavement grinder followed by water makes the road stiff and structurally strong. Analysis of the soil type at the site is necessary to determine the correct ratio of cement or fly ash to soil. In early 2022, the on-line price for Portland cement was about $130 per ton.
Laying down large steel plates over muddy areas so vehicles can drive over them, has also been tried. The steel plates would be re-useable, though steel is very expensive now, and the plates would have to be stored somewhere when not in use. And we would need a lot of plates. A quick look on-line found 5-by-10 ft steel plate (diamond plate) at 3/16 inches thick for $900.
That’s a long-winded way of saying there is no quick or inexpensive fix for Thetford’s 49 miles of dirt roads. The town could try one of the above methods on an experimental basis, choosing a few of the worst areas to see whether these approaches work. And because winters are so variable, the test areas would need observation for five years or so before deciding to treat longer stretches. But the bigger challenge is finding the money for large-scale dirt road refurbishing, since we are paying off a $4 million bond for fixing Route 132, and frequently-used paved roads like Latham and Tucker Hill badly need repair. Even if we are able to mitigate the worst of the worst mud, the expensive upkeep of dirt roads will be ongoing. It’s the price we have to pay for our car-dependent, rural way of life.
Photo credit: Li Shen