By By Dr. Jennifer Jacobs, Ph. D., P.E. and Heather Miller, Ph.D., P.E.
New Hampshire’s local roads are notorious for heaving, cracking, and opening up potholes during the winter, while turning into mud or seeping water during the spring. Public works departments are at the front lines, trying to anticipate and address issues as quickly as possible, often repairing the same site multiple times each year. Some municipalities such as Manchester, Exeter, Dover, and Nashua even have dedicated “pothole hotlines.”
Managing roads that freeze in the fall and thaw in the spring is a continual battle and often quite costly. It is also a fine art with public works employees using their experience to understand which roads are already deteriorated or susceptible to damage and when damage is likely to occur. A refresher on what is new on the topic might help New Hampshire’s cities and towns make the most of their limited resources.
What is the Root of this Evil?
In many northern states, seasonal fluctuations in temperature (known as “freeze-thaw” conditions) can make low-volume roads especially vulnerable to damage from frost action. In the fall, roads begin to freeze, starting at the top and moving deeper. When water has entered the road base soils through cracks in the surface and/or via capillary action, ice lenses are formed, and the soil begins to move upward causing frost heaves and potentially cracking the pavement or even completely breaking it apart. The silver lining is that in the heart of winter, the roads are very stiff and strong because they are frozen and can take additional weight. Some states, such as Maine, take advantage of this increased strength and stiffness and apply winter weight premiums—increases in the allowable weight trucks can haul on these roads. However, New Hampshire does not have a winter weight premium policy.
During the spring, when temperatures rise, the once-frozen roads begin to thaw from the top downward. As the ice lenses thaw, the meltwater is trapped in the soil above the still-frozen lower layers causing the thawed layer to be saturated. In this condition, the structural integrity of the roads is weakened, making them more susceptible to damage from heavy trucks. For unpaved roads, the spring mud season is particularly noticeable because the road surface turns to mud; vehicles driving on the roads can sink to the frozen layer leaving behind large ruts that refreeze and pond water.
Although one early experiment on a New Hampshire silt soil found that a wet, 6-inch thick soil sample expanded to10 inches as it froze, or heaved by 70%, not all roads are impacted so dramatically. The most problematic roads have water in the soils below the paved surfaces, freezing temperatures, and frost susceptible soils. The source of water can come from shallow groundwater tables, precipitation, lakes, streams or wetlands. Typical frost susceptible soils are silts, silty sands and very fine sands. These soils are fine enough to be able to wick water upward (capillary action) yet permeable enough to allow the water to move upward.
How Can Damage be Prevented?
While not a new problem, pavement damage from winter conditions continues to be a pressing issue. American Society of Civil Engineers’ Cold Regions Engineering Division (CRED) partnered with Minnesota Local Road Research Board (MN LRRB) to create a new educational video “Frost Damage in Pavement: Causes and Cures” in 2013. (https://www.youtube.com/watch?feature=player_embedded&v=qnI7T5wbNjc). This video summarizes the causes of damage and engineering design practices to consider for particularly challenging roads. The key is to keep water out of the pavement. There are a number of techniques with a range of costs. Frost susceptible subgrade materials can be removed and replaced with non-frost susceptible materials or existing soils can be blended with non-frost susceptible soil or a stabilizing additive. The frost layer’s downward movement can be prevented by adding an insulate layer below the top layers, but at an adequate depth to prevent surface icing problems. A coarse drainable base or “rock cap” can be added to prevent upward wicking of water. Finally, water can be drained to the road’s side into ditches by a variety of means including specially designed synthetic materials, crowning the subgrade, or adding drainage tiles.
Posting Roads: Balancing Flow of Commerce and Protecting Local Infrastructure
During spring thaw periods (a.k.a. “mud season”), these frozen roads begin to weaken as they thaw, becoming subject to damage from heavy trucks, or even entirely unusable. As a preventative measure, state and local agencies often restrict the amount of weight that can be carried over a roadway by setting seasonal load restrictions (SLRs). Although beneficial for preserving road surface quality, these restrictions can have impacts on local industry and economies. Thus, a key challenge is maintaining a balance between allowing commerce to flow during restricted periods and preventing road damage. While municipalities may favor protecting their infrastructure investment and seek to limit damage to roads, state policies may make that difficult by exempting some industries.
What are the Relevant Statutes Regarding Road Postings?
New Hampshire municipalities can protect those roads by imposing weight restrictions via RSA 231:190 and 231:191. However, exceptions can be granted for “any owner of land or a commercial enterprise served by such highway, who demonstrates that such limits would entail practical difficulty or unnecessary hardship, and who complies with all conditions and regulations concerning bonding and restoration, shall be granted an exemption unless the exemption would be detrimental to public safety.” Furthermore, RSA 236:3-a creates an exception for heating fuel delivery vehicles, trucks delivering processed milk products, trucks carrying sap for maple syrup production, and septic pumper trucks and accompanying supply trucks. RSA 266:24 exempts “implements of husbandry" (farm equipment). Emergency fire fighting vehicles (RSA 266:19-a) and winter maintenance equipment that is owned, leased or rented by the state or any political subdivision of the state (RSA 266:21) are also exempt.
Upon receipt of a request from an impacted business, RSA 231:191 dictates that a “hearing shall be held within 15 working days of receipt of a certified letter by the local governing body from the impacted business requesting a hearing, otherwise enforcement of the maximum weight limits established by that municipality shall be suspended for the remainder of the year or until such hearing is held.”
Tools to Help Set Load Restrictions
Road managers and state Department of Transportation (DOTs) use a variety of tools to judge when roads should be posted. Tools include visual inspections, expert judgement, rudimentary calculations, and weather forecasts. In order to balance the needs of the industry with conservation of the roads, state DOT and municipalities are increasingly looking for scientific approaches to the decision-making process. One of those scientific approaches, developed by the Minnesota Department of Transportation (MNDOT), uses air-freezing and air-thawing indices. Air-freezing indices are accumulated daily indexes of how many degrees the temperature is below 32oF, the freezing point for water. Air-thawing indices are a little more complicated; they incorporate accumulated daily indexes of degrees above 32oF, as well as several other variables. According to the MNDOT, when the cumulative freezing index (CFI) gets up above 280oF-days, the roads should be frozen enough to accommodate winter weight premiums. Conversely, the agency recommends that when the cumulative thawing index (CTI) exceeds 25oF-days, spring load restrictions should go into effect. These CFI and CTI thresholds have been found to be reasonable estimators of freeze and thaw for New England conditions.
A Real-time and Short-term Forecast Road Freeze-thaw Tool
To help New England road managers make road freeze-thaw predictions, the MNDOT indices were turned into a web-based road conditions maps for the Northeast. The Infrastructure & Climate Network (ICNet), including Dr. Jennifer Jacobs at University of New Hampshire and Dr. Heather Miller at the University of Massachusetts–Dartmouth, both Professors of Civil and Environmental Engineering, and NOAA’s Northeast Regional Climate Center (NRCC) led by Arthur T. DeGaetano, Professor of Earth and Atmospheric Sciences at Cornell University, brought together expertise on winter road conditions and weather forecasting to create this tool. The resulting web-based roadway freeze-thaw map interface (patterned after a similar interface developed by Minnesota Department of Transportation) allows road managers in the northeast to better assess when load restrictions should be applied or suspended. The map interface tool can be viewed at www.nrcc.cornell.edu/industry/roads.
The Roadway Freezing/Thawing tool provides daily updates regarding freeze-thaw conditions on a particular day and incorporates weather forecasts to predict conditions for up to 6 days in advance. Up and running since November 2016, the interface has the potential to help road managers make decisions about enacting or removing load restrictions more objectively. Last spring, Maine DOT found that at instrumented test sites, the tool showed thawing at the same time the roads were, in fact, thawing.
Because three to five days advanced notice is needed to enact weight restrictions, being able to see road conditions in advance—to know that, although it’s been warm for a couple days, it’s going get colder, or vice versa, and having a tool to translate weather forecasts to road conditions—should help municipalities operate more efficiently. Better information means a higher level of confidence that roads are posted when the limits are needed. It also helps road managers to more strategically exempt impacted businesses, and handle emergencies such as when someone’s well went down (and they need drills, excavators).
The general sentiment is that just getting funding to maintain roads is a difficult. It is even more challenging to find the funds to reclaim the roads and to stabilize them to withstand freeze-thaw action. NOAA’s new roadway freeze-thaw map interface can help prevent damage by helping road managers to make better informed decisions about posting roads during the spring thaw weakened period.
Cumulative freezing indices showing the freeze onset during December 2017 including forecasts during a long cold snap. MNDOT recommends that the additional weight can be allowed on roads when the 3-day weather forecast indicates that the cumulative freezing index (CFI) will exceed 280°F-days and extended forecasts predict continued freezing temperatures.
Cumulative thawing indices showing the winter melt in late February 2017 where unseasonably warm conditions rapidly made roads susceptible to damage. MnDOT recommends that the SLR be scheduled when the 3-day weather forecast indicates that the cumulative thawing index (CTI) for a zone will exceed 25°F-days and longer-range forecasts predict continued warmth.
Jennifer Jacobs is a Professor in the Department of Civil Engineering at the University of New Hampshire. Jacobs was assisted in this article by Heather Miller and Lee Friess. Jacobs’ research uses new technologies and big data to understand infrastructure response to climate and extreme weather with a focus on extreme precipitation, flooding, and winter snow and freeze-thaw conditions. She may be reached at 603.862.0635 or by email at Jennifer.firstname.lastname@example.org. Miller’s research focus is on freeze-thaw processes in soils, seasonal load restriction protocols, and most recently, the effects of climate change on those processes and policies. She may be reached at 508-999-8481 or by e-mail at email@example.com.
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