A long-standing joke among Vermonters is that the year is composed of the following four seasons: summer, fall, winter and mud. Mud season is that time of year between late March and early May when thawing of frost-susceptible unpaved roads creates conditions even the locals avoid driving. In a really good year, mud season is a time when glittering droplets of water fall daily from postcard-perfect icicles on the edge of the barn roof, the maple sap flows prodigiously from the sugar bush and even the hardest core environmentalists give up cursing 4WD SUVs, at least for a short while. Mud season in Vermont is to local towing companies what the period between Thanksgiving and Christmas is to toy retailers.
Vermont has a total of 12,812 miles of road. Of these, 8,462 miles, or 66%, comprise Class 3 town highways. Portions of these roads in every Vermont town suffer chronic serviceability problems during spring thaw. Because Class 3 roads have a minimum standard to be negotiable by a passenger car under normal conditions all seasons of the year, mud season forces local road commissioners to mitigate deterioration on problem stretches of unpaved roads within their towns. Road crews often repair the affected sections by re-grading and adding more gravel to the surface. Over time, some towns have been successful in reducing the number of trouble spots by incrementally improving the structural section of the roadbed, while other road sections, even after many years of maintenance, still see serious degradation.
Though it’s common knowledge that building with clean, non-frost-susceptible material will help prevent thaw weakening, the practice is expensive, and the old anecdotes about the frugality of Vermonters tend to ring particularly true around town meeting day, when the annual budget is set. Interviews with road commissioners indicate the capital cost of improvements is their primary constraint. Past solutions in many towns have required extensive processing of material from local deposits or hauling in more appropriate material over costly distances. The capital costs of these activities are often unpalatable to town officials and citizens, so town road crews use what is available in local gravel pits with minimal or no processing beyond extraction. Commissioners can seldom obtain budget approval to include engineered materials, such as geosynthetics and soil additives, in their improvement plans.
Finding a cure
To assist town road commissioners in selecting cost-effective remedies to mud-season weakening and to provide cost-benefit information to bolster the case for their budget requests, the Vermont Department of Transportation examined the effectiveness of several long-term remedies. The project was awarded to a consortium that included the University of Vermont Department of Civil & Environmental Engineering (UVM), the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL), GeoDesigns Inc. of Windsor, Vt., and Applied Research Associates Inc. of Royalton, Vt. Dr. James Olson of UVM and Dr. Karen Henry of CRREL were the co-principal investigators.
This team pursued three goals: (1) demonstrating the effectiveness of candidate technical remedies, (2) comparing cost effectiveness of successful remedies and (3) transferring findings and decision support tools to town road officials.
The team solicited interest in the project from road commissioners and obtained participation from the towns of Westford (in the northern part of the state) and Windsor (in the south). They then selected several candidate remedies and developed a protocol for field measurements and instrumentation that they incorporated into alternating test and control sections in the two towns. Since mud-season severity varies from year to year, interspersing untreated control sections with and adjoining test sections facilitated direct comparison between improved and unimproved road performance both within each mud season and at each site. Sections in most cases each spanned 100 ft in length.
Local road crews and private contractors constructed test sections at each site, and the research team monitored the sites and collected performance data during two thaw seasons, after which they assessed performance improvements in the test sections as compared with adjacent control sections and reported the results of these actions.
Understanding the rationale behind the remedies requires an understanding of frost weakening in soils. Unpaved roads are composed of a combination of fine-grained particles and coarse aggregates (gravel). Keeping moisture levels in balance is critical to maintaining the strength of these granular mixtures and thus road stability. Too little moisture causes excessive dust, while too much creates mud. Excessive water is the culprit in mud-season weakening, and a large portion of this water comes from the thawing of soil that has frost heaved. Each remedy in the study aimed to provide some combination of limiting the availability of moisture in the winter, improving drainage during spring thaw and strengthening the upper portion of the road. Each sought to implement, alone or in combination, improved lateral or vertical drainage, reduced capillary rise of water into surface layers of the road using a capillary barrier, separation of underlying frost-susceptible soils from the capillary barrier or draining layer using a geotextile and mechanical stabilization to strengthen the road surface layers. The figure on the left summarizes and depicts the remedies. The GCBD is an invention shown in the laboratory to increase the rate of water removal from pavement base courses after water is applied to the surface.
Once the team had decided on a suite of remedies, construction was planned and completed before the winter of 2001-02. The researchers performed baseline soil characterization prior to and during construction of the sections. Test and control sections were instrumented with thermocouples at multiple depths in the roadway, and soil and road conditions were monitored during spring thaw in 2002 and 2003. The dynamic cone penetrometer (DCP) was used to evaluate soil strength during thaw. The DCP produces a penetration index that correlates to California Bearing Ratio or resilient modulus. Visual records of performance via still photographs and videotape accompanied DCP measurements. Visual observation provided perhaps the most compelling assessment of each section’s performance.
Strong upper body
Sections were monitored for two years including the 2002 and 2003 mud seasons. In the end, the only methods that were found to significantly improve performance during the spring thaw period were those that either permanently increased the strength of the top layers or decreased the water content of the roadbed. The drain improvement by itself did not provide worthwhile improvement to the roads in this study.
In addition, geogrids and geotextile separators provided no observable benefit during mud season. Geogrid and geotextiles are typically placed on weak soil and covered with high-quality aggregate to improve the bearing capacity of the underlying weak soil. Published design guidance relies on this concept. However, in this application the critical weakened soil condition occurs in the top 3 to 6 in. of the road surface. Thus, only improvements that affected this portion of the roadbed produced measurable improvement.
The geowrap and GCBD provided benefit by keeping the upper layers of the soil relatively dry, while the cement and the cellular confinement systems improved the strength of the road surface and upper layers of the road year round in addition to performing very well during mud season. However, the Windsor road crew found on the cement-stabilized surface that if the grader operator doesn’t adjust the grading procedure the surface of the section comes up in slabs. Thus, cement stabilization may be problematic for a regular town road due to the grading difficulties. The town of Westford did not report any problems with maintenance on the cement-stabilized section, but they had added 4 in. of gravel over the original cement-treated section. The additional cover material may have been enough to prevent lifting during re-grading and other maintenance.
The town of Windsor spends about $2,000 annually on emergency repair of deteriorated sections (2004). Repairing the road on which the study occurred consumes about 30% of their maintenance budget, which can surge by a factor of three in extreme years. This cost includes town labor and equipment to haul material and grade the road surfaces. Material is obtained from a town-owned borrow and not included in the estimate. Accounting for 200 cu yd of material annually at an assumed rate of $5 per cu yd, the annual toll for mud-season repair rises to about $3,000.
These repair costs exclude increased travel-time costs for the maintenance equipment and wear and tear on all vehicles that must travel on these deteriorated sections.
