Mitigating Pavement Shoulder Cracking in Northern, Low Volume Highways by Incorporating Tencate Mirafi® H2Ri Wicking geotextile

Action identification was primarily informed by an extensive literature review conducted by researchers from FPInnovations along with those from TenCate. The literature review, through consideration of evidence-based academic articles, sought to reinforce understanding of the root causes of edge cracking, considering both how the roadway was constructed as well as how its ultimately utilized. The review found that edge cracking can be partly credited to under-compacted materials along the shoulder or side of the road. This under-compaction can be attributed to a natural tendency for shoulders and slopes to be difficult to compact (due to their steepness, positioning, etc.). This weakness can be exacerbated by the usage of these roads by heavy vehicles. Under-compacted materials can also allow for higher retention of water further compromising the integrity of the edges. Finally, weakening can also be attributed to over-steepened side slopes as well as frost heave and thawing. From a maintenance standpoint, plowing snow into the shoulder can also have an impact on cracking. Spring thaw combined with the mass of snow on the shoulder creates differential thawing which has a major effect on the integrity of the edge. With the multiple causes of cracking clearly established, the project team was primed to utilize novel techniques such as that of geotextiles to attempt to address the persistent problem.

In October of 2015 the project team selected two sites at which to test the effectiveness of the geotextile (Mirafi H2Ri) in preventing edge cracking. Both sections of road selected were along the Robert Campbell Highway north of the town of Watson Lake, Yukon, and had experienced considerable edge cracking previously. Each chosen road segment featured 43 metre cracks (the entirety of the length of each) along their eastern edge with less substantial cracking along western edges. After the existing road segments were removed, the wicking geotextile was installed within the subbase (comprised of a coarse gravel laid as one of the starting materials) before more gravel of increasing fineness was placed on top and a 4% crown (difference in slope from the centre of the road to each edge) was formed to allow for proper drainage. Once the desired shape was achieved the surface was watered and compacted in place. A bituminous surface treatment (BST) was applied to the gravel surface in June of 2016. Throughout the construction process a number of monitoring instruments were also installed at various depths both above and below the geotextile as well as at the surface.

During construction a number of monitoring devices were installed at various depths both above and below the geotextile as well as at the surface. The instruments were used to collect information regarding temperature (both of ground and air), as well as moisture levels. Road temperatures ranged widely at each site from 38℃ to -30℃. Temperatures also varied significantly between the shoulder and centreline areas of the road with the former being cooler in the fall and spring as more moisture was allowed to infiltrate the less dense materials along the shoulder. Trends reversed with snow fall as snow plowed to the shoulders helped to insulate and regulate temperature along the side. Moisture monitoring found increased moisture at depth on both sites. Thawing occurred slightly faster at the treated sites than at the control site with the centreline thawing several days before the shoulder. At sites containing the geotextile moisture contents fell 1%-1.5% after thawing while untreated shoulders did not drain water away. In addition to temperature and moisture monitoring, visual observations of the test sites and nearby untreated road segments found only minor edge cracking on treated sites while some untreated segments bore cracking 10 centimetres wide and greater than 15 centimetres in depth. Minor cracking on treated sites which was first observed in 2016 led to a field density study which employed a nuclear density gauge. The study found that lower densities along the sides of the road were responsible for the minor observed cracking.

The authors conclude their study by offering a number of steps to be taken to create more resilient roadways in northern climates. Firstly, they recommend the use of wicking geotextile for low volume pavements. In utilizing the geotextile, the authors note that it should be placed closer to the surface or doubled for the most effective moisture wicking effect. When constructing new roadways, the authors suggest that side slopes not exceed a 3:1 ratio to avoid difficulties in compacting and that compacting be done to allow for maximum uniformity between edges and the running surface. Materials selected for constructed should be well graded granular and be low in fine content. Further, it is noted that maintenance practices which look to avoid snow accumulation along the shoulder could aid in reducing edge cracking. Ultimately, geotextiles along with road design and proper winter maintenance practices was suggested to reduce edge cracking.

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