Abstract
Forest road aggregate changes due to traffic. The physical processes that cause these aggregate changes need to be understood for more effective road management that can help reduce maintenance costs and efforts, and negative environmental impacts of forest roads. This study modeled three processes that could change the particle size distribution (PSD) of forest road aggregate: crushing (breaking down the surfacing material), subgrade mixing (moving upward of fine-grained, roadbed sediment), and sweeping (migration of loose aggregate particles to the shoulder and roadside by tire action). There are two types of sweeping: sweeping-out (dislodging large-size particles from tire tracks) and sweeping-in (accumulating large-size particles near the roadside and shoulder). Our study modeled the expected traffic-induced processes based on theoretical concepts and literature to examine how these processes change forest road aggregate PSD. Then the modeled results were compared with the observed PSDs from cross-sectional locations where traffic-induced processes likely occurred. Based on these comparisons, we enhanced the modeling and inferred how much the crushing, subgrade mixing, and sweeping-in processes changed the PSDs, but could not infer the sweeping-out process due to the difficulty in separating the sweeping-out from crushing. This study demonstrates that the traffic-induced processes could be modeled and quantified using the following assumptions: crushing was estimated by assuming a half logarithmic normal distribution with a mean of the crushed particle diameter and higher crushing rates for large-size particles; subgrade mixing was estimated by assuming the move-in of fine-grained subgrade soils from the road bed; and sweeping-in was estimated by assuming the move-in of large-size particles with a logarithmic normal distribution. Our modeling approach can offer insights on how traffic-induced processes affect road aggregate under various road and traffic conditions. This information can be useful in developing cost-effective road maintenance strategies and implementation plans.
Highlights
Forest roads are essential for forest management, providing access for timber harvesting and recreation
In a control volume of forest road aggregate, there is no material-in or material-out and the only aggregate breakdown occurs within the control volume in the process of crushing (Table 1)
Modeling the crushing process consists of crushing the initial particle size fractions (PSFs) and generating crushed fragments
Summary
Forest roads are essential for forest management, providing access for timber harvesting and recreation. Most forest roads are unpaved and are constructed with either aggregate or native soil surface [1,2,3,4]. Forests 2019, 10, 769 construction [5] It is used for surfacing low-volume forest roads to lessen wheel load stress delivered to the subgrade below, reduce maintenance costs, and improve driving comfort, as compared to native surface roads [6]. Aggregate surface roads have higher hydraulic conductivity than native surface roads, helping to reduce soil erosion from forest roads [7,8,9]. Additional road maintenance is needed when aggregate material is lost from the road running surface or when the road is deformed (e.g., forming a rut or pothole) due to vehicle traffic. A better understanding of the processes by which traffic changes aggregate properties can help develop preventive and cost-effective road maintenance strategies
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