Assessment of Unpaved Forest Road Surface Condition based on Slope Steepness and Bearing Capacity

Aims: The technical health of forest road surfaces is essential to access the forest and vehicle traffic safety. This research investigated the effect of longitudinal slope and bearing capacity in different climate conditions on unpaved roads, especially road surface elements. Materials Methods: First, 500-meter road segments were randomly determined in Mediterranean, sub-humid, and semi-arid climates in Shastkalateh, Rezaeian, and Arabdagh forests, respectively. In each climate zone, unit samples on roads were divided into slope classes of 5% and 5%. The unpaved road surface condition index (UPCI) was calculated by considering corrugations, potholes, erosion, oversized aggregate, crown condition, and rutting in the field survey. Three soil samples were collected for California bearing ratio (CBR) analysis in each road segment. Proctor and CBR tests were conducted according to ASTM standards for each slope class. Findings: Results indicated that in slope class 5%, the UPCI value in sub-humid climates was significantly lower than in other climate zones (P 0.05). Maximum UPCI (8.68) was observed for roads located in a semi-arid climate. In addition, an increase in slope classes from 5% to 5% can decrease UPCI by about 6.6%, 11.8%, and 11.1% in Mediterranean, semi-arid and sub-humid climates, respectively. There was a positive and significant correlation between soil CBR and UPCI variables in all climates. Conclusion: In conclusion, road surface in sub-humid climates needs more attention for a maintenance operation, especially in steep slopes of mountainous forests. Besides, more knowledge about slope gradient and climate effects on road surface quality is necessary to decrease road deterioration by traffic on steep slopes.