Abstract
The moisture index of subgrade material directly below a paved highway is typically represented by that of an uncovered slope. However, existing studies have demonstrated the existence of a significant moisture content difference between an uncovered slope and covered subgrade owing to evapotranspiration. Moreover, under the influence of solar radiation, wind, and other factors, soil evapotranspiration persists even at subzero temperatures. This paper presents an improved method for subgrade moisture index calculation for regions that freeze seasonally. Instead of the conventional Thornthwaite method, the Food and Agriculture Organization Penman–Monteith (FAO-56 PM) method was employed to estimate the potential evapotranspiration (PE) of slope soil at subzero temperatures. Based on the moisture balance principle, the PE and water runoff and deficit were used as input parameters to calculate the moisture index of an uncovered slope. After the effect of pavement cover on subgrade humidity was defined through a correction coefficient determined via the matric suction dependence of soil water content, an optimized calculation formula was developed to estimate the moisture index of the subgrade material according to that of the corresponding slope. The results calculated on a typical seasonally frozen region in Northeast China demonstrated the applicability and accuracy of the proposed method for predicting the subgrade moisture. The potential evapotranspiration of an uncovered highway slope soil at subzero temperatures could reach 9.8%–15.7% of the total annual evapotranspiration. The moisture index range for seasonally frozen regions was −14.2–57.3. These findings will have important implications for effective improvements in the design and construction of subgrade in regions that freeze seasonally or face similar climatic conditions.
Highlights
While subgrade material is usually unsaturated, its moisture condition is a critical factor that directly affects the strength and stiffness of the pavement structure [1, 2]
In China, the moisture index is used to calculate the equilibrium suction of a subgrade when the equilibrium moisture is mainly dependent on the climate [6]. is calculation is commonly performed using the ornthwaite method, which is based on two concepts: potential evapotranspiration (PE) and moisture balance
The evaporation rate of snow on an exposed subgrade or plant slope is lower than that of liquid water and forest cover, it is not zero [11,12,13]. us, the use of the ornthwaite method to calculate the PE in seasonally frozen areas produces large errors, which reduces the accuracy of the determined moisture index
Summary
While subgrade material is usually unsaturated, its moisture condition is a critical factor that directly affects the strength and stiffness of the pavement structure [1, 2]. Us, the moisture condition is an important consideration in the design of the structure and material of the subgrade. Is calculation is commonly performed using the ornthwaite method, which is based on two concepts: potential evapotranspiration (PE) and moisture balance. The ornthwaite method assumes that the evapotranspiration from the soil ceases when the average temperature is below 0°C (i.e., subzero) and the calculated PE is zero [7,8,9,10]. E calculation results in the overestimation of the moisture index of the subgrade soil, leading to the adoption of a design value of the subgrade resilience modulus that is significantly high and results in excessive subgrade dehumidification measures. Us, the use of the ornthwaite method to calculate the PE in seasonally frozen areas produces large errors, which reduces the accuracy of the determined moisture index. Temperature-based methods depend only on the air temperature and include the ornthwaite, Hargreaves [15], and McCloud [16] methods
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