Coupled Hydraulic–Mechanical Response of Pavement Base Materials Under Varying Fines and Moisture Conditions

Abstract: The main role of any base material in a pavement structure is to support the asphalt layer above by bearing traffic loads and ensuring proper drainage to remove excess moisture promptly. To perform its intended function well, the base layer must strike a balance between drainability and stability. Factors such as fines content, material gradation, stiffness, and hydraulic conductivity influence the performance of the base layer, which in turn affects the overall pavement performance and its expected service life. This study aims to assess the hydraulic and mechanical properties of base materials with different fines contents and to examine the effect of varying moisture levels on their stiffness. Accordingly, three different base materials were chosen, and their gradations were adjusted by modifying the fines content. A comprehensive testing matrix was then developed, including tests for compaction properties, hydraulic conductivity, and resilient modulus for all gradations. To analyze the impact of different moisture levels, the resilient modulus was first tested at the optimum moisture content. Subsequently, samples were placed in an environmental chamber to initiate drying and simulate moisture levels below the optimum. Finally, moisture content was increased beyond the optimum to emulate the worst-case scenario, representing excessive wetting conditions. Results indicate that parameters such as maximum dry unit weight and optimum moisture content increase with rising fines percentage, reaching a peak before abruptly decreasing. A similar trend was observed for resilient modulus values as a function of fines content. Conversely, hydraulic conductivity was higher at low fines content but decreased as fines increased. These findings aim to identify the fines percentage threshold beyond which base performance deteriorates and to guide the optimization of gradation and moisture control in pavement base design.