Hydro-Mechanical Response of Highly Weathered Phyllite Subgrades Under Long-Term Water Immersion: A Full-Scale Comparative Study of Cement Stabilization

Long-term water immersion can substantially alter the hydro-mechanical behavior of water-sensitive geomaterials used in highway subgrades. This study investigated the responses of highly weathered phyllite (HWP) and 2% cement-improved HWP (CIHWP) using a full-scale subgrade model subjected to prolonged water immersion and intermittent static surcharge loading. The degree of compaction, California bearing ratio (CBR), and dynamic deformation modulus (Evd) were measured during construction. After construction, earth pressure, pore water pressure, volumetric water content, and layered settlement were monitored. Both materials met the required compaction criteria. However, CIHWP exhibited better compactability and higher CBR and Evd values. Following long-term immersion, water content increased across approximately 88% of the HWP profile, while the saturated area accounted for approximately 28% of the profile. Progressive wetting induced aggregate softening, stress redistribution, and increased deformation, with the final surface settlement reaching 9.3 mm under the applied static surcharge. In contrast, only approximately 12% of the CIHWP profile showed moisture changes, and its final settlement was 6.5 mm. These findings provide full-scale experimental evidence for understanding and improving the hydro-mechanical performance of water-sensitive geomaterials in transportation infrastructure, with direct implications for applied geotechnical engineering and subgrade stabilization.