Comparative Study on the Engineering Performance of Lime- and Cement-Improved Argillaceous Siltstone

Argillaceous siltstone is widely distributed along expressways in southern China; however, its strong water sensitivity and slaking properties severely restrict its utilization as subgrade fill, particularly under wet–dry cyclic conditions where bearing capacity deteriorates sharply. Existing studies have predominantly focused on mechanical performance evaluation of stabilizers, while systematic comparisons of lime and cement improvement effects and durability evolution under wet–dry cycles remain insufficiently understood. Drawing on the Yongjin Expressway reconstruction and expansion project, this study systematically investigates the durability of lime- and cement-improved argillaceous siltstone fill. Through unconfined compressive strength (UCS) tests, California bearing ratio (CBR) tests, and five wetting–drying cycles, the evolution differences in strength development, water stability, and durability between the two improvement schemes are revealed. Results indicate that, under identical stabilizer contents (3–7%) and curing conditions, the UCS and CBR of cement-improved soil are significantly higher than those of lime-improved soil. At the same dosage, the strength of cement-improved soil is approximately 1.5–1.7 times that of lime-improved soil, and the absolute strength gap further widens with increasing dosage. Both stabilizers effectively inhibit water immersion swelling, but the swelling rate of lime-improved soil is about 1.3–1.5 times that of cement-improved soil at the same dosage. At 7% dosage, the swelling rates of cement- and lime-improved soils decrease to 0.40% and 0.60%, respectively, both meeting subgrade fill swelling control requirements. After five wet–dry cycles, the UCS retention rate of 7% cement-improved soil is 78.3%, while that of lime-improved soil is 69.0%; the residual strengths are 507.0 kPa and 303.6 kPa, respectively, both satisfying general subgrade engineering strength requirements. However, the 3% lime-improved soil declines to 47.5 kPa after cycling, falling below the engineering threshold. Integrating strength, deformation, and durability indices, high-grade highway roadbeds and other high-load-bearing sections should prioritize 7% cement improvement, whereas general subgrade sections and locations emphasizing crack resistance may adopt 7% lime improvement as an alternative. Low-dosage (<5%) lime improvement is not recommended for argillaceous siltstone subgrade engineering. The findings provide a scientific basis for the engineering application of argillaceous siltstone as subgrade fill and for optimization of improvement schemes.