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In a cost-saving move, the soft rocks composed of highly-weathered phyllites available onsite were used to fill the subgrade in the easte Ankang section of the expressway of Shiyan to Tianshui, China. Cement admixture was used to improve the performance of the weathered phyllites. In order to determine the best mix ratio, values corresponding to compaction performance, unconfined compressive strength, and the Califoia bearing ratio (CBR) were analyzed for variable cement content weight percentages (3％, 4％, 5％, and 6％) using test subgrade plots in the field. Field measurements of resilience modulus and deflection confirmed that the strength of the subgrade increased as the cement ratio increased. In order to further evaluate the cement/phyllite mixture, the performance of the 3％ cement ratio sample was evaluated under saturated conditions (with various levels of moisture addition and soaking time) using both the wetting deformation and resilient modulus values. Results suggest that moisture added and soaking time are key factors that affect the seepage depth, water content, and resilient modulus. The recommend values for the cement addition and for the water content are given out. This study can aid in prevention of highway damage by improving the foundation capacity and lengthening the lifecycle of the highway in phyllite distributed region at home and abroad.