The improved granular mixtures are widely used as the fillings of railway subgrade,and in order to investigate the effect of coarse grain content on granular mixtures,a series of field tests were conducted.The experimental results indicate that the permeability coefficient increases significantly with the increment of granite gravel content,especially in the range of 60%-70%.There exists a coarse grain content limit defined as 53%-58.5% to reform the permeable granular skeleton.Beyond this limit,the permeable granular skeleton is efficiently formed,and the macro pores between the separate gravels are partially filled,which is the explanation for the permeability increase.The investigations indicate the subgrade resistance modulus(k30,Ev2,and Evd) depends on the granite gravel content,and the resistance modulus increases significantly beyond granite gravel content of 50%.The skeletons of granite gravel-clayey sand mixture change in the long-term deformation objected to the train-induced dynamic load,which involves three main repeated and circular deformation stages.Generally,the long-time deformation is explained as the gravel crushing and filling the internal porous space with crushed gravel fragments.Through these investigations,the C40-G60 or C30-G70 is recommended as an optimum soil mixture for the good permeability and high resistance modulus. The improved granular mixtures are widely used as the fillings of railway subgrade, and in order to investigate the effect of coarse grain content on granular mixtures, a series of field tests were conducted. The experimental results indicate that the permeability coefficient increases significantly with the increment of granite gravel content, especially in the range of 60% -70%. There exists a coarse grain content limit defined as 53% -58.5% to reform the permeable granular skeleton.Beyond this limit, the permeable granular skeleton is efficiently formed, and the macro pores between the separate gravels are partially filled, which is the explanation for the permeability increase. The investigations indicate the subgrade resistance modulus (k30, Ev2, and Evd) depends on the granite gravel content, and the resistance modulus increases significantly beyond granite gravel content of 50%. The skeletons of granite gravel-clayey sand mixture change in the long-term deformation objected to the train-induced dynamic load, which involves three main repeated and circular deformation stages. Generally, the long-time deformation is explained as the gravel crushing and filling the internal porous space with crushed gravel fragments. Through these investigations, the C40-G60 or C30-G70 is recommended as an optimum soil mixture for the good permeability and high resistance modulus.