In order to know the variation regularity of the dynamic modulus of red sandstone subgrade in field, the field portable falling weight deflectometer (PFWD) test, the indoor powder X -ray diffraction (XRD) test and the field compactness test are carried out. The effects of mineral contents, water contents, and compactness of different clays on the dynamic modulus of red sandstone subgrade are analyzed. The regression model of dynamic modulus varies with compactness and water content, and the regression model of dynamic modulus varies with cl...