The different states of saturation in sand deposits particularly close to full saturation are introduced, and their effects on the liquefaction (cyclic) resistance, undrained shear strength as well as the initial shear modulus of silty sand are described. In the state of imperfect saturation, the relations between the coefficient of pore water pressure B, the primary wave velocity Vp and the degree of saturation Sr are described. The effects of imperfect saturation on the liquefaction resistance and undrained shear strength are examined based on laboratory triaxial tests. The empirical procedure to appropriately incorporate the effects of imperfect saturation on the liquefaction resistance of silty sand is then developed. In the state of un-saturation, the pore air pressure and pore water pressure as well as the volume change of soil specimens are separately monitored in laboratory triaxial tests. The effects of un-saturation on the cyclic resistance and undrained shear strength of silty sand are then examined. The current state of knowledge on the effects of un-saturation on the initial shear modulus of sand is also introduced.