Permeability is an important property of sand soil. Factors that influence the permeability of sand include density

particle characteristics and properties of the fluid

among which the porosity and particle size are two important ones. However

previous investigations based on natural sand with mixed particles are not able to study the two factors independently. Therefore

constant head permeability tests are firstly carried out on sand soil with single particle size fractions to study the influence of grain size and porosity on the permeability

respectively. For sand soil with the same grain size fraction

the coefficient of permeability is found to increase linearly with the increase in porosity; for sand soil with the same porosity

the coefficient of permeability is found to increase linearly with the increase in porosity the square of average particle size. Based on this

the influence of gradation characteristics on the permeability of multi-particle-size sand soil is studied to extend the results from single particle size fraction to natural sand. The results show that the coefficient of permeability of multi-particle-size sand soil increased linearly with the increase the coefficient of curvature and coefficient of uniformity. The influences of different factors also differ from each other. The change in average particle size leads to change in order of magnitude of the coefficient of permeability. Finally

an empirical equation considering all the above factors is developed for engineering practice.