1, particle size composition and mineral composition of soil. The particle size, shape and gradation of soil affect the size and shape of pores in soil, thus affecting the permeability of soil. The coarser, rounder and more uniform soil particles are, the greater the permeability is. When sand contains more silt and clay, its permeability coefficient is greatly reduced.
2, the combined water film thickness. If the bound water film of soil particles in cohesive soil is thick, it will block the pores of soil and reduce the permeability of soil.
3. Structure of soil. Natural soil layers are usually not isotropic, and so is permeability. For example, loess has large pores in the vertical direction, so the permeability coefficient in the vertical direction is much greater than that in the horizontal direction. Layered clay is often sandwiched with thin silt layer, and its permeability coefficient in horizontal direction is much greater than that in vertical direction.
4. Viscosity of water. The seepage velocity of water in soil is related to the bulk density and viscosity of water, and also affects the permeability of soil.