Permeability anisotropy of natural loess with gradation zone: case study of Q₁ loess in Yan'an New District

In order to study the permeability anisotropy of the natural loess with gradation zone, the early Pleistocene (Q₁) loess containing the loess layer (L19), paleosol layer (S18) and the gradation zone (S18_Z) between them is taken as an example to obtain the permeability coefficients of saturation of various soil layers in 0°, 45° and 90° directions. The microstructural parameters such as porosity n_m, shape coefficient F, morphological fractional dimension D and probability entropy H_m are calculated, and then the influences of the microstructural parameters on the permeability coefficient are analyzed. The results show that: (1) Except the probability entropy, the microstructural parameters of both the pores and the particles are anisotropic, and the microstructural characteristics of the soil layer in the gradation zone are significantly different from those of the adjacent soil layers. (2) Each soil layer has the largest permeability coefficient and the fastest attenuation of permeability during seepage along the deposition direction, and the smallest permeability coefficient and the slowest decay of permeability along the vertical deposition direction. (3) The influence degree of the microstructural parameters on the permeability coefficient of each soil layer is n_m > F (pore) > F (particle) > D (pore) > D (particle) > H_m (pore) ≈H_m (particle), in which the contribution of porosity to the permeability can reach 30%. (4) The permeability of the gradation zone soil is the worst, and water tends to accumulate in the gradation zone, leading to an increase of the water content and weakening the mechanical properties of the soil, which needs to be focused on.