The roles of microstructure and water mobility in pre-Darcy flow formation in saturated clay soil

•The seepage of water in clay deviated from that predicted by Darcy's law.•Macropore and mesopore are the dominant pore types in clay.•The weakly bound water (outer layer) is the main type of water in clay.•The movable water content was closely related to the permeability of the clay. Water flow in saturated clay soil at low hydraulic gradients usually occurs via pre-Darcy flow. The occurrence of the pre-Darcy flow phenomenon is attributed to the adsorption of water on pore walls via surface forces. However, a clear understanding of the amount of water that is movable in saturated clay soil and the distribution of movable/immovable in the microstructure of clay soil is lacking. Furthermore, it is difficult to accurately evaluate the effect of water mobility on the seepage characteristics of clay soil. To investigate the effects of water mobility and pore microstructure on pre-Darcy flow in clay soil, nine clay samples were used to conduct constant head permeability experiments. The results showed that the seepage of water in clay deviated from that predicted by Darcy's law, and with increasing hydraulic gradient, the apparent water permeability first increased and then tended to remain constant. The porosity ϕ, average pore radius rp, sorting coefficient Sp, mean pore radius Dm, dry density ρ and tortuosity τ had major influences on the stable apparent water permeability ks. In contrast, the influences of maximum pore radius and median pore radius on ks were relatively weak. Macropores (>1.0 μm) and mesopores (100–1000 nm) were the dominant pore types in the clay and accounted for almost 80 % of the overall porosity. The water in saturated clay can be divided into four types according to interfacial forces. The results showed that weakly bound water (in the outer layer) was the main type of water in the clay. Among the four types of water, ks had the strongest correlation with the proportion of free water. The mobility of water was affected by coupling between interfacial forces and hydraulic gradients. The movable water content was closely related to the permeability of the clay.