Experimental study on effect of hydraulic gradient on permeability of clay

Permeability is one of the important properties of soil, which is affected by many factors such as porosity, particle size and fluid properties. However, the permeability is generally considered to be independent of hydraulic gradient. By means of the newly developed rigid wall seepage device, the consolidation and permeability tests on clay samples subjected to different consolidation pressures are carried out under the condition of first gradually loading hydraulic pressure and then gradually unloading hydraulic pressure. The results show that the void ratio and permeability coefficient decrease with the increase of the hydraulic gradient for the loaded tests. It can be found that the effective stress of the samples increases after consolidation by seepage force. The theoretical analysis verifies the rationality of the trend of permeability coefficient under increased hydraulic gradient. For the unloaded tests, the void ratio keeps constant, while the permeability coefficient continues to decrease with the decrease of the hydraulic gradient. This phenomenon is related to particle plugging and weak bound water. The larger hydraulic gradient makes the weak bound water flow. With the decrease of the hydraulic gradient, the weak bound water is in the viscous state, which leads to the decrease of pore water conductivity and the decrease of the permeability coefficient. The linear fitting based on the test data shows that the loss in the permeability coefficient can reach half of the initial value.