Non-darcian flow in loess at low hydraulic gradient

Abstract

Determining the mechanism of water flow in loess involves many problems that are vital to science and engineering. Water flow through loess has usually been assumed to follow Darcy's law, but this has not been properly validated. The main objective of this study is to validate the applicability of Darcy's law for loess. For this purpose, we conducted constant-head permeability experiments and mercury injection porosimetry (MIP) on loess specimens of different dry densities. The results indicate that Darcy's law is not suitable for describing water seepage in loess at low hydraulic gradient, since there is a threshold hydraulic gradient (i0) below which no flow is observed and a critical hydraulic gradient (icr) below which the relationship between the hydraulic gradient (i) and seepage velocity (v) is non-linear. The MIP results show that variation in hydraulic conductivity is closely related to the pore size distribution (PSD) in loess specimens. i0 is positively correlated with the content of pores with diameters in the range of 0.01–1 μm and negatively correlated with the content of pores with diameters larger than 3 μm. The existence of bound water on loess is the main reason that its permeability deviates from Darcy's law. The mechanism of water movement in loess can be fully understood by considering the effect of bound water.