The hydraulic conductivity of saturated clays is commonly determined either directly by monitoring water flux or indirectly based on Terzaghi's consolidation equation. Similar results are generally obtained from the two methods, but sometimes a significant difference can be observed, in particular for expansive soils. In this study, the hydraulic conductivities determined by the two methods are first compared based on existing data in the literature. The indirect method is then revisited attempting to explain the difference identified. A modified effective stress, considering physico-chemical interaction between face-to-face oriented particles, is finally introduced to better describe the compressibility of expansive clays and to further improve the indirect method in determining hydraulic conductivity of such soils in the low-compressibility zone. Extra tests were performed on Gaomiaozi (GMZ) bentonite slurry and the results obtained allowed the modified indirect method to be verified.