Two different approaches used to express the tortuosity of water pathways in a solid clay body are known: (1) a static approach based on microscopic measurement in thin sections; and (2) a dynamic approach based on ‘imbibition’ speed. Each is expressed by a corresponding equation. The second method has the following advantages over the first: (a) it enables the testing of a much larger volume than that observed in a common thin section; (b) it measures the effects of all particles, including the finest fragments barely visible under optical microscope which also have influence on hydraulic conductivity or the imbibition speed of water; (c) in the case of expansible clays, the magnitude of k (tortuosity of pathways including the shape of particles) can be compared with k for non-expansible clays using a non-polar liquid, e.g. tetraline; (d) the measurement of the imbibition speed into the clay aggregate through a constant area is automated. Specific properties and processes important for sealing clays are discussed in the connection with the dynamic approach.