E. J. Murray writes: The presence of fissures in clayey soils is rightly identified by the authors as being of major significance to the performance of clays as low permeability barriers in landfill. The hydraulic radius type of permeability equation, such as the Kozeny—Carman equation (see Murray 1995) may be used to demonstrate that the permeability is inversely proportional to the number of void openings. Thus, for a soil of given porosity, a small number of large voids, such as represented by fissures, leads to a greater permeability than a large number of small voids, as where permeability is controlled solely by inter-particle flow. The identification of ‘clod’ size as being a major factor in the presence of discontinuities or fissures goes a long way to explaining why there are often large discrepancies between in situ permeabilities and the lower values obtained from laboratory prepared samples. In this respect, it is considered important to recognize the role of the plastic limit as an indicator of whether a clayey soil, at a given moisture content, is (a) likely to have a fissured structure due to desiccation, and (b) whether it can be moulded by compaction to eliminate discontinuities such as between clods.
The plastic limit of a clay is the moisture content at which it stops behaving as a plastic material and starts behaving as a brittle material. At this moisture content, in accordance with the plastic limit test method, cracks start to appear in thin threads (3 mm) of