J. S. Younger writes: It is interesting to note a re-emergence of interest in the permeability properties of compacted clayey type soils from the earlier work carried out in the late 1950s and 1960s. A large programme of work was undertaken at that time at the University of California into a wide range of properties of compacted fine grained soils under the leadership of the late H. B. Seed and J. K. Mitchell, the authors making reference to the early work on permeability (Mitchell et al. 1965). Much of the interest nowadays has come about due to the need for environmental protection against seepage of contaminated fluids at landfill sites, a key element being the design and performance of compacted clay liners. Some of the authors' references underline this point.

All the work at Berkeley was carried out at full saturation conditions, using a back pressure system in the apparatus designed for the research. I would appreciate the authors' comments on the influence of saturation conditions in their tests undertaken using a falling head permeameter. Barden et al. (1969) indicated significant increases in permeability as saturation increased.

In the second phase of the Berkeley research, work was concerned with influence of hydraulic gradient on the permeability of compacted fine grained materials. It was recognized that there was a significant difference in order of magnitude between normal test procedures and those pertaining in the field, where hydraulic gradients are low. This point is recognized by the authors. In the 1960s there

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