The author has produced some useful data on the influence of aggregation on the index properties of red clays and Keuper Marl, in particular confirming that the apparently silty nature of Keuper Marl results from the aggregation of clay-mineral particles. We would however like to take up some points made by the author with reference to Keuper Marl.

One point on which we wish to comment is the assumption made by the author that in determining the liquid limit, further mechanical working beyond the 10 minutes specified in B.S. 1377 did not increase the liquid limit. This occurred in spite of an apparent increase in the measured clay-size fraction. The author further assumed that prolonged pre-treatment mixing was sufficient to break down all aggregations, leaving some of the clay-mineral fraction having particle diameters larger than 0–002 mm.

In Fig. 1 we show the variation of liquid limit with mixing time for two similar marl samples. Mixing was carried out in a Hobart paddle mixer which could take a sample of about 1/8 cu ft. It is seen that the liquid limit increased appreciably above that obtained using the B.S. 1377 method. Clearly for this particular marl the liquid limit increases with further working, though we hesitate, in view of the variable nature of Keuper Marl, to state that this will be the case for all marls. We do however question the use of the greaseworker as a substitute for mechanical working

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