This paper describes the results of soil classification tests carried out on African red clay soils from Kenya and soils from the Keuper Marl formation in England. It shows that in both groups of soils aggregations of clay particles can occur and give rise to anomalous classifications of the soils when they are tested by standard procedures.

The manner in which the particles are cemented together is different for the two groups of soils. In the case of the red clay soils the aggregations give rise to low plasticity values being obtained whereas in the case of the Keuper Marl soils they give rise to low clay contents being obtained when the particle size distribution is determined by sedimentation techniques.

The reason for the aggregation in the red clay soils is shown to be the presence of free iron oxide cementing the particles together. Silica is shown to be the most likely cementing agent in the Keuper Marl soils.

Apart from the aggregation of the Keuper Marl soils there is evidence that non-aggregated clay particles in excess of 0·002 mm diameter are also present. This is an additional reason for anomalous particle size distribution results being obtained for these soils.

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