Description of Compacted sand/clay mixtures: compaction process and classification of soils.

In earthworks field, soils classifications are partly based on granulometric criteria, and in particular on the fines proportion. Current thresholds have been determined more or less empirically but the influence of the fines on the compaction process has rarely been quantitatively described. In order to palliate this lack of comprehension, compaction tests were carried out on various sand/clay mixtures. At the optimum Proctor, the apparent dry density of the fines and the apparent density of the sand fraction were calculated. The evolution of these parameters with the fines proportion shows that between 0 and 10-20 % of fines, the fines are loose and even don't fill in the voids between sand particles. In this range, the behaviour of the coarse part of the mixture depends on its particle size distribution. Between 10-20 and 35-50 % of fines, voids between sand particles are filled in by the fines and the dry density of the fine fraction increases steadily with its proportion in the mixture. At the opposite, the apparent density of the sand part of the mixture decreases. Moreover, the mixture reaches its highest optimum dry density for 20 to 30 % of fines, this value depending probably on the compaction energy. Between 35-50 and 100 % of fines, the compacity of the fines seems to be little dependent of its proportion. In this case, the mixture behaviour is controled by the fines behaviour. Thus, although sand/clay mixtures can not rigorously represent natural soils, this approach shows that, at the optimum proctor, the dry density of the fines increases with its proportion in three stages. This could explain why many classifications take two thresholds into account for the fines proportion. It also appears that these thresholds depend on the grain size distribution of the sand fraction and on the compaction energy. The comparison between these observations and the thresholds of the current classifications underlines the variety of choices in the definition of soils families. For the covering abstract see ITRD E135448.