Natural materials, called "soils" in the field of road constructions, are judged suitable or unsuitable according as they satisfy or not specifications. Since specifications for the selection of natural materials to be used in embankment construction change considerably according to the country and to the moment, the same soil can be judged suitable or unsuitable. As for as soils are concerned, only very few concepts are widely accepted. At a given compaction energy, for different water contents, a Proctor curve describes well the mass of volume of a soil, while a CBR curve (California Bearing Ratio) describes well its stability. The CBR is not a mechanical property, though for its determination, loads and deformations are measured. The reference compaction energy changes from country to country, but until now on sites it was supposed that the higher the compaction energy of a soil, the higher would be its CBR. On the contrary on many sites it has been found that beyond a certain compaction energy the CBR decreases. Such behaviour was known since long ago: the CBR-curves of a soil traced for different energies intersect themselves. Beyond the water content marking the intersection point the higher CBR is found on the curve traced for the lower compaction energy. Therefore modified Proctor energy doesn't always bring to better mechanical properties of soils than standard Proctor energy. Especially for fine-graded soils the best mechanical properties may be better achieved adopting standard Proctor energy. Soils for the construction of road embankments are selected until now on the base of classification criteria, i.e. on the base of grain-size distribution (a geometric property) and plasticity (a property which defines the aptitude of a soil to be modelled in a wider or narrower range of water content). Grain size-distribution and plasticity have no relation with the mechanical properties of soils. Therefore the compliance of a soil with classification based criteria does not involve its performance for road construction. Finally it is proposed to abandon soil classification and to select soils for road construction according to a CBR based procedure which chooses compaction energy and compaction water content range to achieve good mechanic and volume stability. This procedure should allow the use of almost every soil for embankment, avoid the necessity to take in consideration "materials not compliant with specifications", allow mechanical equivalence of natural and non-natural materials. For the covering abstract see ITRD E135448.
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