Proposal for a New Classification of Common Inorganic Soils for Engineering Purposes

This study proposes a new soil classification based on the different behaviour of two fractions that can generally be distinguished in clayey soils: the “active, binder” fraction composed of clay minerals (conventionally constituted of particles less than 0.002 mm, termed clay fraction) and the “inert” fraction composed of non-clay mineral particles (greater than 0.002 mm). Apart from the soil stress history, the clay percentage (and the type of clay minerals it contains) is of primary importance for the mode of soil behaviour. Hence, inorganic soils are assigned into soil groups based on their clay percentages. Each group includes soils with similar intrinsic geotechnical properties. In describing the soil groups, the terms “grainy” and “fine” are used to distinguish soils with behaviour dominated by the characteristics of their granular and clay phase constituents, respectively. Between these, lie soils (semi-grainy and semi-fine) that are “transitional” between grainy and fine soils, with an intermediate behaviour determined by the increase in the clay fraction. Particularly for common inorganic soils with behaviour dominated by the clay phase, both the soil liquid limit and the clay percentage allow to estimate some soil properties such as the plasticity index, clay activity, intrinsic compressibility and residual shear angle.