Two kinds of improvement of soil properties are dealt with. To simulate the mechanical stabilization, clay was mixed with sand. Specific (unstable) behaviour was observed at the binary system (sand: clay = 1: 1) at low shearing stress level which is explained by macroporosity. Critical ratio sand: clay = 5: 1 was found when the increase of sand contents did not affect the behaviour. Chemical improvement was modelled by mixing sand with portland cement (2 % to 8 % by weight). A bonding surface was defined. The extent of debonding may be described by a ductility index. Brittle shear behaviour is typical for bonded sand, ductile behaviour (loose sand was used) for debonded sand. The model material (sand + cement) depicts well the behaviour of naturally cemented soils where the cementation is, however, nonhomogeneous as a rule.
For the ground improvement, the treatment of structural units and the increase of bonding seem the most viable. Both interventions are, to some extent, interrelated: by implanting some additional bonds, the granulometrical composition should be changed. The nature of structural units may be affected by mixing fine soil (clay) with coarse soil (sand). Such a mixture may reproduce the whole spectrum of soils encountered in situ – sandy clays, loams, clayey sands. Clay-sand mixtures thus deserve wider attention than merely as a procedure of mechanical soil improvement. The last sentence equally applies to sand-cement mixtures (chemical improvement). They may be considered as a way of soil properties improvement but also as a model of structural bonding.
Clay-sand mixture was produced by dry mixing of powdered clay and dry sand in the proportions S:C = 0:1 to 1:0. Theory of such mixtures was developed by Voight and Reuss (Omine et al., 1998).