Structural characteristics of clay-dominated soils of a marsh and a palaeosol in a crossed diagram

Although the shrink-swell phenomenon of clays has been thoroughly studied, the in situ relation of the shrinkage curve to the structure profile is rarely presented from the shrinkage limit to the liquid limit. We studied the consolidated structure of clay-dominated (<2 μm) soils formed on 'pseudo-liquid' marsh sediments in the 'Marais de l'Ouest' (France). The profiles were studied in a grassland field and in a sunflower field from unsaturated surface soils down to deeper, saturated, levels characterized by a very large water content (100% by weight). The consolidation states were quantified recording cone resistance (Qd) profiles using a dynamic penetrometer in successive seasons. These Qd profiles were compared with the associated wet density and gravimetric water-content profiles. Two consolidation depths were evident, the surface soil and a 130-cm deep palaeosol. The seasonal Qd profiles demonstrate the partial irreversibility of the consolidation peaks associated with the surface soil and with the palaeosol. The shrinkage properties were established through drying curves of undisturbed test samples. In the void ratio (e)-water content (W) and water content (W)-saturation index (Sr) diagrams, the profiles as a whole exhibit only one clay soil behaviour from their pseudo-liquid to plastic to solid states. Each Qd profile is represented by a hyperbolic curve in the e/Qd diagram. Represented in a (e - W - Qd --Sr) crossed diagram, the vertical evolution of the successive profiles shows the soil structure behaviour from the initial pseudo-liquid sediment to the consolidated soil. A simple cone resistance recording associated with gravimetric water-content profiles, characterizes the evolution of structural layers of soils for the seasonal drying-wetting cycles, for the over-consolidation associated with the palaeosol, and also for the effect of ploughing.