Shrinkage and Subsidence in a Marsh Soil

Modeling the shrinkage and subsidence of soils is generally based on the laboratory shrinkage curves established from liquid state to shrinkage limit, rarely exhibited in situ. We studied the vertical behavior of clay‐dominant soils from the Marais de l'Ouest (France) in the 20 to 100% water content range. The consolidation states were quantified by recording profiles of wet density (γ b ) and gravimetric water content ( W ) down to the depth of 2.50 and 2.00 m in a sunflower ( Helianthus annuus L.) field and a grassland, respectively. Under the evident surface consolidation, a paleosol was observed at 1.3‐m depth in the sunflower field. The W profiles show two superimposed layers: in the upper layer, W increased from the shrinkage limit ( W s ) to the plasticity limit ( W p ), the W profiles bounded by the wet and dry season profiles; in the subjacent layer (100% > W > W p ), the W profiles were quite constant. The depth of W p marks the end of the downward progression of the shrinkage cracks. The properties of shrinkage were established through drying stages on intact samples. In the W s to W p domain, the linearity of the volume–water content relation allows the modeling of the interseasonal volumetric distribution of the macroporosity due to the shrinkage cracks. The preliminary model of porosity behavior proposed agrees with the two superimposed layers: the W s to W p domain characterized by isotropic shrinkage (shrinkage geometry factor r = 3), and the W > W p domain characterized by subsidence only ( r = 1).