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Soil Physics

Shrinkage and Subsidence in a Marsh Soil

Measurements and Preliminary Model

^a UMR HYDRASA 6532 du CNRS, Ecole Supérieure d'Ingénieurs de Poitiers, 40, Av. du Recteur Pineau, 86022 Poitiers Cedex, France
^b INRA Domaine Expérimental de St Laurent de la Prée, 17450 St Laurent de la Prée, France

^* Corresponding author (patrick.dudoignon{at}univ-poitiers.fr)

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 (_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).