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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

View larger version (114K): [in a new window]   Fig. 1. (a) Prismatic structure of the consolidated Bri induced by the shrinkage cracks; the ancient cracks are filled with recent clay-rich material deposit (depth of 0.50 m); (b) contact between the beige Bri and the black and organic paleosol. The vertical cracks crosscut the paleosol level (depth of 1.30 m).

View larger version (30K): [in a new window]   Fig. 2. Schematic representation of (a) the volumetric shrinkage of the unitary prism (a = prism side dimension, h = prism height dimension, a and h = decrease of prism dimensions), and (b) crack opening (horizontal shrinkage) and subsidence (vertical shrinkage) phenomenon in the grassland field. Layer 1 = solid state (W [gravimetric water content] < Wp [plasticity limit]), crack opening and subsidence operate simultaneously. Layer 2 = plastic to "pseudo liquid" state (W > Wp), only subsidence operates (b = wet density of the clayey matrix, = microporosity of the clayey matrix, fracture area = horizontal shrinkage (cracks) at the soil surface, a1/3 = calculated aperture of crack, WSTS = wet season topographic surface, DSTS = dry season topographic surface).

View larger version (20K): [in a new window]   Fig. 3. Scheme of the shrinkage lines of the fraction <400 µm (Circle 1 to Circle 2) and shrinkage lines including the volume of cracks (Vc): hatched area = Vc; dry and wet black points = state of the soil at surface (depth = 0) for the dry and wet seasons, respectively; r = shrinkage geometry factor; Circle 1 = initial high water content sample; Circle 2 = zero water content sample.

View larger version (17K): [in a new window]   Fig. 4. Representation of (a) the shrinkage curves, in the void ratio–water content (e–W) diagram, of 1.65- (), 1.80- () and 1.96-m-deep (•)intact samples (lozenges = fraction <400 µm, full straight line = e linear regression calculated in the 20 to 40% W domain, ZS = zero shrinkage, RS = residual shrinkage, PS = proportional shrinkage, SS = structural shrinkage); and (b) height–water content of the intact samples (full curved line = polynomial height calculation according to the isotropic shrinkage, Ws = shrinkage limit, Wp = plasticity limit).

View larger version (34K): [in a new window]   Fig. 5. Vertical profiles of (a) wet density and (b) gravimetric water content in the sunflower field (P1) recorded in July 2003, January 2004, and March 2004, and (c) wet density and gravimetric water content in the grassland field (P2) recorded in February 2003, July 2003, March 2004, and May 2004 (straight line = simplified dry water content profile, dashed line = simplified wet water content profile).

View larger version (18K): [in a new window]   Fig. 6. Vertical profiles of (a) the volumetric shrinkage (% v/v) between the wet and the dry water content profiles, and (b) the half crack width (cm) calculated by difference between the wet and dry water content profiles of grassland (prism side a = 7 cm) and sunflower (prism side a = 13 cm).

View larger version (16K): [in a new window]   Fig. 7. Vertical profiles of (a) the volumetric shrinkage (% v/v) for the wet and dry water content profiles referring to the plasticity limit (Wp) in the grassland field, and (b) the half crack width (cm) calculated for the wet and the dry water content profiles referring to the plasticity limit (Wp) in the grassland field (prism side a = 7 cm).