Enregistrement scientifique n : 730 Symposium n : 4 Présentation : poster. BRUAND Ary, COUSIN Isabelle, BASTET Gilles, QUETIN Philippe

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1 Enregistrement scientifique n : 730 Symposium n : 4 Présentation : poster A mechanistic model for estimation of the water retention properties of clayey soils Un modèle mécaniste pour estimer les propriétés de rétention en eau des sols argileux BRUAND Ary, COUSIN Isabelle, BASTET Gilles, QUETIN Philippe INRA Orléans, Unité de Science du Sol - SESCPF, Ardon, France Introduction Earlier studies on the water retention properties of clayey soils showed that the water retained at each value of matric potential is related to the clay fabric and content (Tessier et al., 1992; Bruand, 1990; Bruand et al., 1994). These studies showed also that the clay fabric which was defined by Brewer and Sleeman (1960) as the spatial assemblage of clay particles can be expressed numerically using the pore volume associated with the packing of clay particles (Bruand et al., 1994). Bruand and Zimmer (1992) demonstrated that this pore volume increases with the cation exchange capacity of the clay particles and proposed a morphological model for the fabric of the elementary particles. A practical consequence of these studies was the establishment of pedotransfer functions (PTFs) which enable the prediction of the water retained at different matric potentials and the use of the soil bulk volume as single estimator (Bruand et al., 1996; Bastet et al., 1998). The objective of this study was to show that these PTFs which are regression equations are consistent with a water extraction process which induces a deformation of the clay fabric, the deformation intensity depending on the pore volume when the soil is near to field capacity. Material and methods The soils were located mostly in the Paris basin and developed on marl, sedimentary clays, limestones and alluvial deposits (Bruand et al., 1996). 110 subsoil horizons were collected. The clay content ranged from 30 to 98 % and the bulk density from 1.1 to 1.8. The organic carbon content ranged from 2 to 10 g kg -1 except for a few horizons which exhibited a higher carbon content. The cation exchange capacity of the clay phase ranged from 18 to 57 cmol kg -1. Undisturbed samples cm 3 in volume were collected in winter when the soil was near to field capacity (Hall et al., 1977). The samples were stored at 5 C in sealed plastic containers to avoid water loss, and clods 5-8 cm 3 in volume were separated from 1

2 them by hand. The field bulk volume (Vf in cm 3 g -1, reciprocal of the bulk density) of the clods was measured using the kerosene method (Monnier et al., 1973). Water contents (W, g of water per g of oven-dried soil) at -1, -3.3, -10, -33, -100, -330, and kpa matric potentials were measured using pressure membrane or pressure plate apparatus. Clods were placed on a paste made of < 2µm particles of kaolinite to establish continuity of water between the clods and the membrane or the porous plate of the apparatus (Tessier et al., 1992; Bruand et al., 1996). Water contents were expressed with respect to the dry mass of the sample after oven-drying at 105 C for 24h. Fifteen clods were used for each sample to determine the mean values of Vf and water retained at the different values of matric potential. Results and discussion Theory We assumed that water is extracted from pores which result from the packing of clay particles in the whole range of studied matric potential, and that the shrinkage which results from water extraction when the matric potential decreases from Ψ1 to Ψ2 can be described by the following relationship: vp2 = k(ψ1,ψ2) x vp1 [1] with vp1 and vp2, the pore volume (cm 3 per g of oven-dried soil) at the matric potentials Ψ1 and Ψ2, respectively and k(ψ1,ψ2), the shrinkage parameter from Ψ1 to Ψ2. When water is extracted from the clay fabric from Ψ1 to Ψ2, the packing of clay particles becomes closer and the pores remain saturated by water. Thus, equation [1] gives: w2 = ρ w x k(ψ1,ψ2) x vp1 [2] with w2, the water content (g per g of oven-dried soil) at Ψ2 and ρ w the mass of water per unit volume (g per cm 3 of water, usually 1 g cm -3 ). Equations [1] and [2] can be illustrated by the evolution of the clay fabric which is presented on the figure. The latter was established by refering to the studies which proposed a model for interbedded clays (Nadeau et al., 1984a and b). At high matric potential, the fabric of clay particles would be a loose packing of the elementary particles. The figure shows that when the water matric potential decreases, water is extracted and the clay particles move. Thus, there is a rearranging of the clay particles which corresponds to a new fabric. 2

3 Y 1 Ψ2 v p1 v p2 Figure: Schematic representation showing the change in the fabric of the clay particles for water extraction from Ψ1 to Ψ2. Experimental results Results showed that the water content (W) at each value of matric potential ranging from -1 to kpa was closely related to the field bulk volume (Vf) of the horizon at field capacity and maximum swelling as follows: W = (a x Vf ) - b [3] where a and b were two coefficients which decreased with the matric potential (Table 1). Vf resulted from the variation of the soil porosity alone since the particle density of the studied soils can be considered as constant. Thus, equation [3] gives: W = [a x (Vp + Vs)] - b W = (a x Vp) + (a x Vs) - b [4] with Vp, the volume of pores within the soil in cm 3 of pore per g of oven-dried soil and Vs, the volume of the solid phase in cm 3 per g of oven-dried solid phase (0.377 cm 3 g -1, reciprocal of 2.65 g cm -3 ). 3

4 Table 1. Regression equations between the gravimetric water content (W) and the bulk volume (Vf) of the horizons near to the field capacity. Matric potential (kpa) Regression equation r² n -1 W 1 = (Vf) W 3.3 = (Vf) W 10 = (Vf) W 33 = (Vf) W 100 = (Vf) W 330 = (Vf) W 1000 = (Vf) W 1500 = (Vf) Table 2. Transformation of the regression equations using the pore volume as indicated by equation [4]. W 1 = (Vp) W 3.3 = (Vp) W 10 = (Vp) W 33 = (Vp) W 100 = (Vp) W 330 = (Vp) W 1000 = (Vp) W 1500 = (Vp) The regression equations with Vf were transformed using equation [4]. The resulting equations can be written as follows: with: W = a x (Vp) + b [5] 4

5 b = (a x Vs) - b [6] b being a residual term which was positive and roughly increased with the matric potential (Table 2). Comparison with the model The equations presented in Table 2 differ from equation [2] because of the term b. This term can be discussed if we consider that the actual volume of the solid phase (Vs ) differed from Vs because of the iron content of the clay phase. The experimental relationships shown in Table 1 enabled calculation of Vs by considering: b = 0 when Vs is replaced by Vs. Thus: The calculated Vs are shown in Table 3. (a x Vs ) - b = 0 Vs = b / a [7] Table 3: Values of Vs which gave b = 0 in equation [5]. Matric potential Vs kpa cm 3 g Vs varied between and cm 3 g -1 for - 33 Ψ -1 kpa and then decreased for Ψ -100 kpa. These calculated Vs can be discussed as follows: (i) for - 33 Ψ -1 kpa, Vs was roughly constant. This would indicate that if this Vs value is reasonable, the model which was proposed can be applied. The mean value of 5

6 Vs was cm 3 g -1 and corresponded to a particle density (ρs) of 2.79 g cm -3. This value of ρs which corresponds to variable skeleton - clay mixtures is consistent with the values of ρs which were published earlier. Tessier (1984) showed that ρs of reference clay minerals ranged from 2.67 to 3.19 g cm -3 with their iron content. In addition, iron oxy-hydroxides were also associated to the clay particles within the clay phase of the studied horizons and ρs = 4.37 g cm -3 for the goethite. Consequently, the volume of the solid phase can be estimated as close to cm 3 g -1 and the model would valuable for - 33 Ψ -1 kpa, indicating that water extraction would induce an ideal deformation of the clay fabric ( W = Vp) (ii) for Ψ -100 kpa, Vs decreased and corresponded to values of ρs which are not acceptable because they are too great (ρs > 2.92 g cm -3 ). These values of Vs can be decomposed as follows: Vs = r with r, a residual term which resulted from a non ideal deformation of the clay fabric as described by equations [1] and [2]. This non ideal deformation would result from interactions between the clay particles which limit their displacement and induces the development of air volume within the clay fabric ( W Vp). The increase in r when Ψ decreased would indicate that the soil behaviour differed more and more from the model with the decrease in the matric water potential when it was smaller than -100 kpa. Conclusion Our results showed that the regression equations which were earlier established between the water content at particular values of matric potential and the field bulk volume can be related to shrinkage and consequently to change of the clay fabric. These regression equations can be used for estimation of water retention at the corresponding values of matric potential. They are consistent with a model which considers that water extraction from -1 to kpa matric potential results from shrinkage of the clay fabric without any contribution of other pores. Finally, these relationships would enable also development of a model for the water retention curve by using the variation of the parameters of these equations with the matric potential. References Bastet, G., Bruand, A., Voltz, M., Bornand, M., Quétin P., Performance of available pedotransfer functions for predicting the water retention properties of French soils. In: Characterization and Measurement of the Hydraulic Properties of Unsaturated Porous Media, October 1997, Riverside, California. Brewer, R., Sleeman, J.R., Soil structure and fabric: their definition and description. J. Soil Sci., 11: Bruand, A., Improved prediction of water retention properties of clayey soils by pedological stratification. J. Soil Sci., 41:

7 Bruand, A., Zimmer, D., Relation entre la capacité d'échange cationique et le volume poral dans les sols argileux: incidences sur la morphologie de la phase argileuse à l'échelle des assemblages élémentaires. C. R. Acad. Sci. Paris, t. 315, Série II, Bruand, A., Baize, D., Hardy, M., Prediction of water retention properties of clayey soils: validity of relationships using a single soil characteristic. Soil Use and Management, 10, 3, Bruand, A., Duval, O., Gaillard, H., Darthout, R., Jamagne, M., Variabilité des propriétés de rétention en eau des sols: importance de la densité apparente. Etude et Gestion des Sols, 3, 1, Hall, D.G., Reeve, M.J., Thomasson, A.J., and Wright, V.F., Water retention, porosity and density of field soils. Techn. Monograph No. 9. Soil Survey of England and Wales, Harpenden. Monnier, G., Stengel, P., and Fiès, J.C., Une méthode de mesure de la densité apparente de petits agglomérats terreux. Application à l analyse des systèmes de porosité du sol. Annales Agronomiques, 24, Nadeau, P.H., Tait, J.M., McHardy, W.J., and Wilson, M.J., 1984a. Interstratified XRM characteristics of physical mixtures of elementary clay particles. Clay Miner., 19, Nadeau, P.H., M.J. Wilson, W.J. McHardy, and Tait, J.M., 1984b. Interparticle diffraction: a new concept for interstratified clays. Clay Miner., 19, Tessier, D., Etude expérimentale de l organisation des matériaux argileux. Hydratation, gonflement et structuration au cours de la dessiccation et de la réhumectation. Thèse d Etat, Univ. Paris 7, 361p. Tessier, D., Lajudie, A. and Petit J.C., Relation between the macroscopic behavior of clays and their microstructural properties. Applied Geochemistry, Supplementary Issue No 1, Key words: mechanistic model, clayey soils, water retention, clay content, mineralogy, porosity, clay particle Mots-clés : modèle mécaniste, sol argileux, rétention en eau, teneur en argile, minéralogie, porosité, particule d argile 7