Infiltration en milieu naturel argileux interprétation physique des phénomènes

¹ Ingénieur à la Compagnie d'Aménagement des Coteaux de Gascogne, responsable de la section Hydrologie.
² Directeur du Bureau d'Etudes et de Recherches Géotechniques.

Résumé

The "Compagnie d'Aménagement des Coteaux de Gascogne" has equipped the representative Mielan basin in the Gers Departement of France with devices for flow measurement at the various stages of the water cycle with a view to the solution of water supply prediction problems solely with the aid of series of climatic data. As a means of checking water movements in the ground the basin has also been provided with a network of neutronic measurement stations for soil moisture determination and a field is reserved for tensiometric measurements. By measurement of the water in the soil the main behaviour pattern of a clay soil in the Gascony climate has been determined as regards infiltration and evaporation, which is as follows : - (i) The volume of moisture in the ground varies down to a depth of about 3.5 m underground during the hydrological cycle. (ii) The maximum variation in the amounts of water stored between the surface and a depth of 3.5 m is considerable, being equivalent to 270 mm of water, about 40-60 per cent of which is due to the surface zone (i.e. down to 60 cm depth). (iii) The surface zone becomes saturated in winter and then only discontinuously. (iv) The kinetics of wetting and drainage due to meteorological accidents occur at an extremely rapid rate. The mechanisms involved in these moisture transfers arc as follows: (i) Superficial rainfall running off under gravity or due to suction. (ii) Evaporation of soil water, also involving the above forces. (iii) Volumetric soil variations connected with piezometrie variations in the underlying confined aquifer. (iv) Moisture transfers connected with temperature gradients within the clay ground. (v) Osmosis effects connected with electrolytic concentration variations in the water in the ground, on the surface and in the aquifer. The soil volume variations connected with piezometric variations and the moisture transfers connected with temperature gradients can be ignored in the considered case. It seems, therefore, that the only possible full or at least partial explanation for the observed transfers must be suction forces. By recording interstitial pressures with the aid of pressure pick-ups and establishing relationships between capillary potential and water content, therefore, it should be possible to confirm the effects of these phenomena on the spot and the observed moisture content variations. Such measurements are in progress at the time of writing.