PHYSICAL AND CHEMICAL VARIATIONS WITHIN PRISMS OF A SALINE POLDER CLAY IN RELATION TO SEASONAL-CHANGES

The prismatic structure and soil behaviour were studied on a saline cl ayey soil in the polder of the Marais de l'Ouest (France). Three profi les with different histories were examined, and the structure under we t and dry conditions, ionic concentrations, and changes in void ratio and water suction were measured on prisms sampled between 5 and 30 cm deep, in four places (base, centre, top and side). Seasonal dynamics w ere accompanied by reorganization within the prismatic structure, chan ging both the size and distinctiveness of the blocky substructure. Che mical and physical characteristics were closely related to the sharpne ss of these two imbricated structures. In the absence of blocky substr ucture the salinity increased horizontally from the centres of the pri sms to their sides (the latter acting as exchange surface between the soil and the atmosphere), and no physical gradient was apparent. These results emphasized the permanence of lateral hydraulic continuity in individual prisms. On the ether hand, where the blocky substructure wa s well developed there was no lateral variation of salinity within the prisms, whereas the swelling potential varied from the centres to the sides: the sharpness of the substructure therefore caused a decrease in lateral water transfer. The internal structure of the prisms appear ed to determine the seasonal behaviour of the soil.