Influence of moisture content on several properties of Upper Cretaceous chalk

It is now well known that the mechanical behaviour of chalk is directly governed by its moisture content. The saturation degree induces suction and influences the constitutive law and related parameters. Besides the strict mechanical properties (strength, stiffness…), the seismic properties (seismic velocity of compression and shear waves) depend on the saturation degree of the chalk. The relationship between those various properties is also influenced by the saturating fluid (wettability, physico-chemical interactions) and by the chalk structure. This paper aims to present the so-far unpublished results obtained in the framework of Masters and PhD theses supported by the Université Libre de Bruxelles and the University of Mons. It presents the basic mechanical properties (strength) and experimental studies (methods and results) leading to a new model of wave velocity as a function of fluid content. The paper first describes the studied chalks and relates them to the general framework of the relationships between structure and behaviour. Then the general mechanical behaviour of the tested chalks is presented, including parameters values and their variations with moisture content. The third part of the paper is devoted to the analysis of the seismic wave velocities as a function of the saturation degree. Previous results are presented in introduction to the new studies. Experimental results are analysed and an original model is presented. It generalises Gassmann’s model, using a theoretical fluid with weighted average of water/air content. The model defines a critical saturation ratio for each rock which is directly linked to the shape of the porosity distribution graph.