Effective stresses and estimations of the apparent Biot coefficient in stacked clay layers

In materials with nanometric pores, such as clays, confinement effects alter the way the pressure of fluids is transferred to the solid. Even for liquid-saturated porous materials with poorly compressible solid skeleton, this can translate into apparent Biot coefficients (defined as the partial derivative of the stress orthogonal to the clay layer with respect to the bulk pressure of water at fixed volume) of adsorbing materials being higher than 10 or even negative. In this paper, molecular simulations of saturated clay layers are presented. From these simulations, the apparent Biot coefficient is derived as a function of the basal spacing. It is observed that the strains of an infinite stack of clay layers are generically not governed by the Terzaghi effective stress; but apparent Biot coefficients remain in the range 0-1 for stable basal spacings larger than 1.5 nm. For the sake of comparison, results of molecular simulations of a confined Lennard-Jones fluid are also provided. In this case, Biot coefficients vary from a negative value up to more than 15 according to the pore size.