This paper presents an optimal estimation method for mixture models to describe the mechanical behaviour of (biological) materials. The a priori knowledge of the non-linear behaviour is taken as a starting point. The method determines estimates of unknown model parameters and of the displacements of a number of points in the system. The method is applied to a numerical simulation of the confined compression test. The unknown parameters relate to stiffness and permeability. In the simulation model the permeability is deformation dependent whereas in some of the identification models this dependency is neglected. It is shown that this method makes it possible to detect a structural model error with respect to the permeability.