A model structural cell of special (isometric) form with singular boundary conditions is proposed and investigated within the framework of large-deformation theory for calculation of the effective elastic properties of granular composites. Effective moduli are calculated over a broad range of filler contents (from zero to 0.55), and it is shown that the agreement between calculations and experimental results is quite good. The proposed model cell appears promising for study of exfoliation of the matrix from the filler under deformation.