In recent years, the mechanics of porous media has been a subject of intensive study. In Ref. 106, Goodman and Cowin established a continuum theory of granular materials with interstitial voids. The basic concept underlying this theory is that of a material for which the bulk density is written as the product of two fields, the density field of the matrix material and the volume fraction field. This representation was employed by Nunziato and Cowin [258] to develop a nonlinear theory of elastic materials with voids. The intended applications of the theory are to elastic bodies with small voids or vacuous pores which are distributed throughout the material. The linear theory of elastic materials with voids has been established by Cowin and Nunziato [57]. The theory of porous elastic materials was explored in various papers (see, e.g., Refs. 25 and 304, and the literature cited therein).