Porosity dependence of the elastic modulus of lithophysae-rich tuff: Numerical and experimental investigations

The influence of porosity on the mechanical properties of rock has received much research attention. Portions of the Yucca Mountain high-level nuclear waste repository may be placed in tuff units containing lithophysal cavities, which are large, generally noninterconnected cavities that can be considered a form of macroscopic porosity. This paper presents the results of numerical modeling and uniaxial compression testing of analog models and tuff rock, in order to assess the relationships between elastic modulus and porosity. The first part of the paper presents numerical simulation of uniaxial compression testing to calculate the elastic modulus of two-dimensional models containing randomly distributed circular holes in plane strain. The range of porosities investigated is approximately 5-40%. In the second part, the elastic modulus determined from the uniaxial compression testing of analog models and tuff specimens is presented. The analog specimens were made of plaster of Paris containing varying amounts of spherical shaped Styrofoam® inclusions to simulate a cavity structure similar to tuff. The results from the numerical analysis and analog material testing show an exponential decrease in elastic modulus with increasing porosity, whereas the elastic moduli of tuff show a linear decrease. The difference in the two behaviors can be attributed to the nonuniform cavity shapes in the tuff specimens.