Compressive strength and failure modes of lithophysae-rich Topopah Spring Tuff specimens and analog models containing cavities

The presence of lithophysae in some units of Topopah Spring Tuff at Yucca Mountain, Nevada, the U.S. high level nuclear waste repository, have a detrimental effect on the engineering properties of the rock mass and its performance. The lithophysae were formed by pockets of gas trapped within the falling volcanic ash that formed the tuff units. The porosity associated with the lithophysae is termed macroporosity because of the large pore size as compared with traditional rock pores. In this paper, lithophysae-rich tuff and analog models (both cylindrical and cubic) made of plaster of Paris containing artificially created cavities were tested to assess the effect of macroporosity on both the uniaxial compressive strength and failure modes of the specimens. As expected, compressive strength decreases with increasing porosity due to lithophysae in tuff and cavities in plaster analog specimens. Failure modes of cylindrical specimens were also investigated. The failure modes observed were grouped into four distinct categories: spalling, axial splitting, shear failure and web failure. The failure modes transition from spalling through web failure as the percentage of macroporosity within the specimen increased.