Engineering properties of rock have been shown to be influenced by defects including porosity (Talesnick et al., 2001, Avar et al., 2003, Gates, 2008). Strength and stiffness of rock is affected by macroporosity as demonstrated in previous studies (Avar et al., 2003, Gates, 2008, Al-Harthi et al., 1998). The quantified effects of macropore spacing on the unconfined compressive strength of synthetic rock analog material and the effect of macropore spacing on failure mode are described in this investigation. Fifty-four 4” cubic specimens made of HydrocalTM and Plaster of Paris were tested in unconfined compression for strength, and the failure mode was observed. The cubic specimens have cylindrical voids extending from the front of the specimen through the back. The laboratory results are used as validation of 2D numerical simulation of unconfined compression testing of square specimens with circular holes. Strength data appear to fall between two bounds: an upper bound that displays increasing specimen strength as distance between macropores increases, and a lower bound that suggests decreasing strength followed by increasing strength as distance between macropores increases. There is also a trend observed in the failure mode of specimens, showing that as macropore spacing increases, the failure mode changes from tensile cracking to tensile cracking accompanied by shear failure. At the minimum macropore spacing the macropores act as a single “mega-macropore” and at maximum macropore spacing peak strengths are relatively high and specimens fail as macopores react to loading individually.


Understanding the effect of macropores on the engineering behavior of rock is important to practical engineering applications. Macroporosity has been shown to influence strength and stiffness of rock as demonstrated in previous studies (for example Avar et al., 2003, Gates, 2008, Al-Harthi et al., 1998). In general, as rock porosity increases, strength decreases (Goodman, 2003, Sowers, 1996). Investigating the influence of macroporosity on rock behavior is not just an academic endeavor; a significant construction project which encounters macroporous lithophysal tuff, is that of the Yucca Mountain deep geologic disposal for nuclear waste (US DOE, 2003). Buildings can settle as the result of compaction if constructed over porous media, such as that of the Miami Limestone (Sowers, 1996). In the limestone of Kuala Lumpur, Malaysia, the installation of foundation pilings can create the potential for failure if pile foundations rest on macroporous rock cavity roofs (Tan, 2004).

It is often assumed that microscopic porosity is a uniformly distributed characteristic of rock. There is a positive linear correlation between increasing percent porosity and homogeneity of void space distribution. Uniform loading of specimens with microscopic pores creates stress field interactions that consistently produce smooth shear failure curves. Increases in microporosity are indicated by decreases in Young’s modulus and specimen strength. The failure of macroporous rock is less predictable. Void space distribution of macroporous rock is typically non-homogeneous. This change in void space distribution accompanied by an increase in the ratio between void size and specimen size from the microscopic to the macroscopic condition produce non-uniform stress field interactions.

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