Strength and failure mode as a function of macropore spacing: Experimental and numerical investigation

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 Hydrocal™ 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.