An on-going project at the University of Arizona is using Kartchner Caverns in Benson, Arizona as a natural analog to study time-dependent rock failure with subcritical crack growth modeling. Various material properties of the Escabrosa limestone composing the caverns are required for input into the damage model. Central among these properties are the subcritical crack growth parameters n and A, which can be calculated from modes I, II, and III fracture toughness tests conducted at different loading rates. This paper presents the results of modes I, II, and III testing on Escabrosa limestone, providing the material properties necessary for the larger goal of modeling breakdown in Kartchner Caverns and applying the model to the long-term stability of rock excavations. Additionally, fracture test results are compared with a previous study by Tae Young Ko at the University of Arizona, which tested Coconino sandstone and determined that the subcritical crack growth parameters were consistent among modes. This study expands upon Ko’s work by adding the characterization of a second rock material in all three modes; preliminary results indicate that for Escabrosa limestone the subcritical crack growth parameters are not consistent among modes.
Time-dependent rock failure is an important aspect in the analysis of long-term rock stability for slopes, dam and bridge foundations, and underground storage facilities. Under short-term loading, a crack will propagate when the stress intensity factor at the crack tip exceeds a critical value: the fracture toughness. Subcritical crack growth is the propagation of a crack at values of the stress intensity factor smaller than the fracture toughness. This type of growth happens under long-term loading and is environmentally-assisted, occurring through mechanisms such as stress corrosion, diffusion, microplasticity, dissolution, and ion exchange [1, 2]. There are three modes of crack tip displacement in critical and subcritical crack growth: mode I (tension), mode II (in-plane shear), and mode III (out-of-plane shear).