Abstract:
Assessing long-term rock stability is an important aspect in the analysis of slopes, dam and bridge foundations, and other infrastructure. At such long time scales, rock failure occurs through time-dependent methods such as subcritical crack growth. Natural analogs, such as caves, where rock breakdown has occurred undisturbed over tens to hundreds of thousands of years, can be used to study such failure. An on-going project is reconstructing the process of natural cave breakdown at Kartchner Caverns through LIDAR scanning of the cave geometry and rockfall accumulation and 3D damage modeling of the resulting point clouds. Two modeling methods are compared in order to examine the importance of including time-dependence when analyzing rock behavior over long time scales. The first method is implemented as a fracture mechanics model through user subroutines in Abaqus. Damage occurs through calculation of decreasing rock bridge size from subcritical crack growth. The second method is implemented as a bonded particle model in PFC3D, using the flat-joint contact model for intact rock and the smooth-joint contact model for joints. Damage occurs through strength reduction of the material properties. The trade-offs in the two models (relative computational simplicity vs. more direct inclusion of time dependence) are discussed.
Introduction
Assessing long-term rock stability is an important aspect in the analysis of slopes, dam and bridge foundations, and other infrastructure. Rock behavior over tens to thousands of years must be anticipated when predicting the performance of, for example, an underground nuclear waste storage facility. At such long time scales, the time dependence of rock failure, typically ignored in short time scale analyses, has a significant effect and must be included in the analysis (EPRI, 2006). Time-dependent rock failure is thought to occur along pre-existing discontinuities through the failure of rock bridges (Kemeny, 2003). A rock bridge is a section of intact rock along an otherwise persistent discontinuity in a rock mass. Some examples of rock bridge geometries are shown in Fig. 1. The scale of rock bridges can range from centimeters to meters, but typically the rock bridge area is only a small percentage of the total fracture surface area (Frayssines and Hantz, 2009). Over long time scales, crack growth initiating at the tips of the pre-existing discontinuity slowly decreases the size of a rock bridge. Once the rock bridge size reaches zero (i.e., the former intact portion is fully cracked), the stability provided by the rock bridge ceases and the discontinuity fails (Kemeny, 2003).
