Limestone Cave Collapse Evolution As an Analog for Long-Term Rock Behavior

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Long-term rock behavior is important for understanding the degradation and failure of rock slopes, underground excavations, dam and bridge foundations, national monuments, and other critical rock structures. Natural underground caverns are an ideal geologic analog for studying long-term rock behavior, because of the relatively constant climate and the preservation of rockfalls that accumulate on the floor of the cave over thousands of years. In this study, the evolution of rock degradation and failure in the Kartchner caverns in southern Arizona will be investigated, using a unique combination of radiometric dating and state-of-the-art geomechanics techniques. First of all, to date the evolution of rockfall that has occurred in the cave over the last 100,000 years, both U-TH and Carbon dating will be used. Because Kartchner is a live cave, speleothems occur on many of the rockfall blocks, and small cores will be taken at the base of these speleothems for dating. Secondly, a new generation ground-based LIDAR scanner will be used to obtain detailed three-dimensional point clouds of the cavern geometry and also of the rockfall fragments on the floor of the cave. The detailed point clouds along with the radiometric data will be used to reconstruct the cave geometry over the past 100,000 years. Finally, three-dimensional modeling will be conducted using the Abaqus finite element program. A time-dependent damage model based on subcritical crack growth will be implemented in Abaqus to simulate cave collapse evolution. Seismic loading of the cave will also be considered, since southern Arizona is susceptible to small to medium levels of seismic hazard.Kartchner caverns provides a unique opportunity to understand long-term rock behavior, and the results should be of great interest to rock mechanics scientists and engineers as well as karst and cave specialists. The results of this research will be used to develop and validate long-term strength models or subcritical crack growth models to correctly predict long-term rock behavior. Kartchner caverns is also an ideal platform to communicate geomechanics to the general public, with public access to 2.4 miles of passages and attracting over 150,000 visitors each year. As part of this research, an interactive geomechanics display case will be established in the Kartchner visitor center, and cave tour guides will provide geomechanics information on their daily tours. The results of this research will hopefully provide new findings in the areas of cave breakdown and sinkhole development. The results of this research might reveal the timing of past seismic events in southern Arizona, and therefore will be of interest to seismologists and also geologists studying Quaternary faulting.