Time-Dependent Instability in Rock Masses: Understanding, Prediction and Prevention

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The instability of rock structures in areas of human habitat is important and is responsible worldwide for the loss of lives and large financial damages. Examples include landslides and rockfall, the collapse of underground tunnels and mine drifts, dam and bridge foundation failure, sinkholes, etc. This three-year project will investigate the effects of time on rock instabilities, and in particular, time-dependent crack growth (modes I, II, and III) leading to (or assisting with) field-scale rock mass deformation and failure. In general the time-dependent growth of tensile and shear fractures is an important and poorly-understood aspect of field-scale rock instabilities. The research will focus on the time-dependent degradation of rock masses when exposed in slopes or underground excavations, and will consider specific triggering methods for rock instability such as pore pressure or freeze-thaw as necessary. Also, the research will focus on "crack tip" rock mass degradation as opposed to the overall weathering of rock masses that may occur due to dissolution and other processes. This project consists of the following four research tasks:1. Advanced discontinuum numerical modeling. One of the key tasks will be to implement time-dependent fracture mechanics into a three-dimensional discontinuum code. This will be an extension of previous research where time-dependent fracture mechanics was implemented into a two dimensional discontinuum code (Kemeny, 2005). The time-dependent fracture mechanics will allow for time-dependent rock bridge failure and the progressive crack growth that results in the comminution of rock blocks. Overall this will allow realistic simulations of the time-dependent degradation of actual field-scale rock masses.2. Ground-based LIDAR and high-resolution digital imaging. Case studies will be conducted where LIDAR and digital imaging are utilized for detailed rock mass characterization. This also includes the use of semi-automated software for processing the LIDAR and digital imaging data. The detailed rock characterization data will provide state-of-the-art geometric and parameter information on field scale rock masses, including the characterization of rock bridges and small features that may contribute to rock block comminution. The new imaging technologies can be used to characterize pristine rock masses as well as degraded rock masses where failure has taken place. This information, along with laboratory testing, will then provide the basis for the numerical modeling described in task 1. 3. Development of tools and strategies for prediction and prevention. The results from tasks 1 and 2, along with ongoing theoretical fracture mechanics relationships, will be used to better understand and improve the monitoring of rock structures and the interpretation of monitoring results. Many techniques are now being developed to "sense" impending rock failure, such as microseismic monitoring, displacement monitoring, seismic tomography, and other techniques. The basis for all of these techniques is a basic understanding of the spatial and temporal nature of the rock failure process in rock masses. As our understanding of time-dependent rock failure increases through this work, these techniques can be improved and new techniques can be developed. 4. Dissemination of research results. Results from this research project will be disseminated in several ways, including publications and presentations, distance-delivered courses, and collaborations with industry and government and academic institutions.This work fits in very well with the proposed rock mechanics activities for DUSEL (Deep Underground Science and Engineering Lab). Collaboration will take place between this project and design, construction and operation activities at the selected DUSEL site. This collaboration may include case studies at the selected DUSEL site (tasks 1 and 2 above), the collaboration with other DUSEL projects and researchers, particularly those projects having to do with underground monitoring and the interpretation of monitoring results (task 3 above), and publishing research related to DUSEL projects and activities (task 4 above).