Development of an Efficient Probabilistic Approach for Risk Assessment of Geotechnical Applications, 20-R9726

Principal Investigators
Amitava Ghosh
Fernando Ferrante
Michael Enright
 

Inclusive Dates:  07/01/07 – Current

Background - In several engineering fields, there is an increasing demand for reliability quantification analysis to address the current trend to include probabilistic considerations in design codes and standards. In particular, applications involving an excavation in a rock mass have to deal with an extremely complex material: natural rock that cannot be constructed to project requirements. In addition, properties of the rock are determined by site investigations, generally limited to a small set of samples from a few predetermined locations, which leads to considerable uncertainties in estimation. A result from this significant inherent randomness is that traditional approaches such as the factor of safety may not be generalized with sufficient confidence between different sites and applications. This study proposes to develop a probabilistic framework for reliability assessment of complex engineering problems with an application in geotechnical engineering. Therefore, by addressing a field where risk is commonly assessed using deterministic analyses and engineering judgment, this research will demonstrate the benefits of both the probabilistic approach and the use of efficient probabilistic techniques.

Approach - This study focuses on a rock slope stability model coupled with uncertainty analysis and overall system reliability modeling. Discontinuum modeling is used to simulate the response of a fractured rock mass. The rock mass is represented by an assembly of discrete blocks with fractures as the interface between adjacent blocks. Material properties, geometrical information, and constitutive laws specified for blocks and joints control the solution scheme. Traditionally, only single values of these parameters are considered in the slope stability analysis (e.g., the mean or worst-case values). However, a more complete spectrum of possible rock joint configurations and material property distributions can be achieved using uncertainty analysis. This approach will provide critical insights regarding the most risk-significant parameters, sensitivity of multiple failure modes, and quantified risk values. As the computational effort for this problem may be nontrivial, an optimal sampling method that can substantially reduce the number of simulations required to achieve a specified accuracy level will be incorporated. This method, based on system reliability principles, allocates an optimal number of Monte Carlo samples to individual failure modes based on initial estimates of the associated block failure probabilities.

Accomplishments - All the parameters necessary to describe the rock slope stability problem have been identified. A literature survey is currently being conducted to develop the appropriate range for each parameter. Measured values of parameters describing strength and stiffness properties of natural rock fractures in one type of rock (Apache Leap tuff rock) have been obtained. Literature is being searched to identify similar information for other rock types. Characterization of the variability after removing the strong influence of the normal stress is currently under way. A sensitivity analysis for each parameter on the failure of the slope will be developed using a discrete element code to identify the most important parameters. Additionally, a global mining company was contacted for a potential slope stability case study problem to be used in this project. The response was very positive and discussions are currently in progress to include higher relevance to mine applications as part of this project.

2007 Program Home