Sensitivity and probabilistic slope stability analyses to understand sources of geotechnical uncertainty in Open Pit Mining
Creators: Velarde Herencia, Gustavo A
Publisher: name: University of Alberta Library
Year: 2024

<p>The open pit mining industry is an essential sector around the globe, playing a crucial role in energy production and the development of renewable energy technologies. To this end, open pit mining is increasing efforts to implement best engineering practices to develop sustainable mining that aims to balance economic benefits with safety and environmental aspects. This balance is currently assessed through open pit slope design that seeks to implement reliable, cost-effective, and safe slopes that meet the needs of the operator and other stakeholders. Current practice of open pit slope design encompasses slope stability analyses and the adoption of Design Acceptance Criteria (DAC). The results of the slope stability analysis are compared against the selected DAC to decide whether the slope design can be implemented or if modifications are required to meet the selected DAC. Furthermore, this decision-making process is influenced by uncertainties, both epistemic and aleatoric, that can potentially lead to an increased risk for the operator. <br>
Different sources of uncertainties associated with the design of open pit slopes have been identified and classified in geotechnical literature. In a reliability-oriented approach, these sources of uncertainties are formally managed by adopting sophisticated monitoring technologies, modelling techniques, and leveraging enhanced knowledge derived from experience in the open pit operation. <br>
This approach outlines the reliability level in slope design to be leveraged or increased for future design phases. Consequently, reliability analyses are becoming increasingly important in slope designs, which also requires the adoption of design acceptance criteria that accurately reflect the reliability level. To this end, Reliability-Based Design Acceptance Criteria (RBDAC) were proposed by Macciotta et al. (2020). The 2020 RBDAC adopt the concepts of reliability and slope stability approaches to meet the economic risk appetite in slope design. However, the 2020 RBDAC need testing against different reliability levels to validate the assumptions behind these. This thesis aims to test assumptions behind the developed 2020 RBDAC and to demonstrate its practicability and flexibility through analyses conducted in an open mine sector under specific site conditions.<br>
The thesis is structured as a paper-based thesis. An introductory chapter presents the knowledge gaps and objectives of the thesis. A succinct literature review is then presented to cover materials essential for understanding the work in the following chapters but avoiding repetition with the literature review in the subsequent paper-chapters. The research then followed with a parametric study to characterize the uncertainty of the rock mass strength properties and of the geological discontinuities under three scenarios with different reliability levels targeting the 2020 RBDAC. The research also focuses on uncertainty associated with modelling geological discontinuities for different reliability levels.  The uncertainty of the rock mass strength properties was defined by Probability Density Functions (PDFs) while the uncertainty of the geological structures was defined through kinematic analysis. These were input parameters for performing probabilistic slope stability analyses using 2-dimensional Limit Equilibrium (LE) method and Monte Carlo technique. The results obtained from the three scenarios in terms of Factor of Safety (FoS), Probability of Failure (PoF), and the associated Coefficient of Variation (COV) of the resultant FoS (COVFoS) match the ranges defined in the 2020 RBDAC. The flexibility of the 2020 RBDAC is also demonstrated with a slope design that compares a proposed pushback leveraging the information analyzed and compared with a design that targets the current DAC proposed by Read and Stacey (2009). The results show a decrease in mining additional volume while adopting a safe design.<br>
The other aspect of the work was to investigate the impact of uncertainty associated with fracture normal stiffness. To this end, sensitivity analyses were performed using a continuum model and discontinuum model that test the epistemic uncertainty and its impact on the slope design. This impact is observed in the results of FoS or Strength Reduction Factor (SRF), suggesting that the fracture normal stiffness may act as possible trigger for progressive failure by decreasing the magnitude as a result of the inherent excavation process and low stress environment generated by the excavation process. The probabilistic and sensitivity analyses constitute powerful design tools for managing uncertainties and implementing reliable slope designs. These tools can be leveraged in the slope design process along with the 2020 RBDAC.  </p>