Numerical and Experimental Analysis of the Effect of Faults in Open-Pit Mining Stability

Abstract

This thesis addresses two main issues in slope stability: 1) effect of faults in the large open pit mines and large rock slopes, and 2) improving stability of tailing dames using Hydro-Jex® technology.Section 1 - Effect of faults in the large open pit mines and large rock slopes: Deep open pit mines and large rock slopes expose many diverse rock lithologies and geological structures in a short period of time. Faulted rock masses reduce the integrity of slopes and can impose significant design challenges for geotechnical engineers. Numerical modeling is a powerful tool for simulating fault response. However, there are few guidelines and methods for calibrating/validating and implementing faults in a numerical model. This study presents a novel laboratory method to calibrate numerical models and highlights the challenges in simulating faults. The weak zone, ubiquitous-joint, and interface techniques are the widely-used methods in the modeling to capture fault slip mechanisms. One of the main issues in reliable modeling of faulted rock structure is the scarcity of experimental analyses in the laboratory under the controlled conditions.Moreover, a comprehensive evaluation of the effect of using the conventional fault modeling methods on the stability of rock structures is required, as well as a benchmarking between theoretical and experimental results. This research combines theory and experiment, in order to fill the existing gaps, using numerical simulation and laboratory measurements. In the numerical simulations, sensitivity and comparative analyses are carried out to investigate the stability of rock slopes on large and small scales (overall open pit slope and bench slope), and the fault zones, employing the methods as mentioned earlier in the FLAC3D software. The factor of safety of the slope is monitored upon variation of the design parameters, such as fault and rock mass mechanical properties, fault types, and modeling framework (e.g., mesh density, convergence ratio). Also, parameters such as shear displacement and shear stress are investigated to deduce the failure mechanism of the studied models. Finally, a laboratory test is performed to calibrate the modeling results and approximate the agreement between theoretical and experimental results. It needs to be mentioned that these analyses/tests are not to favor one method over the other, but rather to emphasize on pros and cons of each within the assumptions of this study. Section 2 - improving stability of tailing dames using Hydro-Jex® technology: Hydro-Jex® is a new enhanced method for heap leach treatments with significantly higher mineral recovery. In this study, the Hydro-Jex operation is case-studied at the Los Filos mine in Mexico for heap leach stability. This is pursued by numerical modeling of the heap leach pad stability and well monitoring of the phreatic surface before and after the Hydro-Jex operation. Experimental and numerical results both indicate an improvement in mineral processing under the influence of Hydro-Jex. Numerical results show an insignificant decrease and significant increase of stability during and after Hydro-Jex, respectively, while the integrity of the liner is maintained in both cases. The injection pressure is found to be negligible compared to the dimensionality of the heap and stability of the structure is not exposed to any significant risk, even up to 25% above the overburden pressure.On the other hand, the heap leach factor of safety increases after Hydro-Jex attributed to the breakdown of the build-up water solution and thus a decrease in the phreatic surface. The injection wells are suggested to be drilled in a wise manner based on accurate geophysical data; in locations where over-compaction of the heap material results in water solution build-ups and pore pressure enhancement. Compared to traditional heap leaching, the Hydro-Jex technique could not only expedite the mining process by increasing the chemical extraction, but also increase the stability of the heap by unclogging drains.