August 21, 2013
Anddes Asociados S.A.C. recently dealt with a difficult project in northern Perú involving a three dimensional (3D) slope stability analysis of a waste dump. The stability analysis for this facility involved a complex mine waste rock layout and many different soil layers at the foundation, most of them affecting the waste dump stability. This problem was successfully solved by using SVSlope®3D.
Due to the mining expansion and the increment of production required by a mining company in northern Perú, the waste dump needed a redesign to get a capacity of 113 million tones (66 million of cubic meters), with an area of 118 Ha. The angle of repose of the waste rock will form a slope of 1,5H:1V, and with an overall slope of 2,5H:1V. A referential area of this facility is shown in Figure 1.
Anddes Asociados S.A.C., an innovative and rapidly growing Peruvian consulting company, was hired by the mining company to carry out the waste dump design, whose configuration is shown is Figure 2. The work included an extensive geotechnical investigation program for waste rock and soil foundation characterization. This investigation involved a large quantity of field data which included: 123 test pits, 31 dynamic penetration tests (DPL), 91 standard penetration tests (SPT), 30 large penetration tests (LPT) and 21 geotechnical boreholes among many laboratory tests needed to model accurately the soil and rock materials present.
As a result of the geotechnical investigation, it was determined that the waste dump foundation was conformed by very heterogeneous soil layers, most of them compromising the stability of this facility, so that there could be sections with high factor of safety and others nearby with poor stability conditions, as shown is Figure 3.
As a first approach, to analyze the overall slope stability of the dump, the two most critical sections were analyzed, as shown in Figure 2. Both sections represent quite different foundation conditions. For instance, one section failure was governed by soft alluvial clayey soils and the other by loose residual soils. Both sections had a 2D factor of safety lower than the minimum required by the design criteria using the Spencer limit equilibrium method.
A reliable 3D slope stability program was required that could deal with issues such as: variability of strong and weak layers in the waste dump toe foundation, actual dike geometry, and actual 3D geometry. SVSlope®3D was chosen as the modeling tool as it met these criteria.
Since the geotechnical information provided along the waste dump foundation was abundant, two models were created for the analysis. 15 geotechnical sections were developed for Model 1 and 19 for Model 2 by Anddes´ geotechnical and geological engineers. Each section was separated by 50 m. It is important to mention that both models were created around the two most critical sections previously analyzed. Figure 4 presents both areas modeled in SVSlope®3D.
All the point data contained in each section was uploaded to the program using Excel files. The program managed to interpolate over 30 sections in both models. During this process 11 surfaces were created, showing geotechnical complexity related to this case. Moreover, lenses of loose soil layers represented in the geotechnical sections were easily processed and they could be smoothly found in the models. SVSlope®3D adequately represented the 3D waste dump stacking as if it were modeled in any CAD software.
The Spencer rigorous limit equilibrium method was used as well to determine the exact relation of 2D and 3D factor of safety. The search method used to determine the failure surface was the grid and tangent method. Many analysis were performed, each one increasing the grid density in the most critical zones and using different eccentricity ratios for the ellipsoid geometry. Figure 5 shows the most critical surface found in Model 1.
The results were successful in the sense that the 3D factor of safety overcame the minimum required by the project. Moreover, the 3D factor of safety was around 10% larger than the 2D one. This can be explained by considering that the 3D analysis encounters strong layers in addition to weak layers and takes into account the 3D topography of the site, both of which contribute to the overall stability of the facility. The soil layers cut by the 3D slip surface can be seen in Figure 6.
Almost two months ago, we overcame some issues concerning cross-sectional data entry with the help of you and your development team, and this is the final result of our work. We hope we can keep working together."
Andrés Reyes Parra, Geotechnical Engineer, Anddes Asociados S.A.C., Lima, Perú