Consolidation and deformation analysis for the stability assessment of a heap leach pad

The rapid growth of mining operations in Peru has brought many new challenges to the already difficult task of designing, building and operating heap leach pads in the aggressive Andes terrain. Consultants usually deal with delicate issues such as specific construction techniques, complex terrain geometry, high seismic activity, and limited suitable locations. This paper presents a case study of a heap leach pad designed on top of an existing mine waste dump, which itself was built over a heterogeneous and complex deposit of alluvial and residual soft clayey soils. Among other geotechnical issues, it was necessary to assess the short-term stability of both structures, as well as determine the settlements due to consolidation of foundation soils and deformation of mine waste, which threatened the overall stability and operation of the heap leach pad. Using a large set of geotechnical information, several cross-section, finite-element staged construction analyses were carried out to estimate consolidation times and settlements. This evaluation led to a detailed construction schedule to allow pore pressure dissipation and use of buttresses for early stages of construction, and to include precambering to guarantee both stability and optimal operation conditions, given the mine waste settlement and its influence on the liner system. The analysis and design showed the importance of the assessment of both short-term stability and deformation of soils. Furthermore, the construction schedule for both structures, which ensured their short-term stability, highlighted the importance of planning and coordination between the designers and mine operators. Introduction Since the beginning of the 21 century, the mining industry in South America, particularly in Peru and Chile, has experienced rapid growth. Most of the mine sites in Peru are located in the Andean region, above 2,500 m.a.s.l. , where the geological conditions allow mining companies to extract gold, copper and other minerals, making the Peruvian mining industry one of the top gold, silver and copper producers in the world. Recent development of mining operations in Peru has been related to extensive use of the heap leaching technique. Since 1998, when the Pierina heap leach project was constructed, major heap leach HEAP LEACH SOLUTIONS, 2015 ● RENO, USA 136 pads (HLP) have been designed, constructed and operated in the Peruvian Andean region. However, in the last five years several issues have arisen that have complicated the civil and geotechnical design, as well as the construction work. Special construction techniques and criteria for the use of geomembranes, geosynthetics and earthworks have been addressed by César et al. (2013), Garay et al. (2014) and César et al. (2014). The limitations of the 2-D approach and the advantages of 3-D slope stability analysis due to the aggressive Andes terrain have been discussed by Reyes et al. (2014). However, issues such as the need to design HLP on poor foundation areas given the lack of adequate terrain, or even on top of other earth structures, have not been extensively analyzed. Moreover, the increasing need for larger and higher HLP has caused geotechnical designers to evaluate the resistance of both leach ore and liner material interface for confining pressures from 1 to 3 MPa, thus highlighting the need for larger and more powerful laboratory devices. Finally, the high seismic activity in Peru, which is generated by subduction of the Nazca plate beneath the South American plate, almost always defines the design of most earth structures. HLP are particularly sensitive to seismic forces due to the economic and environmental consequences of leakage of pregnant solution through tears in the geomembrane that can be caused by seismically induced permanent displacements. This paper presents a case study of a HLP that, given the lack of an adequate location, was designed on top of a mine waste dump (MWD) that itself was built over a heterogeneous and complex deposit of alluvial and residual clayey soils. Challenges such as a staged construction plan due to consolidation of foundation soils, estimation of the settlements of mine waste resulting from the ore loading, and related geotechnical analysis are explored in detail. Case study general background The case study presented is a HLP design project, located in northern Peru, developed for one of the most important gold mines in the region. A new HLP with a capacity of 40 Mm and a maximum height of over 100 m was needed in the short term for the future development of the mine. However, there were limited suitable locations, forcing the designers to place it on top of an existing MWD. This facility was also in design stages: phase 1 was under the last stage of construction and phase 2, with a maximum height of 140 m, was still in design. Previous geotechnical investigations had shown that the foundation of phase 2 was composed of large and heterogeneous deposits of alluvial and residuals soils that were over 80 m deep. Clayey, silty, sandy and gravelly soils were distributed over the area; consequently, settlements due to consolidation of these soils, as well as settlement caused by the deformation of the mine waste, were expected. Preliminary stability analysis showed that for short-term conditions, which included undrained resistance of clayey soils, phase 2 of the MWD and the HLP were unstable. It was decided that, in permanent coordination with the mine owner, a staged construction analysis was needed to develop a detailed construction plan for both structures. Thus, complementary site investigations as well as HEAP LEACH SOLUTIONS, 2015 ● RENO, USA 137 laboratory tests were carried out to define in detail the geotechnical properties required for the shortterm stability assessment and staged construction analysis. The long-term stability condition of both structures, particularly when including seismic forces, is dealt with by Reyes et al. (2015). Geotechnical site investigations A large complementary geotechnical site investigation program was carried out in order to properly determine the spatial variability and geotechnical properties of the foundation soils. Previous site investigations and stratigraphy were described by Reyes and Parra (2014). A total of 210 test pits, 41 dynamic penetrations tests, 115 standard penetrations tests, 65 large penetrations tests and 26 geotechnical boreholes were included within an area of over 1,000 Ha. Laboratory tests included a total of 6 undrained (UU), 14 consolidated undrained (CU) and 7 consolidated drained (CD) triaxial tests, as well as one-dimensional consolidation and permeability tests on most soils. A range of geophysical investigations was also performed. All of this information allowed the authors to describe and discretize the foundation soils, mine waste and leached ore in more detail. Foundation soil stratigraphy and properties for phase 1 of the MWD were described by Reyes and Parra (2014). However, foundation soils of phase 2 of the MWD proved to be softer and/or looser and more heterogeneous, and required more and longer geological-geotechnical cross-sections and additional laboratory tests to properly describe them. For the particular case of residual and alluvial clayey soils, laboratory tests were performed on undisturbed samples obtained from large test pits and boreholes. The particle size distribution (PSD) of mine waste and leached ore was determined in the field by several excavations in both the existing MWD and HLP, respectively, determining maximum sizes of over 12 inches for both materials. In each excavation, almost 1 ton of mine waste or leached ore was evaluated, first weighting the whole sample and then separating particles with sizes smaller than 3 inches. Using large sieves, as presented in Figure 1, the PSD of all material larger than 3 inches was determined in situ. Complementary laboratory tests on the smaller size particles completed the global PSD curve. Figure 2 shows the average global PSD of both mine waste and leached ore. Due to the large size of mine waste and leached ore particles, two parallel gradation curves to the original PSD curves were developed for each material, which are shown in Figure 2, using a maximum particle size of 1 inch, so that no scalping would be needed when tested on a 6 inch diameter drained triaxial test. The technique used to develop the curves was first developed by Lowe (1964), and then extensively used by Marachi et al. (1969), Thiers and Donovan (1981) and Varadarajan et al. (2003) to perform drained triaxial tests on rockfill, crushed rock and alluvial soils. In the last decade, many researchers, particularly Gesche (2002), De La Hoz (2007), Dorador (2010) and Ovalle et al. (2014), and practitioners such as Linero et al. (2007) and Palma et al. (2009), have used this technique when testing alluvial and waste rock materials. The laboratory tests performed for this case study were used HEAP LEACH SOLUTIONS, 2015 ● RENO, USA 138 to obtain the shear strength properties for phase 1 and provide representative stress-strain curves for the numerical models. Figure 1: In situ determination of the global particle size distribution of coarse granular materials Figure 2: Global and parallel particle size distributions of the mine waste and leached ore Geotechnical properties Table 1 and 2 show all materials involved in the short-term analysis of the HLP and their geotechnical properties for both limit equilibrium and finite element analyses, respectively. Since translational failures through the liner system of the HLP were not critical and were not related to the short-term behavior of foundation soils, their properties were not included in this paper. HEAP LEACH SOLUTIONS, 2015 ● RENO, USA 139 Table 1: Geotechnical properties for short-term limit equilibrium analysis Materials Total unit weight (kN/m3) Saturated unit weight (kN/m3) Drained shear strength Undraine