As a result of a mining expansion, an underground and open pit mining project located in Bolivia has required utilization of heap leach pad sites other than natural ground. Then, the heap leach pad with a crushed and agglomerated ore was proposed to place over the surface of unconsolidated mine waste material 55 m thick which fill an existing open pit.

Due to the loose nature of the foundation material and existing long holes from underground mining, there is concern about probable settlements that may compromise the serviceability of the leach solution collection system and the integrity of the liner system. In order to determine the shear-strength/elastic parameters (critical for settlement analysis), a comprehensive geotechnical site investigation program was carried out. The geotechnical investigations included a range of geophysical investigations (MASW, MAM, seismic refraction and electrical resistivity testing) aside from several boreholes.

Geotechnical investigations indicated that the material that comprises the foundation is a coarse material (10 m depth) and a fine material (45 m depth). Since the coarse material contains particles sizes up 200 mm (8”), laboratory tests using the fine fraction were carried out applying the parallel gradation method (Lowe, 1964). This methodology is intended to achieve that shear-strength/elastic parameters be as closely as possible between parallel gradations regardless the maximum particle size.

This paper presents the immediate settlement analysis of a heap leach pad founded on an unconsolidated mine waste material with the FEM (Finite Element Method). The behaviour of the materials was modelled using both linear elastic perfectly plastic Mohr-Coulomb model as well as Hardening Soil model.

Total and differential settlements over the liner system were calculated. To evaluate and compare the abilities of LLDPE geomembrane to withstand differential settlements the co-energy concept was applied (Giroud, 1995). The initial and the final grading of the liner system are presented. Finally, a grading design plan addressing settlement was proposed, thus improving solution recovery.