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Table 7.2: Factors of safety and failure type for a typical tailings dam downstream slope using varying assumptions on the foundation properties and apparent cohesion. Foundation type No foundation 2 m clay 2 m alluvium 1 m clay and 1 m sandstone 2 m sandstone Foundation properties Ïâ² (Â°) Î³ (kN/m3) - 30 28 - Earth fill cÊ¹ (kPa) Factor of safety 5 2.279 Shallow toe circle 0 2.082 Shallow toe circle 5 1.914 0 1.846 5 1.826 0 1.759 19 18 30 19 5 2.052 36 21 0 1.901 5 2.179 0 2.070 36 21 Failure type Deep toe circle through foundation Deep toe circle through foundation Deep toe circle through foundation Deep toe circle through foundation Deep toe circle tangential to sandstone Deep toe circle tangential to sandstone Deep toe circle through foundation Deep toe circle tangential to foundation When the foundation was not included in SLOPE/W, the software has assumed a stiff stratum, causing a shallow toe failure arc and relatively high factor of safety. This was also evident where the clay and alluvium foundations had failure surfaces tangential to the base of the assumed two metre thick foundation (since the foundations were weaker than the earth fill). This may not be representative of a real failure surface since the properties and depth of the foundation are unknown. If the foundation properties at more locations beneath the embankment and soil properties below the foundation were known then a more realistic failure surface could be determined. If foundation properties are not known then an assumption should be made for analysis purposes, if using SLOPE/W, to avoid unrealistic factors of safety being calculated. The weathered sandstone had the densest bulk unit weight of all materials in the case study embankment, it acted as firm stratum below the embankment and so the critical failure surface did not penetrate through it for the cases of 1 m clay and 1 m sandstone, and 2 m of 83