Original EA Amendment

12 February 2020

20137096-009 Rev A

Parameter

Design Input

Reference

Finger blanket underneath TSF embankment, and seepage collection trenches located along the outer embankment toe-line

Seepage drainage

Golder

Embankment zoning and provision of high seepage discharge chimney

Internal drainage

Golder

TBC and to be dependent on the water level from the PIT

Depth to original groundwater

Clearance required

About 3 m to be excavated within the footprint of the embankment to RL 272 m No allowance between the pit and the final downstream toe of the embankment

Tailings re-mining

Assume

Causeway

Assume

Closure criteria

TBC

3.3 Stability requirements The Factors of Safety that the embankment has to have are presented in the ANCOLD (2012) guidelines and are presented in Table 2. These requirements generally match International standards and guidelines.

Table 2: Stability Factor of Safety guidance (ANCOLD 2012).

Loading condition

Minimum FoS

Description

Drained conditions are adopted for all materials (effective stress drained shear strength) and a steady-state seepage/pore pressure regime is applied where there is:  No rapid change in the phreatic surface/pore water pressure  All the excess pore pressure generation due to placement of any fill, loading or unloading has had time to dissipate  A slow rate of shearing occurs during failure so that no shear-induced excess pore pressures are generated. This condition could represent long-term or post-closure static stability of the facility but may not be applicable in certain cases such as those described in ANCOLD (2012a, pp. 43-44). In those cases, an undrained loading condition below is recommended. Undrained shear strengths are adopted for one or more materials where excess pore water pressure is generated due to rapid loading/unloading and/or during shearing with no time to dissipate, or where pore pressures are developed due to the contractive nature of the tailings and/or embankment and foundation materials. Coarse grained, free draining materials and dilative materials, which are not expected to sustain any

1.5

Drained

Undrained (or partially drained) - potential loss of containment

1.5