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Table 10-3 Recommended FOS and shear strength parameters
Stage Loading case Recommended min. FOS (ANCOLD)
Shear strength parameters Slope to be evaluated
Comments
Starter embankment
Short term 1.3 (no loss of containment)
Consolidated undrained strength Upstream and downstream
The clay core immediately post construction is assumed to have excess pore pressures due to the size and placement rate and therefore undrained parameters are assumed. The low strength foundation material encountered in the foundations south of the site are assumed to behave as undrained due to rapid loading of the starter embankment
Long term 1.5 Effective strength Downstream All material parameters are assumed to be drained for a design check, even though short life for starter.
Post seismic 1.0 – 1.2 Post-seismic shear strength (reduced parameters by 20% and use undrained parameters for clay)
Downstream The low strength foundation material encountered is conservatively assumed to be undrained in southern embankment. Based on CPT data the low strength material is not considered potentially liquefiable material (refer to Section 10.8.1) and therefore 20% reduced strengths is considered appropriate.
First raise Short term 1.3 (no loss of containment)
Consolidated undrained strength Upstream The raised portion of clay core is assumed to be undrained immediately post construction, however the older starter embankment clay core is assumed to have effective parameters.
Final embankment
Long term 1.5 Effective strength Downstream All material parameters are assumed to be drained.
Post seismic 1.0 – 1.2 Post-seismic shear strength (reduced parameters by 20% and use undrained parameters for clay)
Downstream The low strength foundation material is assumed to have consolidated as the embankment is raised in stages over 20 years.
GHD | Report for Talison Lithium Limited - Talison TSF4 Detailed Design, 6137226
10.7.3 Phreatic surface
The phreatic surface adopted for stability assessments were obtained from the seepage assessments
as described in Section 10.6.4. The stability analyses consider stability with upstream toe
underdrainage but also considers the stability if the underdrain system were to fail.
10.7.4 Material parameters
General
Material properties adopted for the stability analysis are shown in Table 10-4, Table 10-5, Table 10-5
and Table 10-7 (material parameters for each loading case). Discussion on the material parameters
adopted are provided in the following sections.
Zone 1A and mine waste
The clay core and mine waste materials are consistent with previous embankment raise designs for
TSF 2. The material parameters adopted are based on previous investigations and laboratory testing.
Tailings
The tailings strength parameters are generally based on the CPTu investigations carried out in 2014
as described in Section 8.3. The initial filling of the cells was considered to have lower strength
parameters assuming the beach lengths are too short to allow for adequate segregation of the tailings.
The subsequent raises are expected to have sandy tailings close to the embankment and finer tailings
beyond 10 m from the embankment. Based on the CPTu investigation (2014) the sandy tailings close
to the embankment for the subsequent raises are expected to be free draining with zero cohesion.
Triaxial shear testing was carried out on two samples, one being “whole of tailings” from the discharge
pipe with no segregation and the other a sample on the upper beach where the coarser grains
segregate and an area more likely to be involved in a failure plane. These results were used to define
effective stress tailings properties.
Foundations
Foundation 1 and Foundation 2 layers were assumed to be consistent in all cross sections analysed.
Foundation 1 comprises silty/clayey gravel and sandy gravel, which will be ripped and compacted
during foundation preparation and is approximately 2 m thick. Foundation 2 layer is hard lateritic clay
material into which the clay core will be keyed. This material is expected to have high strengths based
on SPT refusals on this material across the site. This material is expected to be about 2 to 6 m thick.
Foundation 3 and Foundation 4 layers are layers above the competent bedrock. The thickness and
strengths of these layers vary across the site and were assigned strengths based on recent
investigations as described in Section 5.
The two cross sections analysed to the south of the site encountered low strength materials and CPT
data was used to determine the strength of Foundation 3 and Foundation 4 layers. These materials
are potentially low permeability and hence consolidate slowly. Undrained parameters were used
except for the long term case when consolidation was assumed to be nearly complete. This will
require monitoring with piezometers during the facility development. The strength parameters for
Foundation 3 and Foundation 4 layers for the two cross sections analysed to the north of the site were
based on SPT N60 values from the closest boreholes.
Foundation 5 consists of sandy clay, clayey silt and silt (mafic bedrock) as well as high plasticity
gravelly sandy clay and clay soils (acid bedrock). Foundation 6 is formed from extremely weathered
basement rock, predominantly silty sand above mafic rock.
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Table 10-4 Material unit weight
Material Unit weight (kN/m3) Zone 1A - Clayey materials (embankment core) 19 Mine waste rock 21 Tailings (Inner sandy silt) 14 Tailings (Outer beach sand) 14 Foundation Layer 1 (laterite) 21 Foundation Layer 2 (lateritised saprolite) 20 Foundation Layer 3 (pallid saprolite) 18 Foundation Layer 4 (soft layer) 17 Foundation Layer 5 (saprolite) 18 Foundation Layer 6 (weathered basement rock) 21
Table 10-5 Material parameters for short-term loading conditions
Table 10-6 Material parameters for long-term loading conditions
Material Angle of friction, ∅ (°)
Undrained strength, Su (kPa)
Zone 1A - Clayey materials (embankment core) - 120 Mine waste rock 40 - Tailings (Inner sandy silt) 30 - Tailings (Outer beach sand) 34 - Foundation Layer 1 (laterite) 40 - Foundation Layer 2 (lateritised saprolite) 28 - Foundation Layer 3 (pallid saprolite) - 150 Foundation Layer 4 (soft layer) - 40 Foundation Layer 5 (saprolite) - 95 Foundation Layer 6 (weathered basement rock) - 180
Material Angle of friction, ∅ (°) Zone 1A - Clayey materials (embankment core) 28 Mine waste rock 40 Tailings (Inner sandy silt) 30 Tailings (Outer beach sand) 34 Foundation Layer 1 (laterite) 40 Foundation Layer 2 (lateritised saprolite) 28 Foundation Layer 3 (pallid saprolite) 26 Foundation Layer 4 (soft layer) 23 Foundation Layer 5 (saprolite) 26 Foundation Layer 6 (weathered basement rock) 27
GHD | Report for Talison Lithium Limited - Talison TSF4 Detailed Design, 6137226 | 46
Table 10-7 Material parameters for post-seismic loading condition
Material
Short term (Starter) Long term (Final) Angle of friction, ∅ (°)
Undrained shear strength, Su (kPa)
Angle of friction, ∅ (°)
Undrained shear strength, Su (kPa)
Zone 1A - Clayey materials (embankment core) - 96 - 96 Mine waste rock 32 - 32 - Tailings (Inner sandy silt) 24 - 24 - Tailings (Outer beach sand) 27 27 Foundation Layer 1 (laterite) 32 32 Foundation Layer 2 (lateritised saprolite) 22 22 Foundation Layer 3 (pallid saprolite) 120 21 Foundation Layer 4 (soft layer) 32 18 Foundation Layer 5 (saprolite) 76 21 Foundation Layer 6 (weathered basement rock) 144 22
10.7.5 Stability results and discussion
The stability analyses plots are included in Appendix E (refer to Figures 1.05 to 1.17 and Figure 2.05
to 2.17). The results are summarised in Table 10-8 for Cell 1 cross sections and Table 10-9 for Cell 2
cross sections.
All analysed cross sections and cases meet the recommended FOS.
However, post seismic cases for the starter embankment returned FOS values close to the required
minimum therefore additional sensitivity analyses were conducted to assess the sensitivity to the post-
seismic strength parameter. These sensitivity analysis and suggested improvements are detailed in
Section 10.7.6.
The underdrainage is expected to lower the phreatic surface, improving the FOS in some cases.
Although the embankments were assessed as stable even for cases with high phreatic surface
(without underdrainage), the underdrainage is required to reduce the likelihood of liquefaction of the
sandy tailings (refer to Section 10.8.3 for seismic assessment of the tailings material). If the sandy
tailings were to liquefy it could cause differential settlement of future centreline raises, where the
footprint extends by approximately 6 m over the tailings. In particular, this could cause cracks along
the engineered clay portion of the embankment. Therefore it is imperative for the design the sandy
tailings do not become saturated. Monitoring of the phreatic surface within the tailings and
embankment is described in detail in Section 10.6.4.
The stability results show the benefit of centreline construction using waste rock materials in a wide
downstream zone. Nevertheless, critical conditions such as pore pressures, underdrain performance
and beach drying should be checked prior to the design of each lift.
Local stability of the upstream embankment toe was analysed for the first raise where the
embankment extends over the tailings beach. All local stability slopes met the minimum FOS of 1.5 as
this minimum FOS is recommended for failures where there is potential for loss of containment.
GHD | Report for Talison Lithium Limited - Talison TSF4 Detailed Design, 6137226
Table 10-8 Cell 1 Stability analysis results
Stage & Load case Section location Upstream toe underdrainage
(Y/N)
Slope analysed and failure
mode
Calculated FOS
Minimum recommended
FOS
Acceptable (Y/N)
Figure reference
Starter Embankment Short term
Cell 1 South Y D/S global 1.3 1.3 Y Figure 1.05
Cell 1 North East Y D/S global 2.9 1.3 Y
Cell 1 South Y U/S global 1.4 1.3 Y Figure 1.06
Starter embankment Long term
Cell 1 South Y D/S global 1.9 1.5 Y Figure 1.07
Cell 1 North East Y D/S global 3.0 1.5 Y
Cell 1 South N D/S global 1.9 1.5 Y Figure 1.08
Cell 1 North East N D/S global 3.0 1.5 Y
Cell 1 South Y D/S local 2.1 1.5 Y Figure 1.09
Cell 1 North East Y D/S local 2.9 1.5 Y
Start embankment Post seismic
Cell 1 South N D/S global 1.0 1.1 Refer to Section 10.7.6
for details
Figure 1.10
Cell 1 North East N D/S global 2.2 1.1 Y
Cell 1 South Y D/S global 1.1 1.1 Y Figure 1.11
Cell 1 North East Y D/S global 2.2 1.1 Y
First raise Short term
Cell 1 South Y U/S local 3.7 1.3 Y Figure 1.12
Cell 1 South N U/S local 3.7 1.3 Y Figure 1.13
Final embankment Long term
Cell 1 South Y D/S global 1.8 1.5 Y Figure 1.14
Cell 1 North East Y D/S global 1.8 1.5 Y
Cell 1 South N D/S global 1.8 1.5 Y Figure 1.15
Cell 1 North East N D/S global 1.8 1.5 Y
Cell 1 South Y D/S local 2.1 1.5 Y Figure 1.16
Cell 1 North East Y D/S local 1.8 1.5 Y
Final embankment Post seismic
Cell 1 South Y D/S global 1.3 1.1 Y Figure 1.17
Cell 1 North East Y D/S global 1.4 1.1 Y
GHD | Report for Talison Lithium Limited - Talison TSF4 Detailed Design, 6137226 | 48
Table 10-9 Cell 2 Stability analysis results
Stage & Load case Section location Upstream toe underdrainage (Y/N)
Slope analysed and failure mode
Calculated FOS Minimum recommended FOS
Acceptable (Y/N) Figure reference
Starter Embankment Short term
Cell 2 South Y D/S global 1.8 1.3 Y Figure 2.05 Cell 2 North West Y D/S global 1.3 1.3 Y Cell 2 South Y U/S global 4.3 1.3 Y Figure 2.06
Starter embankment Long term
Cell 2 South Y D/S global 1.8 1.5 Y Figure 2.07 Cell 2 North West Y D/S global 1.6 1.5 Y Cell 2 South N D/S global 1.8 1.5 Y Figure 2.08 Cell 2 North West N D/S global 1.6 1.5 Y Cell 2 South Y D/S local 2.0 1.5 Y Figure 2.09 Cell 2 North West Y D/S local 1.8 1.5 Y
Starter embankment Post seismic
Cell 2 South N D/S global 1.4 1.1 Y Figure 2.10 Cell 2 North West N D/S global 1.1 1.1 Y Cell 2 South Y D/S global 1.4 1.1 Y Figure 2.11 Cell 2 North West Y D/S global 1.1 1.1 Y
First raise Short term
Cell 2 South Y U/S local 3.7 1.3 Y Figure 2.12 Cell 2 South N U/S local 3.7 1.3 Y Figure 2.13
Final embankment Long term
Cell 2 South Y D/S global 1.7 1.5 Y Figure 2.14 Cell 2 North West Y D/S global 1.7 1.5 Y Cell 2 South N D/S global 1.7 1.5 Y Figure 2.15 Cell 2 North West N D/S global 1.7 1.5 Y Cell 2 South Y D/S local 1.5 1.5 Y Figure 2.16 Cell 2 North West Y D/S local 1.5 1.5 Y
Final embankment Post seismic
Cell 2 South Y D/S global 1.3 1.1 Y Figure 2.17 Cell 2 North West Y D/S global 1.2 1.1 Y
GHD | Report for Talison Lithium Limited - Talison TSF4 Detailed Design, 6137226 | 49
10.7.6 Sensitivity analyses for soft foundation layer
Stability analyses presented in Section 10.7 are based on recommended soil strength parameters
sourced from Geotechnical Investigation Report (GHD, 2019). Post seismic cases analysed for starter
embankment returned FOS values between 1.0 to 1.1 as shown in Figure 10-7 and Figure 10-8.
These show that the FoS was only marginally greater than the minimum required and slightly less than
the targeted 1.1. Hence, the factors influencing these factors of safety were considered in more detail
as follows.
Figure 10-7 Cell 1 South, starter embankment, post seismic stability analyses
Figure 10-8 Cell 2 North, starter embankment, post seismic stability analyses
Consideration of the failure paths of the critical slip circles shows the failures to be heavily influenced
by the soft foundation layer F4 even though the profile of other foundation layers are different in each
case. A sensitivity analysis was carried out on the strength parameter in the expected range from
40 kPa to 50 kPa being the lower bound strength range estimated from CPT investigations. Graphs
presented in Figure 10-9 and Figure 10-10 illustrate results of sensitivity analyses.
GHD | Report for Talison Lithium Limited - Talison TSF4 Detailed Design, 6137226 | 50
Figure 10-9 Sensitivity graph for cohesion in Foundation Layer F4 in Cell 1 South
post seismic case
Figure 10-10 Sensitivity graph for cohesion in Foundation Layer F4 in Cell 2 North post seismic case
The expected strength range makes only marginal difference to the FOS for Cell 1 South but could
provide an acceptable FOS for Cell 2 North. An alternate design approach is required to give added
margin, unless future in situ testing demonstrates that increased strength occurs shortly after loading.
It is recommended to implement the construction methodology described in Section 14 to include
continuous placement of waste rock on the downstream side of the embankment. If surplus waste rock
is used to form a counterweight buttress, the post-seismic stability of the starter embankment will
improve as the failure plane is forced to follow a longer path.
Placement of a 5 m high and 20 m wide berm on the southern wall of Cell 1 and northern wall of Cell 2
where the cross sections are at their maximum height was shown to result in acceptable FOS. The
GHD | Report for Talison Lithium Limited - Talison TSF4 Detailed Design, 6137226 | 51
stability analysis are shown in Figure 10-11 and Figure 10-12. These, or wider, berms should extend
lengthwise until they meet higher ground levels which would give berm lengths approximately equating
to the extent of underlying soft clay. Higher berms to accommodate ongoing rock placement should be
checked for local stability at the leading face of the rockfill.
Figure 10-11 Cell 1 South post seismic stability with downstream berm
Figure 10-12 Cell 2 North post seismic stability with downstream berm
10.7.7 Consolidation of soft layer
Consolidation testing on the low strength material presented was undertaken to determine if the extent
of consolidation during the first years of operation would give adequate increase in strength to allow
raising of the dam to continue in a safe manner.
Consolidation parameters determined from triaxial testing indicated coefficient of consolidation values
between 6 m2/year to 30 m2/year. However, two oedometer tests carried out on undisturbed samples
indicated lower consolidation rates (from approximately 1 m2/year to 6 m2/year). Taking the lower
values from the oedometer test results, the degree of consolidation was estimated following the
completion of starter embankment raise. The calculations assume the starter embankment will remain
for two years followed by ongoing construction of raises at a rate of 3 m/year. The maximum thickness
of the soft layer was 5 m, and one-way drainage to the top was assumed to give a more conservative
(slower) consolidation.
Using the lower bound consolidation rates, the soft material will achieve approximately 60%
consolidation two years post construction of the starter embankment. In the foundation beneath the
crest where the load is greatest, the undrained shear strength is expected to increase from 40 kPa to
114 kPa following 60% dissipation of excess pore pressure and from 40 kPa to 167 kPa following
100% dissipation of excess pore pressure. Closer to the toe of the embankment, the undrained shear
strength of the soft layer is expected to increase from 40 kPa to 50 kPa following 60% dissipation of
excess pore pressure and from 40 kPa to 57 kPa following 100% dissipation of excess pore pressure.
The consolidation rates were based on the lowest test result but rates were higher in all other samples
tested, which would suggest consolidation is likley to be faster than used in calculations.
The strength gains from 60% consolidation occur before additional raises are added and will improve
post seismic stability from a FOS of 1.05 shown in Figure 10-7 to a FOS of 1.5 shown in Figure 10-13.
GHD | Report for Talison Lithium Limited - Talison TSF4 Detailed Design, 6137226 | 52
Layer 5 (saprolite) was conservatively assumed to have similar incremental strength increase as layer
4 due to consolidation.
Figure 10-13 Starter embankment post seismic stability after 2 years of consolidation
Each new raise adds further load and initiates further consolidation with the ongoing strength gains
during construction, which provides stability for each lift. However, if the construction of the
subsequent three lifts are carried out too rapidly for further dissipation to take place by lift three, the
FOS again becomes marginal as shown in Figure 10-14, but could be improved by a counterweight
berm as discussed above. This shows the future raises are dependent on the consolidation of the
foundation layers in particular foundations layers described in the stability analysis as F4 (soft layer)
and F5 (saprolite).
Figure 10-14 Third lift with strength gains from starter embankment load only
It is critical that the actual degree of ongoing consolidation is checked regularly using the three
piezometers set into the F4 low strength foundation layers. Further piezometers will be required to be
installed in both F4 and F5 foundations before construction commences to check if consolidation rates
of the critical foundation zones are at or higher than the rate assumed for the design.
10.8 Seismic assessment
10.8.1 Foundation
The foundations were generally not considered to be liquefiable due to being well graded material with
significant clayey fines. In the low lying areas of the site relatively poorly graded sands were
encountered close to the natural surface. These sands can be distinguished by their grey colour and
are considered potentially liquefiable material. These sands will be removed from within the
embankment footprints during foundation preparation (refer to Section 10.3).
The CPTu investigation data was used to confirm the low strength material encountered beneath the
hard lateric clay foundations has low potential for liquefaction under the design earthquake loads. The
CPT data was interpreted using CLiq software and the liquefaction potential index plots are included in
the TSF4 geotechnical report (GHD, 2019).
Although not liquefiable, all foundation strengths were reduced by 20% to recognise strain softening
for post seismic stability analysis as recommended in USCE, 1984.
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10.8.2 Embankment
The compacted clay embankment is not considered to be liquefiable material as it is has been
compacted in layers to maximum density. It is also allocated lower post-earthquake strength.
10.8.3 Tailings
The particle size distribution of the majority of tailings sampled from the 2014 CPTu probes was found
to be generally uniform and gradings are within the liquefiable zone as shown in Figure 10-15.
Saturation of the tailings, and a relatively low in-situ density, provide the conditions for liquefaction to
be a risk.
The 2014 CPTu tests were analysed to determine the cyclic resistance ratio (CRR) of the tailings and
the cyclic shear stress (CSS) under the proposed OBE and SEE events. Both the methods proposed
by Robertson (1998) and NCEER (1996) were considered.
The results showed that in the event of applied earthquake loading, the tailings would most likely not
liquefy under the OBE, but could potentially liquefy under the SEE.
The liquefaction potential of the tailings was also demonstrated by the consolidation and maximum
density test results, indicating that shaking of saturated tailings results in a significant change of void
ratio. The reduction of the void ratio during shaking is an indicator of pore water pressure generation
as the soil particles move into the voids thus displacing water.
The uniform particle distribution and the consolidation test results confirmed that consolidation of the
tailings under their own weight would not result in sufficient tailings densification to reduce the
liquefaction potential. However, this design considers that the major portion of tailings near the
embankment will be drained by the drawdown of the phreatic surface by underdrainage (refer to
Section 10.11). Liquefaction is not expected as long at the material remains drained. Piezometers will
be included to monitor the phreatic surface against the embankment.
Figure 10-15 Tailings PSD results (GHD, 2014)
10.8.4 Deformation analysis
For materials neither prone to significant strength loss (greater than 20%) nor liquefaction, the
performance during a seismic event may be analysed by estimating earthquake induced deformations.
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The Makdisi & Seed, 1978 method was developed for estimating the deformations of the earth fill
embankments due to seismic loadings and it is applicable to TSF 4 embankments at the Talison site
as the embankments will be constructed using centreline construction method. Deformation analysis
was carried out for the highest section of the final embankment profile. The deformation assessments
were carried out using the same embankment geometry and material properties as for stability
analysis (refer Sections 10.7.2 and 10.7.4).
The SEE, as specified in Section 2.5, was considered in the estimation of deformation under seismic
loading conditions. The peak ground acceleration (amax) of 0.25g was adopted and a magnitude of 6.
The yield seismic coefficient (ky) was calculated for different circular slip surfaces with the
embankment.
The shear wave velocity of at the foundation of the embankment was assumed to be 200 m/s. The
maximum average acceleration at the base of the embankment was amplified to a peak crest
acceleration of 0.6g.
A deformation assessment of TSF 4 was carried out using a range of empirical and simplified methods
to estimate the embankment crest deformation caused by an earthquake.
The estimated earthquake induced displacements were based on the normalised curves produced by
Makdisi & Seed, 1978 for various magnitude earthquakes (Figure 10-16). The maximum embankment
crest displacement was calculated to be less than 1 mm for the final embankment when subjected to
SEE ground motion.
Figure 10-16 Normalised deformation curve
The estimated settlements are considered minor and will not affect the overall integrity of the
embankments. The results of estimated deformation under various methods of analysis are
summarised in Table 10-10.
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Table 10-10 Deformation analysis results
Method Estimated crest settlement Swaisgood (2003) – Empirical <5 mm Pell and Fell (2003) – Empirical 12 mm Seed & Makdisi – Simplified <1 mm
10.9 Settlement estimation
Sigma/W as a package of Geostudio 2012 was used to estimate the settlements within the
embankment. Two cases were considered:
Settlement of the starter embankment
Settlement of the final embankment
For both scenarios, the tailings were assumed to be partially saturated silty sand material with a
relatively low stiffness.
The soil Elastic Young’s modulus (elastic modulus) were adopted from previous studies and were
typical for the material types. However, the elastic modulus for the critical low strength material was
based on the DMTs undertaken in this foundation layer (F3 and F4). The elastic modulus was
estimated to be 80% of the DMT modulus (MDMT). The average MDMT value from the DMTs was
45 Mpa and therefore F3 and F4 were assigned a conservative stiffness parameter of 35 Mpa.
The material parameters considered in Sigma/W are summarised in Table 10-11.
Table 10-11 Stiffness parameters adopted
Material Unit weight (kN/m3)
Elastic modulus at starter embankment height (MPa)
Clayey materials (starter embankment and clay face)
19 30
Mine waste rock 21 80 Sandy silt tailings 14 5 Foundation Layer 1 (top) 21 30 Foundation Layer 2 20 40 Foundation Layer 3 18 36 Foundation Layer 4 18 36
The results on two sections taken in the southern part of the site are summarised in Table 10-12.
Much of the settlement for final embankment will occur during the successive raisings over many
years. However, an additional 100 mm was added to the freeboard allowance to account for this.
Table 10-12 Settlement estimates
Section Starter embankment (mm) Final embankment (mm) Cell 1 65 217 Cell 2 41 160
10.10 Freeboard
10.10.1 General
Water control is a key element for the safe management of TSF4. Cell 1 was designed to hold a
decant / storm pond at the central to northern portion of the facility, whereas Cell 2 was designed to
hold a decant / storm pond in the centre of the facility.
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Figure 10-17 Decant pond locations for starter embankment
10.10.2 Recommended freeboard requirements
Freeboard requirements were selected in accordance with ANCOLD Guidelines on Tailings Dams
(2012). ANCOLD Guidelines recommend allowing for freeboard for a TSF based on the Consequence
Category. This requires additional freeboard to account for the design storm above the Normal
Operating Level.
The DMIRS Guidelines (DMP, 2015) also define freeboard for TSF in addition to the storm surcharge.
The DMIRS define an “operational freeboard” and a “beach freeboard” which together forms a total
freeboard as shown in Figure 10-18.
The normal operating pond level refers to the maximum level of the decant pond, excluding any rainfall
runoff. Operational freeboard refers to the vertical distance between the maximum tailings beach level
and the embankment crest. The beach freeboard is defined by the vertical distance between the pond
level after the design storm event and the maximum tailings beach level. The total freeboard is a
combination of both the beach and the operational freeboards.
Figure 10-18 Definition of Freeboard for TSF (DMP, 2015)
The 1 in 100 year 72-hour design rainfall depth was estimated using the Bureau of Meteorology (BoM)
IFD data as 159 mm (Figure 10-19).
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However, according to ANCOLD Guidelines (ANCOLD, 2012), the storm storage of a High B
consequence category TSF should safely contain rainfall and runoff of 1 in 1,000 year, 72-hour storm
event. The 1 in 1,000 year 72-hour design rainfall depth was estimated using Figure 10-19 to be
217 mm. This would create a 300 mm rise in pond water level.
In order to comply with the freeboard requirements outlined above, it is necessary to maintain the
normal operating pond level not less than 0.9 m below the embankment crest. The minimum freeboard
criteria are summarised in Table 10-13. Based on the freeboard criteria, Table 10-14 presents the key
levels for the TSF.
Figure 10-19 IFD Data for Greenbushes WA (BoM)
Table 10-13 Freeboard criteria
Operational freeboard (m)
Beach freeboard (m)
Additional freeboard (settlement) (m)
1,000 year storm event (m)
Total freeboard required (m)
0.3 0.2 0.1 0.3 0.9
Table 10-14 Key TSF levels
Level Starter embankment (Elevation RL m) Cell 1 and 2 crests 1265.0 Maximum tailings beach (DMIRS) 1264.7 Maximum operating pond level (ANCOLD) 1264.1
10.10.3 Spillway
A spillway has not been incorporated into any stage of the TSF4 design due to the associated risks of
erosion impacting the embankment. Instead, the freeboard allowance can accommodate significantly
larger storms in the short period when the cell reaches target filling height before the next lift. The
design allows for storage of an extreme storm event (1 in 1,000 year, 72 hour) which equates to
217 mm of rainfall. The catchment areas for Cell 1 and Cell 2 are 850,000 m2 and 640,000 m2,
respectively. The stormwater storage volume of required for the starter embankment is estimated to be
185,000 m3 for Cell 1 and 139,000 m3 for Cell 2.
Tailings deposition models for both cells of the starter embankment indicates that the facility can safely
contain the extreme storm event. Stormwater volumes were modelled on two beach slopes of 0.5%
and 1%.
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10.11 Underdrainage
10.11.1 General
The toe underdrainage system was designed to control the development of the phreatic surface within
the embankment and to improve the consolidation of the tailings and will have sufficient capacity to
drain the seepage flow estimated in the seepage analysis. Segregated sandy tailings deposited
adjacent to the embankment were identified to be potentially liquefiable if saturated (refer to Section
10.8.3). As the embankments will be raised by centreline methods, the upstream toe of each lift will
extend over the tailings beach by approximately 6 m from the crest. The intent of the underdrainage is
to drain this area of sandy material, reduce the phreatic surface and reduce to potential for
liquefaction.
The underdrainage system design is shown on Drawings 61-37226-C021 and 61-37226-C022.
10.11.2 Underdrainage pipeline
The underdrainage system will comprise of slotted flexible drain coil pipes buried in a trench along the
upstream toe of the embankment, located a minimum 3 m from the toe. The trenches will be graded to
ensure a continuous cross fall towards the outlet pipes and backfilled with gravel (geofabric wrapped)
as shown on 61-37226-C022.
Outlet pipes are proposed at low points along the perimeter embankments, which will feed into
seepage collection sumps (refer to Section 10.11.5). The outlet pipeline trenches will be backfilled with
clay under the Zone 1A clay with three bentonite collars as shown in the cross section on Drawing 61-
37226-C022.
The flow captured in the drainage sumps will be pumped back into the storage or if required can be
pumped back to plant as required.
Vibrating wire piezometers (VWPs) will be installed at various locations upstream of the perimeter
embankments to confirm the underdrainage is drawing down the phreatic surface adequately near the
perimeter embankment. Monitoring of these pore pressures will dictate the requirement for additional
drainage in the future raises. Additional VWPs are also proposed within in the embankment clay core
and mine waste zones as detailed in Section 10.12.
10.11.3 Downstream drainage blanket
A 400 mm thick sand blanket is included on the foundation downstream of the clay core to reduce the
risk of piping of the embankment toe or foundation if seepage develops in these areas. A 3 m thick
selected permeable waste rock base layer was included beneath all rock areas to allow the rockfill to
drain freely and to allow escape of any foundation seepage.
10.11.4 Downstream finger drains
To facilitate consolidation of the low strength foundation material and improve drainage for the sand
blanket, a series of rockfill finger drains were included along the southern side of starter embankment
extending to a 500 m wide section in the southern section as presented on drawing 61-37226-C021.
Finger drains will be spaced at 5 m centres and discharge into the toe seepage trench. The
requirement of these drains will be reviewed based on the further investigations proposed in
Section 5 and based on the behaviour of the low strength material during placement of the starter
embankment.
10.11.5 Seepage collection sumps
Three seepage collections sumps were included at low points along the final embankment toe as
presented on the underdrainage plan Drawing 61-37226-C021. Two Northern outlets from the
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underdrainage system will be captured into separate sumps, while southern outlets will be connected
via series of inspection pits to a combined seepage sump. The combined seepage sumps allows for
an additional 10% for freeboard and to accommodate flows from downstream finger drains.
A summary of seepage collection sump sizes for starter and final stages is presented on Table 10-15.
The seepage trenches will be constructed for the anticipated flows for the initial five years. The flows
will be monitored and the seepage sumps will be modified as required to their final sizes. The sizes
required at the final stage are indicative only and should be reviewed based on seepage flow rates
during initial five years.
Table 10-15 Seepage sumps
Seepage sump location Size required at starter (m) Size required at final stage(m) Cell 2 North West 10(W) x 15(L) x 1.5(D) 15(W) x 60(L) x 1.5(D) Cell 1 North East 15(W) x 10(L) x 1.5(D) 15(W) x 50(L) x 1.5(D) Cell 1 and 2 South (combined)
20(W) x 22(L) x 1.5 (D) 20(W) x 120(L) x 1.5(D)
10.12 Monitoring instrumentation
10.12.1 Vibrating wire piezometers
A series of six vibrating wire piezometers (VWPs) are proposed at intervals along the starter
embankment as shown on Drawing 61-37226-C029. In section, the proposed VWPs are located in
three locations and shown on Drawing 61-37226-C029 for the following objectives:
Upstream VWPs in tailings to confirm phreatic surface against the embankment, the efficiency of
the underdrainage and the requirement of additional underdrainage in future raises.
VWPs within clay core to confirm the phreatic surface within the clay.
VWPs downstream of clay core to confirm a phreatic surface within the waste rock has not
developed within waste rock zone and to confirm the effects of loading in the low strength
foundation layer.
The trigger levels will be specified in the Operating Manual once the as constructed elevations and
locations of the VWPs are available.
10.12.2 Settlement survey markers
Survey markers will be installed on the Zone 1A (clay embankment) crest at 200 m centres. Monitoring
of the survey marker movements should be carried out regularly at a minimum of three monthly
intervals. In addition to regular monitoring, the markers should be surveyed after any major event
occurring on the site (flood, earthquake).
The settlement of the crest should be monitored weekly during construction of subsequent raises by
adding an extension rod to the survey point for the subsequent raise. The subsequent raise should
include additional survey markers which can be extended for the following raise. To support the
extended rod during the raise a 2 m diameter fine rock cone will be formed around the extended rod
during construction as shown on Drawing 61-37226-C030.
10.12.3 Groundwater monitoring bores
Groundwater monitoring bores will be included to monitor the groundwater elevations and quality.
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11. Water balance 11.1 General
A water balance model was developed to determine the volumes of rainfall runoff, supernatant liquor
released, decant water as the tailings settle and evaporation from the decant pond.
The water balance model was used to calculate the volume of water within TSF4 over time and under
a range of climatic conditions. The decant pond volume was converted to a pond area and pond level
from the pond stage storage curves. The decant water was planned to be returned to the process
plant for reuse.
The monthly average inputs and outputs were plotted for average rainfall, and for typical wet and dry
year conditions. The decant return rates required to maintain an adequate normal operating pond level
were estimated.
The water balance model considered the following inflow and outflow streams (Figure 11-1)
Inflows:
Water in the tailings slurry
Rainfall.
Outflows:
Evaporation loss
Seepage loss (based on estimated seepage rates – refer to Section 10.6.4
Water retained in the tailings
Decant water
Figure 11-1 Schematic representation of water balance
11.2 Data
The average, dry and wet rainfall data is presented in Table 11-1.
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Table 11-1 Rainfall and evaporation data
Month Decile 1 Rainfall (mm)
Median Rainfall (mm)
Decile 9 Rainfall (mm)
Evaporation (mm)
January 1 18 41 196 February 0 10 28 159 March 3 26 65 132 April 9 49 117 79 May 54 114 158 47 June 53 127 193 32 July 74 158 227 36 August 62 128 163 54 September 59 101 148 81 October 25 50 82 123 November 6 37 57 154 December 2 22 59 187
11.3 Key Assumptions
The key assumptions for the water balance are listed in Table 11-2. The storage curve for TSF4
developed for the deposition schedule was used for the water balance (Section 9).
Table 11-2 Water balance assumptions
Parameter Value Catchment area 1.48 Mm2 Tailings production rate Annual tailings production rates used are as presented in
Table 2-1. Slurry density Slurry density is 30% w/w for Chem Grade and TSF1
retreatment. For Tech Grade, the slurry density is 4% w/w for the first 4 years and then the percent solids is increased to 30% w/w.
SG tailings 2.65 Rainfall run-off from the exposed tailings surface, decant pond area and surrounding catchment
Rainfall from recorded years with monthly total for wet, average and dry years as presented in Table 6-2. Runoff coefficient = 0.8
Evaporation loss Evaporation coefficient = 0.8 Pond area of total tailings beach area
Seepage The permeability of the foundation (clay) is 1 x 10-8 m/s, and the seepage is to the foundation only. The interstitial water assumes the tailings is fully saturated. Wet up of the tailings is not considered following a dry period.
Water retained in the tailings Assumes 1.4 t/m3 average settled density Underdrainage Water from the seepage sumps collected by underdrainage
system is pumped back into the TSF
11.4 Results
To maintain the normal operating pond level of one third of the total tailings beach area, an average
decant return rate of for average (median) rainfall conditions:
850 m3/h is required (between FY19-FY25)
180 m3/h (between FY26-FY33)
470 m3/h (between FY34-FY36)
850 m3/h (between FY37-FY38)
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Given the variation in decant return rates required for wet and dry conditions and the short operating
life of TSF4 the return system should be designed to operate at a flow rate of 900 m3/h. If wet
conditions result in the level of the decant pond increasing above the minimum operating level an
additional temporary pump may be required for short durations.
Figure 11-2 Monthly Water Volume for Dry and Wet Conditions
Figure 11-3 Decant Pumping Rate for Average, Wet and Dry Conditions
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12. Deposition and water reclaim 12.1 General
Deposition modelling was carried out for the filling of both TSF4 cells to starter embankment elevation
(RL 1265 m). The modelling shows the development of the tailings beach and migration of the return
water pond during commissioning and initial filling.
12.2 Decant location optimisation
Centralised decant pond locations were considered the Base Case for the development of the two cell
TSF (Figure 12-1). Two alternative decant pond location options were considered to remove the
requirement for a decant accessway structure (Figure 12-2).
Figure 12-1 Base Case – centralised decant pond location
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Figure 12-2 Alternative decant pond location options
The two alternative options considered were compared against the Base Case in Figure 12-1. A
summary of the key comparisons made against the Base Case are presented on Table 12-1. The
centralised decant location would require a decant accessway structure that equates to approximately
165,000 m3 earthworks volume for each cell. The tailings storage volume loss incurred due to the
volume of the decant accessway structure required in the Base Case has been considered in the
assessment for additional embankment height required to recover the loss of tailings storage capacity.
The two alternative options will also require the Cell 2 northern embankment to be designed as a
water retaining structure. This can be achieved for most of the north embankment by applying a clay
lining to the existing TSF1 embankment, which is acceptable for Option 1. At the western end of this
embankment there is no TSF1 embankment and a water retaining embankment is required for Cell 2
in Option 2. This is likely to comprise a downstream raised embankment with a significantly wider clay
core and filter on the downstream side of the clay core. This would also increase the risks at this part
of the embankment.
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Table 12-1 Alternative decant pond options against Base Case
Item Option 1 Option 2
Additional tailings storage volume loss
incurred at final raise (per cell)
850,000 m3 85,000 m3
Earthworks saving by deleting
accessway (per cell)
172,000 m3 172,000 m3
Additional embankment height required
to account for volume loss (per cell)
1.2 m (approx. 450,000 m3
of additional earthworks)
0.2 m (approx. 50,000 m3
of additional earthworks)
Add additional earthworks
for Cell 2 north
embankment
Reduction in return water pipe length
(per cell)
1,700 m 1,300 m
The assessment determined Option 2 with the pond against the north embankment is the more
economical alternative as it saves building an accessway and reduces the earthworks volume for
additional raising to compensate for storage loss.
The Option 2 layout was considered suitable for Cell 1 as the pond will be against the existing TSF1
downstream slope and lined with a 7.5 m wide clay zone.
For Cell 2, a pond against the northern embankment would require the more expensive water retaining
construction for this embankment, and the risks would increase. Hence, a conventional central decant
is proposed for this cell. There is a significant ridge on the western side of this cell such that a floating
decant pump can be accessed along this ridge in the initial phases. A physical accessway is not
required until tailings reach the level of this ridge, which is about Raise 2, thus deferring capital for
approximately three years.
12.3 Deposition modelling
The deposition modelling was undertaken using Muck 3D software based on the design model of
starter embankment. Due to the topography, the initial filling required deposition modelling for
assessment of pond migration and establishment. Once the TSF floor is covered in tailings and the
long-term pond is developed cyclic deposition assumed to maintain an even beach.
The objectives of the deposition modelling were as follows:
Maintain dry beach adjacent to the embankments that will be raised by centreline methods and
avoid ponding of free water against them
Divert water towards preferred pond locations (where they can be easily accessed to reclaim water)
Estimate when embankments will be required to be raised
Optimise number of required spigots
12.4 Tailings production data
Deposition rates were based on the annual tailings productions and input into the software as average
monthly rates. The following stages were modelled:
Stage 1 – One year of production deposition into Cell 1 (Partially filling starter dam)
Stage 2 – Deposition into Cell 2 to design capacity (Starter embankment RL 1265 m)
Stage 3 – Deposition into Cell 1 to design capacity (Starter embankment RL 1265 m)
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12.5 Modelling criteria
The input parameters used for the deposition modelling are presented in Table 12-2 and are based on
the parameters in Section 8.
Table 12-2 Tailings deposition input parameters
Parameter Value Source/Comments
Dry density of deposited
tailings
1.40 t/m3 -
Beach slope 0.5 - 1% Survey of TSF1 and TSF2 indicates existing
beach slopes of 1.0%-1.5% on average however
due to topography limiting beach length a flatter
beach is expected initially
Crest elevation (starter
embankment)
RL 1265 m Design drawings
Beach freeboard RL 1264.7 m 0.3 m below crest level
Decant pond size Min 1 m depth -
12.6 Storage capacity and staged development
Deposition modelling indicated the storage capacity and sequencing of the deposition would vary
significantly based on the beach slope achieved as shown on Table 12-3.
Table 12-3 Starter embankment storage capacity
Estimated storage capacity (m3) 0.5% beach slope 1% beach slope
Cell 1 5,000,000 4,800,000 Cell 2 1,300,000 1,100,000
12.7 Beach development and pond migration
The deposition sequence described as follows for the starter embankment (RL 1265 m) Cell 1 and
Cell 2 assumes a 1% tailings beach. The deposition into Cell 2 and Cell 1 (second filling) is expected
to be extended by approximately one month if a flatter average beach of 0.5% is achieved, prolonging
deposition into Cell 2.
Deposition in first month from the south embankment of Cell 1 utilises natural topography to push the
ponding area away from the embankment as presented on Figure 12-3.
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Figure 12-3 Tailings and pond in July 2019
In the subsequent months until end of FY20, deposition from about nine spigots spaced at
approximately 100 m centres will drive the pond to a temporary pond location against the hill where it
can be easily accessible for water reclaim as presented on Figure 12-4 and Figure 12-5.
Figure 12-4 Tailings and pond in December 2019
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Figure 12-5 Tailings and pond in June 2020
Tailings in following financial year (FY21) should be deposited to Cell 2, allowing the partially filled
Cell 1 tailings to dry and consolidate. The spigots spaced at 100 m centres along the full length of
embankment (including corners) allows two ponds to develop against the hill to the west of the cell as
presented on Figure 12-6. Cell 2 is expected to reach capacity in approximately five to six months.
Figure 12-6 Tailings and pond November 2020
As Cell 2 reaches capacity, tailings deposition will be reverted to the partially filled Cell 1. Deposition of
tailings will be from the perimeter and divider embankments to drive the pond to its long-term position
against the clay lined TSF1 embankment as presented in Figure 12-7.
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Figure 12-7 Tailings and pond December 2021
12.8 Typical beach profile
The anticipated 1% beach slope was modelled for the raises after the tailings beach has covered the
entire base of the cell impoundments as shown on Figure 12-8. The diagram shows the long-term
location of the return water pond for each cell. The return water pipeline arrangement and associated
civil structures are detailed in Section 12.10.
Figure 12-8 Typical beach profile and pond location
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12.9 Tailings delivery pipeline
The tailings delivery pipeline arrangement for the starter embankment deposition is shown on Drawing
61-37226-C020. Detail design of the pipeline is beyond the scope of this report.
The proposed tailings pipeline arrangement is based on the design decant pond location for each cell
(refer to Section 12.2). In Cell 1, tailings will be deposited from three sides - North, West and East – to
concentrate ponding area against the northern embankment. In Cell 2 tailings pipeline will be installed
along the full perimeter of the cell to centralise the decant pond.
The spigots will be installed approximately every 100 m along the tailings distribution pipe, with longer
gaps for the corner spigots to avoid tailings build up.
12.10 Return water
The arrangement of the return water pipeline is shown on Drawings 61-37226-C019 and 61-37226-
C020. Decant water from both cells will be pumped north to the new Clear Water Pond via a new
return water pipeline. Detail design of the return pipeline and pumps will be completed during the
detailed design phase of the tailings deposition pipeline. The return water pipeline corridor to the new
clear water pond at this stage is proposed to run along the TSF2 buttress and will require
infrastructure to cross Maranup Ford Road.
The capacity of the Cell 1 and Cell 2 decant pumps will be designed as per the rates estimated from
the water balance (refer to Section 11). The pumps will be skid mounted and driven by a diesel engine
and have self-priming capabilities. A suction pipe will be supported by floats and will keep the coarse
screen above the consolidated tailings and prevent suction of tailings during operation.
The Cell 1 pipeline will initially extend to the temporary decant location in the centre of the cell where
the pond can be pumped from the east side of the cell as described in Section 12.7. Once the Cell 1
starter capacity is reached, the pond is expected to have migrated to its final location against the TSF1
embankment, where it can be accessed via a short access ramp.
The Cell 2 pipeline will initially extend to the centre of the cell as presented on
Drawing 61-37226-C020. Two flexible pipelines will be included to access the separate ponding areas
described in Section 12.7. In subsequent raises, the two ponds are expected to merge into one and a
single pipeline will run along a decant accessway.
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13. Surface water management 13.1 Downstream toe drain
Downstream toe drains will be installed to collect runoff from the embankment and surrounding
catchment and divert it to a free outlet. The catchment areas of the toe drains are shown in
Figure 13-1. The eastern toe drain will be directed around the underdrainage sump to the low lying
area located east of TSF1. As the proposed waste dump in this area develops, runoff is expected to
become trapped in this location and at which point the drain will be directed to the underdrainage
sump.
Figure 13-1 Catchment areas
The key parameters assumed for the toe drain design are summarised in Table 13-1.
The toe drain size was optimised to nominal 1 m wide and 1 m deep channel at the alignments
indicated on Drawings 61-37226-C006 and 61-37226-C007 .
A maximum slope of 8% was calculated to limit the maximum water velocity to approximately~4 m/s.
This requires rock lining for erosion protection (nominally 0.5 m thick) as detailed on Drawing 61-
37226-C009.
Table 13-1 Toe drain design parameters
Parameter Value Design rainfall event 1 in 100 year event (1% AEP) Time of concentration 5 min Rainfall Intensity 168 mm/hr Runoff coefficient 0.35 Manning’s number 0.025 Maximum design catchment area 56.5 Ha
13.2 Sedimentation pond
The tenement boundary is located to the south of the TSF4 site (approximately 100 m south of the
final embankment toe). To reduce turbidity of the runoff water from the TSF4 embankments and
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catchments, the toe drains flowing to the southern valley will be directed into a sedimentation pond
prior to release off site. To allow adequate time for settlement of the particles, the size of the
sedimentation pond was designed with a volume equivalent to a 1 in 10 year, 24 hour storm event
assuming 5% runoff coefficient.
The sedimentation pond location and sizing is shown on Drawing 61-37226-C005.
13.3 Embankment crest
The embankment crest comprises minimum 500 mm high safety windrows along the upstream and
downstream edge of the crest. To provide trafficability and drainage along the crest, a wearing course
material graded from the centre of the embankment crest will be placed to form a crossfall of 2% to the
upstream edge of the embankment. The downstream side of the crest is waste rock and is considered
to be permeable material and therefore does not require wearing course or crossfall. To allow
adequate drainage into the TSF, windrow breaks will be formed at 25 m centres in the windrow along
the upstream edge of the crest.
Erosion protection should be placed locally on the upstream slopes at windrow break locations to
avoid scouring.
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14. Construction methodology 14.1 General
A detailed design of the raises beyond the starter embankment will be required based on the initial two
year performance of the TSF. This will require review of monitoring instrumentation data and testing of
insitu parameters.
The following construction sequence is applicable to the TSF4 perimeter embankments only. Zone 1A
is the engineered clay fill and the freeboard of the facility is measured from the elevation of the Zone
1A crest.
14.2 Starter embankment
The starter embankment to RL 1265 m will be constructed by placement of the Zone 1A to the crest
elevation. This work will be completed by civil contractor using plant to suit the design requirements.
The mine waste rock zone will be placed on the downstream side of Zone 1A by mining fleet. To allow
for dual operating lanes (one truck loaded and one truck empty) and a safety windrow, the following
recommended bench widths were included (provided by Talison):
Cat 777 used in the current fleet: 32 m bench width and 1.4 m windrow height
Cat 785 for possible future use: 34 m bench width and 1.6 m windrow height
The downstream mine waste rock zone will be placed in layers by the mining fleet at a slope of
3(H):1(V) at the final embankment profile for progressive rehabilitation and embankment stability. The
minimum bench width of the mine waste rock depends on the type of equipment being used and
should consider the applicable height of a safety windrow as shown in Figure 14-1.
Figure 14-1 Minimum bench width
TSF4 will be raised in 3 m lifts using centreline raising methods. The future raises are subject to the
tailings deposition schedule and further detailed designs, however, the downstream mine waste rock
can continue to be placed during this period up to the extent of the final embankment profile. This
offers Talison some flexibility to place material on the downstream side of the proposed starter
embankment geometry early if required to suit mine schedules (refer to Figure 14-2). However, the
mine waste crest must remain ahead of the scheduled raises.
Due to the construction sequencing proposed, the maximum recommended height to which the mine
waste rock can be placed is two lifts ahead of the Zone 1A crest elevation as shown in Figure 14-3.
Zone 1A starter to RL 1265 m Mine waste rock
32-34 m
1 3
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Figure 14-2 Possible staging of downstream mine waste rock
Figure 14-3 Maximum recommended height of mine waste rock placed ahead
14.3 Subsequent raises
The proposed construction sequence following the initial development of the facility for each
subsequent raise is as follows:
Step 1: Raise the Zone 1A by centreline methods by 3 m (civil contractor to complete) as shown in
Figure 14-4:
Figure 14-4 Raise of first Zone 1A embankment
Step 2: Fill the void between the 3 m clay embankment and mine waste rock by pushing material from
the crest of the mine waste rock crest to prepare a surface for the subsequent 3 m raise as shown in
Figure 14-5:
Figure 14-5 Filling of void between Zone 1A and mine waste
32-34 m
RL 1265 m
RL 1268 m RL 1271 m
3 1
RL 1271 m RL 1268 m
Zone 1A 3 m clay core
Fill gap with mine waste
RL 1271 m
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Step 3: Raise the Zone 1A by centreline methods by another 3 m (contractor to complete) as shown in
Figure 14-6:
Figure 14-6 Raise of second Zone 1A embankment
Step 4: Fill the void between the clay and the mine waste rock with mine waste material by pushing
material from the mine waste crest as shown in Figure 14-7:
Figure 14-7 Filling of void between Zone 1A and mine waste (2)
Step 5: The mine waste rock can be placed at a maximum height of 6 m above the TSF:
Figure 14-8 Raise of mine waste 2 raises ahead of Zone 1A
Repeat Steps 1 to 5 until the final embankment landform is achieved as shown in Figure 14-9:
Figure 14-9 Staged raising to final embankment profile
RL 1271 m RL 1271 m
RL 1271 m
RL 1277 m RL 1271 m
RL 1295 m
Zone 1A 3 m clay core
Fill gap with mine waste
Raise mine waste to a maximum of two lifts above clay core
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15. TSF1/2 seepage management Cell 2 of TSF4 is adjacent to the area where seepage has been observed coming from the junction of
TSF1 and TSF2. A seepage collection trench has been installed to collect and divert the seepage in
the location shown on Figure 15-1. This section describes the civil works required at this location prior
to the construction of the TSF4 Cell 2 north embankment (Photograph 1).
Figure 15-1 Starter embankment existing seepage trench location
Photograph 1 – Seepage trench from ramp to TSF2 (looking north)
Ramp to TSF2
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The starter embankment downstream toe just encroaches the south-east section of the trench.
However, since this portion of the trench carries little water, this section can be blocked off and no civil
works are specified for the starter embankment stage.
The final embankment footprint is shown on Figure 15-2 and will cover the seepage trench. The
seepage will require capturing before embankment construction to avoid blocking the flow and
pressurising the system. Detailed design of a replacement trench will be required at this stage, which
will include a significant trench into the fill east of the existing trench and buried pipework before the
trench is backfilled and then covered by rockfill.
To reinforce the junction between the three TSFs, it is proposed that this area be progressively infilled
with rockfill as shown on Drawing 61-37226-C007. This will stabilise this corner such that if seepage is
blocked by construction in this area then any consequent rise in phreatic surface will not affect
stability. It will also facilitate the construction of access ramps in this area.
Figure 15-2 Final embankment existing seepage trench location
EXISTING TRENCH
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16. Closure design When deposition ceases, there will be an inverted cone shape in the TSF with a trapped remnant
decant pond. Pumping from the decant pond should continue until the surface has consolidated and
dried to allow for the closure work.
Once the tailings strength has increased, all the tailings delivery and return water pipes, valves and
spigots should be removed from the TSF. The inverted cone will then be filled with waste from the
mining operation to form a convex surface. Fine mine waste rock will be tipped in to the inverted cone
to create a dome shape which will shed away runoff towards discharge points around the perimeter.
This will cover the tailings surface and will minimise dust exposure. Another option to fill the inverted
cone void is to discharge tailings from central locations within the storage, by gradually advancing
discharge spigots inwards, but with due consideration for storm capacity.
Specially designed drainage systems would be required to handle normal run off and storms up to the
Probable Maximum Precipitation (PMP) event.
The surface would be topsoiled and rehabilitated together with erosion control systems.
The exterior surfaces will not require further reshaping as a closure design slope of 1(V):3(H)
(approximately 18%) has been adopted for the TSF4 design. The design is in accordance to standards
referenced in Mine Closure Plan 2016; Greenbushes Mineral Field 01, 29 September 2016.
It is proposed that these external slopes will be developed as the dam rises and will be progressively
rehabilitated. Long-term erosion on these slopes will be monitored and any adjustments made in the
detailed closure plan. Closure details should be finalised towards the final years of the TSF operation
once the final tailings geometry within the TSF are confirmed.
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17. Safety in design The TSF was designed to provide safe, long-term storage of tailings with minimal environmental
impact. The design of the TSF was tailored to site conditions, to promote safety and minimise the
environmental impacts, which promotes reduction in total project costs.
Safety was considered in each aspect of the design. Some examples of safety features in the design
are:
Use of the more stable centreline construction in lieu of upstream construction
Use of two cells instead of one (or three cells when TSF1 is recommissioned) such that there is
always a storage available in the event of a problem with one storage
Decant pond located centrally in Cell 2 to reduce risk of water against northern embankment.
Upstream underdrainage system
Sand blanket and gravel finger drains downstream of the clay core
Safety berms to be provided on either side of the embankment crests, for each stage of
construction
Adequate freeboard is provided in the tailings storage facility
Embankments are designed for adequate stability
Instrumentation for ongoing monitoring of performance
The TSF was designed to ANCOLD guidelines.
Safety in design workshop was held on 18 October 2018 where design aspects were discussed and
additional potential control measures were added as required. The complete register is included in
Appendix F.
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18. References ANCOLD, 1998. Guidelines for Design of Dams for Earthquake, August 1998.
ANCOLD, 2003. Guidelines on Dam Safety Management. August 2003.
ANCOLD, 2012 a. Guidelines on Tailings Dams. May 2012
ANCOLD, 2012 b. Guidelines on the Consequence Category for Dams. May 2012.
BoM, 2018. Rare Rainfall IFD Data System for Greenbushes WA. Commonwealth of Australia Bureau
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