Table 10-3 Recommended FOS and shear strength parameters€¦ · The tailings strength parameters...

GHD | Report for Talison Lithium Limited - Talison TSF4 Detailed Design, 6137226

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.


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.

GHD | Report for Talison Lithium Limited - Talison TSF4 Detailed Design, 6137226 | 45

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


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.


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 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




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 N D/S global 1.8 1.5 Y Figure 1.15


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




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


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.


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


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.


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.


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.


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.


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.


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).


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%.


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


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.


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.


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)




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


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


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.


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)


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.


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


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.


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


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.


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


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.


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


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


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


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


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


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.


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.


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

of Meteorology. 2018

Fell R, MacGregor P, Stapledon D, and Bell G (2005) “Geotechnical Engineering of Dams”, A.A.

Balkema, Leiden, The Netherlands.GHD, 2015, Talison TSF2 Development Feasibility Study report,

May 2015

GHD, 2017. Tailings option study for expanded production. November 2017 (Ref. 6136445)

GHD, 2018 a. TSF4 Option study update. Revision 0 issued 3 May 2018 (Ref. 6136890)

GHD, 2018 b. Hydrogeological Investigation 2018 site-wide hydrogeology report. August 2018 (Ref

6136958)

GHD, 2018 c. Assessment of acid and metalliferous drainage, July 2018. (Ref 6136960)

GHD, 2019. Talison TSF4 Geotechnical Investigation (Ref. 6137155)

Talison, 2012, Talison Lithium Surface Water Management Plan, March 2012

USCE, 1984. Rationalizing the Seismic Coefficient Method. July 1984

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