Tailing Storage Conference 2020 Birchwood Hotel, 12 – 13 February 2020 The Southern African Institute of Mining and Metallurgy

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3D seepage modelling in tailings storage facility analysis and design for low permeability lined basins

W. Rouncivell, S. Imrie, G.C. Howell and H.A.C. Meintjes

SRK Consulting (Pty) Ltd, South Africa

Tailings Storage Facility (TSF) stability is significantly affected by the pore pressures in the outer embankments. As more TSFs are using low permeability basin barrier systems for environmental compliance, the potential for mounding of the phreatic surface, with the associated increased stability risk, needs to be understood. Three-dimensional (3D) seepage modelling supports two-dimensional (2D) analyses and provides additional insight in cases where the internal drain geometry cannot be represented accurately using 2D cross-sections. Two case studies with HDPE lining systems are presented. The first is an existing TSF where tonnage creep and excessive water storage on the TSF have led to high phreatic levels. A calibrated 3D model was developed to assess different drain design interventions. The second case study is a new TSF where an extensive footprint drainage network, supplemented by a vertical curtain drain, has been designed to minimise not only the location of the phreatic level but also the rate of pore pressure build-up with depth to significantly below hydrostatic.

INTRODUCTION Tailings Storage Facility (TSF) stability is significantly affected by the pore pressures in the outer embankments. Two-dimensional (2D) finite element seepage analyses are becoming standard in the industry to estimate pore pressure build-up during the life of the facility. As more TSFs are using low permeability basin barrier systems for environmental compliance, the potential for mounding of the phreatic surface, with the associated increased stability risk, requires innovative design approaches with more rigorous analysis. Three-dimensional (3D) seepage modelling supports 2D analyses and can provide additional insight in cases where the internal drain geometry cannot be represented accurately using 2D cross-sections, as well as for appropriate representation of uneven topography. To meet these challenges, 3D groundwater modelling software is used to understand and optimise TSF design. The software has been adapted to successfully model typical TSF geometries and boundary conditions. Two case studies are presented in this paper; each has an HDPE lining system. The first study looks at an existing TSF, where tonnage creep and excessive water storage on the TSF (due to restrictive regulatory factors) have led to higher than expected phreatic levels. Cone penetrometer testing with pore pressure measurements (CPTu) and drain flow data were used to calibrate the 2D and 3D models. A sophisticated elevated drain drainage system was designed to ensure future embankment slope stability until final design height. The second TSF is a new design. Due to relatively steep terrain and a 1V:3H outer slope (required to ensure design life storage capacity), an extensive footprint drainage network, supplemented by a vertical curtain drain, has been designed to manage seepage and minimise not only the location of the phreatic level but also the rate of pore pressure build up with depth to well below hydrostatic.

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MOTIVATION FOR 3D MODELLING The importance of water management of TSFs is emphasised in ANCOLD’s (2012) list of ‘Past Lessons Learnt’ regarding TSF incidents and failures including, inter alia, lack of control of the water balance, a general lack of understanding of the features that control safe operations, poor water control, slope instability and seepage. Basu et al. (2013) includes reliability, denoting a system that is sufficiently far away from its predictable failure states, as an essential part of a sustainable engineered system. This implies being able to define its possible future states. Garrick et al. (2014) emphasise the importance of dimensionality and scale in modelling TSF hydraulic performance to assess leakage rates through the base of TSF facilities where 3D hydrogeological models are essential for certain TSF configurations. This also applies to modelling the TSF itself. Martin (1999) commented that in the TSF design stage, the definition of pore pressure conditions that will exist within an upstream TSF is at best a difficult undertaking and not possible in many instances. TSFs are complex systems that in the past have been designed and managed using a combination of geotechnical design and geographical specific heuristic knowledge. This has largely worked in the past but due to numerous factors, including high profile TSF failures and more stringent environmental requirements, TSF designers have had to re-evaluate certain design and analysis methods. A framework for the characterisation of mine tailings for TSF design, based on engineering mechanics, has been systematically set out by authors such as Been (2016), which includes a comprehensive description of laboratory and in-situ tests, interpretation techniques, as well as appropriate constitutive models such as Nor-sand (Jeffries, 1993) for specific tailings. The current work is expected to contribute to developing a framework for the categorisation of pore pressure and saturation regimes within TSFs. CASE STUDY OVERVIEW The case studies are from existing projects; the scenarios presented were chosen to demonstrate the usefulness of the 3D modelling and are not necessarily the final designs adopted for the respective projects. The first case study facility (TSF1) is a ring dyke impoundment over an undulating terrain with one side against a hill. It has an outer slope of 1V(vertical):5H(horizontal) to a design height 35 m above the starter wall. TSF2 is situated against a hill and has an outer slope of 1V:3H to a design height 102 m above the starter wall. Both facilities are constructed using the upstream method with spigot and cyclone deposition on TSF1 and TSF2 respectively. Each model was formulated in the 3D finite element software package FEFLOW 7.1 to simulate water movement in both the horizontal and vertical planes in porous media, assuming variably saturated conditions. Fluid flow was simulated as steady state (long-term equilibrium) for all scenarios and Richard’s equation for variably saturated flow was solved using the preconditioned conjugate-gradient (PCG) solver method. The numerical flow modelling methodology is based on that of Applied Groundwater Modelling by Anderson et al. (2015), with the model acceptance criteria and confidence classifications according to the Australian Groundwater Modelling Guidelines by Merz (2012). TSF1 Case Study Due to a mounded phreatic level throughout TSF1, an intervention is required to reduce this level, or at least prevent it from rising. If the TSF1 continues to be operated in the same way, the seepage zone on the outer walls will rise and pose a greater risk to stability. The phreatic level will be controlled by draining water from the tailings along the perimeter wall section using elevated drains within the tailings. The position of these drains was determined by the results of seepage modelling where drain geometry is shown to effectively intercept infiltration from the tailings beaches and the saturated zone towards the pool. The stability of the TSF1 could then also be assessed

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to show the effectiveness of the solution. Furthermore, the accessibility on the tailings surface is taken into consideration. Model Setup The TSF1 was modelled in 2D and 3D using two different software programs, Slide 2018 (Rocscience, 2018) and FEFLOW 7.1 (DHI-WHASY GmbH., October 2017). The model parameters were aligned to make direct comparison between the results. Steady state conditions were applied for the condition of the TSF1 at the time of the CPTu test period for calibration purposes. A 2D section along a CPTu line was chosen to analyse in detail since it is representative of the phreatic levels in the TSF1 at the time. The 3D model, however, allowed assessment of additional sections. For the Slide 2D model setup, the geometry of a section through the TSF1 was imported into Slide and the appropriate material zones delineated and hydraulic properties assigned. Since the facility is lined, an effectively impermeable base layer was used. The existing basal drain was modelled as a higher hydraulic conductivity (K) material and a discharge section was included to check the outflow for calibration purposes and to assess other scenarios. For the FEFLOW 3D model setup, the 3D geometry of the facility (including the basin survey) up to the respective TSF1 elevations were setup using approximately 4.6-million triangular prism elements and 25 layers. Mesh refinement is applied along the locations of the under-drains, the slopes and the CPTu cross sections. Hydraulic properties were assigned for each material zone. CPTu sections and drains are included in the super-mesh for accurate modelling and result extraction (see Figure 1).

Figure 1. Basal drain showing CPTu sections and basin shape.

Fundamental to this seepage modelling is understanding the K of the tailings and calibrating the model based on known flows into and out of the tailings. The elevation of the TSF1 was at 71.5 m at the time of the CPTu testing. Using the CPTu-interpreted phreatic levels, the model was calibrated at this elevation. The above-liner drain flows were determined around the time of the CPTu testing for the specific areas of the cross sections modelled. The flows for all the drains were also compared in the 3D model to check the mass water balance and flow volumes within the system. In the initial phase of deposition, below the starter wall crest, the rate of rise was at times very high, with open ended deposition and large pool. This would have resulted in minimal segregation and thus

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more homogenous layering. This was represented in the model by horizontally extending the lower K material properties over the drain towards the starter wall. This K was further calibrated using a comparison of modelled to observed drain flows. As the TSF1 rose, beaches became more prominent and pool management reduced the pool size. This resulted in more segregation and is represented in the model by five material zones (see Figure 2), with K ranging from 2.5 x 10-7 to 6.0 x 10-7 m/s. The laterite outer zone has a lower K than the adjacent tailings. The bottom of the model is effectively impermeable to represent the HDPE lining. The 3D and 2D models were aligned in TSF1 to validate the output of the 3D model.

Figure 2. Elevated drain to 91.5 m K zones (FEFLOW section shown top, and Slide 2D shown bottom).

To determine the actual flux (infiltration) on the tailings beach that recharges the saturated zone, different infiltration profiles were considered, as shown in Figure 3. Another characteristic of spigotted tailings is layering, which can form horizontal coarse/fine layers which cause higher horizontal than vertical K. The ratio of horizontal K to vertical K was determined to be 20 for all the material zones.

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Figure 3. 3D flux profile.

The observed pool area changes constantly in response to rainfall and deposition. The pool edge thus varies between 280 m and 520 m from the walls. However, there is a highly saturated zone of tailings that extends to within 125 m from the outer laterite wall. This location was determined by extrapolating the phreatic surface interpreted from the CPTu’s towards the centre of the TSF1. This interpreted intersection with the beach from 2D was used to delineate the saturated area of the beach in plan. This ‘effective’ pool area and edge were used in the model in addition to the observed pool location. The saturated beach is represented as a total head at beach level. The CPTu results were interpreted to estimate K. The value of K ranged between 2.3 x 10-10 m/s and 2.1 x 10-6 m/s. It is noted that empirical factors for calculating K from CPTu data were not developed for tailings, but for more clayey materials. Since the TSF1 tailings is of a sand to sandy silt nature these values were taken as a guide for estimating K during model calibration, to develop a representative picture of K across the TSF1 which took drain flows into account. Model Results The K values and horizontal to vertical K ratio, together with the high beach flux, maintains the particularly high phreatic levels in the TSF1. In addition, local mounding may result from non-uniform flow and drainage above the highly undulating base. A drainage system was designed to prevent this phreatic level rising in the future (see Figures 4 and 5). The elevated drain will be built up from around the 71.5m beach level, therefore does not penetrate the tailings further down. Although the drain will impact the phreatic level above this level, the phreatic level below will not be lowered sufficiently, therefore this will need to be controlled/drawn down using another method, possibly a borehole/well system along an existing bench. The drain design consists of two parts, the 1V:5H portion running parallel to the perimeter, and the paddock extensions (‘fins’ or ‘fingers’) which extend perpendicularly and/or diagonally from the 1V:5H portion outwards.

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Figure 4. 91.5 m elevation model setup (5x vertical exaggeration).

Figure 5. Elevated drain to 91.5 m elevation design option 1 used for 3D seepage model. Multiple model predictive scenarios were used to assess the seepage regime within the TSF1. It was shown that the phreatic level will continue to rise and increase the stability risk of the facility if no

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interventions are constructed. An elevated drain has therefore been designed to manage the phreatic level. Without elevated drains installed, the mounded phreatic surface in some areas at final height is apparent from the cross-section shown in Figure 6. 3D modelling to include the elevated drain design (scenario 2) is shown to resolve the mounded phreatic level concerns along this same cross-section (Figure 7).

Figure 6. Scenario 1 (without elevated drains): pore water pressures in cross-section.

Figure 7. Scenario 2 (with elevated drains): pore water pressures in cross-section.

TSF2 Case Study The second case study, TSF2, is currently following a phased design and construction process. The construction of Phase 1 is effectively complete and the design of Phase 2 is in progress and required the

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development of a 3D seepage model for use in running and post analysing predictive scenarios under varying 3D drain designs at final height. Model Setup For the FEFLOW 3D model setup, the planned 3D geometry of the facility (including the basin survey) up to the full TSF2 planned elevations were set up using highly refined vertical triangular prismatic elements of approximately 1 m along each edge. Being a new design, material hydraulic properties from an established similar facility were adopted. Material zones are assumed to be divided between underflow, overflow and curtain drain material. The location of these zones along a cross-section are shown in Figure 8 in green, purple and red, respectively.

Figure 8. 3D view through cross-section 1a – vertical K.

The drains shown in Figure 9 are grouped into the main drains and two 20 m wide blanket drains adjacent to the curtain drain. Basin drains cover the rest of the basin upstream of the main drains. The curtain drain starts at 1115 mamsl, with outlet structures at 50 m centres along the drain. The drain extends almost to the underflow/overflow material zone interface. TSF2 Results Figure 10 shows the pore water pressure distribution for scenario 1 (main and curtain drains) and scenario 2 (main, basin and curtain drains). The coloured zones are shown in the legend as pressure in units of kilopascals (kPa). The phreatic surface and 50 kPa isoline are shown as a white and green line respectively. The location of the curtain drain is shown in dark orange. The vertical light orange lines represent the locations of the depth-pressure output graphs.

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Figure 9. Plan view of drain geometry used for 3D seepage model.

The following information could be derived from the model results: • The phreatic surface is successfully drawn down to the base of the curtain drain at 1115 mamsl. • The pore water pressure gradients increase below the phreatic surface, at a significantly lower rate

than the hydrostatic pressure gradients (shown in red) due to the flow towards the drains. This modelling result confirms the design intent of the drainage systems.

• The main drains, which include the blanket drains on either side of the curtain drain, aid in reducing the vertical pore pressure gradient, but do not completely lower the phreatic surface to near the liner beneath the curtain drain.

• The phreatic surface is lower in this 3D model compared to the 2D model. This is mainly a result of the increased relative dissipation geometrically in 3D compared to 2D due to the ratio of wall area to beach area being higher in 3D than in 2D. In other words, the infiltration surface to total TSF volume ratio is lower in 3D than along a single 2D cross-section. The implication of this result is that the design undertaken based on the 2D seepage analyses is a conservative design.

The applied flux on the TSF2 is relatively high and therefore there is a continuous flow of water from the beach down to the drains. Below the phreatic surface the saturation is 100% of the connected pore space. Above the phreatic surface the saturation doesn’t simply decrease with distance from the phreatic surface; instead, it is a function of the flux infiltrating from surface and the hydraulic properties and behaviour of the unsaturated zone. Although saturation does not have a direct impact on stability, it does influence the rapidity at which the system could alter (e.g. change in the phreatic level) in response to a change of inputs or outputs (e.g. increased infiltration following a high rainfall event). Figure 11 shows a cross section of saturation for scenario 1 and scenario 2. As per the legend, the coloured areas represent zones of a 20% range in saturation.

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Figure 10. Pore water pressures for scenario 1 (top) and scenario 2 (lower) – cross-section 1a. The following points are noted: • Both scenarios show a high saturation area (80 – 100%) directly below the top surface of the TSF2.

This corresponds to the zone where pore pressures are near 0 kPa. • There is a decrease in saturation near the curtain drains. • Although the phreatic surface is a lot lower in scenario 2, there is still a high saturation area directly

below the top surface of the TSF2. The extension of this saturated area towards the TSF2 walls is lowered by the implementation of the curtain drain.

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Figure 11. Saturation for scenario 1 (top) and scenario 2 (lower) – cross-section 1a.

Mass Water Balance The mass water balance for the two predictive scenarios are shown in Table I. The following main points are noted: • The applied infiltration/flux is a maximum value. Depending on the degree of saturation and the

hydraulic properties of the tailing’s material, only a portion of the applied flux may infiltrate. The remainder is assumed to be decanted away at surface via the penstock systems and is simulated in the model using a maximum head constraint of 1190 mamsl.

• For all scenarios, the main drains provide significant drawdown of the phreatic surface and this is further demonstrated by the high flow portion through these drains.

• The curtain drain flow is relatively low for scenario 1 but does draw down the phreatic surface and decrease pore pressures near the wall.

• For scenario 2, the basin drains have a significant flow rate and are well positioned to keep the phreatic surface low. Subsequently, the flow through the main drains and especially the curtain drain are reduced in these model scenarios. However, it is important to note that the saturated K in the “overflow” material zone is assumed to be constant with depth as well as towards the pool area. If the K decreases with depth (or towards the pool) then this would further inhibit the flow to the basin drains, resulting in an increase of flow to (and reliance on) the main drains and curtain drains.

• Table I. Mass water balance for scenarios 1 and 2.

TSF2 - Flow (m3/d) Scenario 1 Scenario 2

Applied Flux to Surface 748 748 Applied Inflow

Pumped from Surface 464 394 Pool Outflow

Main Drains 253 83

Drain Outflows Curtain Drains (Chimneys) 31 0

Basin Drains 0 271

748 748 Total Outflow

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The 3D seepage modelling has increased the confidence level of the results (i.e. decreased the potential range of uncertainty) for the inputs (particularly pore water pressures) used in the TSF stability and drain design calculations. Of significance is the lowered phreatic surface in the 3D modelling compared to the 2D modelling due to the geometric dissipation effect. Also, cross-sections and associated pore pressure distributions can be exported from multiple locations within the 3D model thus providing a further check on the 3D drain design within the context of the 3D basin geometry and TSF volume. CONCLUSION 3D seepage modelling was successfully used to increase the confidence level of the drainage designs of the two case studies. Seepage flows within the tailings are an important input for stability analyses (pore pressure distribution), environmental impacts (contaminant transport via leakage to groundwater) and water efficiency. 3D seepage modelling can provide valuable input to these disciplines. With practically unlimited computing power and established and adaptable finite element modelling software, the use of 3D modelling is an extremely valuable tool for future TSF design. REFERENCES ANCOLD (2012). Guidelines on Tailings Dams. Planning, Design, Construction, Operation and Closure.

ANCOLD, Australia. pp. 1-70. Anderson, M., Woessner, W. and Hunt, R. (2015). Applied Groundwater Modeling: Simulation of Flow and

Advective Transport, 2nd edn, ISBN: 9780080916385, Academic Press. Basu, D., Misra, A., and Puppala, A. (2013). Sustainability in Geotechnical Engineering.

Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering. Paris International Conference Centre, 2-6 September. Paris: Technical Committee 307. pp. 3171-3174.

Been, K. (2016). Characterizing mine tailings for geotechnical design. Geotechnical and Geophysical

Site Characterisation 5, 1 (1), 41-56. Garrick, H., Digges La Touche, G., Plimmer, B. & Hybert, D. (2014). Assessing the Hydraulic

Performance of Tailings Management Facilities. Proceedings of the 17th Extractive Industry Geology Conference. Edge Hill University, September. Nottinghamshire: Golder Associates (UK) Ltd. pp. 125-134.

Jeffries, M.G. (1993). Nor-Sand: a simple critical state model for sand. Geotechnique, 43 (1), 91-103. Martin, T.E. (1999). Characterization of pore pressure conditions in upstream tailings dams.

Proceedings of the 6th International Conference on Tailings and Mine Waste. Colorado State University, 24-27 January. Colorado: Balkema, A.A. pp. 303-314.

Merz, S.K. (2012). Australian groundwater modelling guidelines. Technical Report 82, Waterlines

Report Series. The following references appeared in-text but not in the list: DHI-WHASY GmbH, October 2017 and

Rocscience, 2018

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Wesley Rouncivell Civil Engineer SRK Consulting Wesley has 6 years of work experience in Civil Engineering, particularly Tailings Storage Facility (TSF) projects focussing on geotechnical stability and water management. His Masters dissertation in geotechnical engineering investigated interface shear strength characteristics of lining systems for waste containment facilities. Currently he is involved with developing advanced numerical seepage modelling techniques for TSFs. These assist with optimising internal drain designs and understanding related water balance impacts over the life of these facilities. Sheila Imrie Principal Numerical Modeller / Hydrogeologist SRK Consulting Sheila has 21 years of work experience in applied mathematics, with 8 years specialising in IT software development and statistics, followed by 13 years specialising in water related projects and groundwater numerical modelling. She has worked and studied in both South Africa and the UK, from where she worked on numerous international projects. Sheila has a particular interest in solving problems on the interfaces of disciplines, such as often occurs between science and engineering.

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