SARATHY GEOTECH & ENGINEERING SERVICES PVT LTD JUNE 2021 (ISO 9001:2015 & ISO 45001:2018 CERTIFIED COMPANY)

TAILING DAM STABILITY ANALYSIS Tailings are the waste mineral that have been removed from the natural ore during the refining process. Tailings are usually in slurry form at the end of processing operation. This slurry is transported to the tailing disposal structure, typically, a pond or a dam. It is the function of tailing impoundment to retain the slurry for an extended time so that suspended tailings settle and fall at the bottom of impoundment. The decant system is used to remove the classified slurry water from the impoundment either for disposal or for ’reuse’ or reclamation purpose.

Types of Tailing There are two basic types of structures used to retain tailings in impoundments, the raised embankment and the retention dam. Raised embankments are more popular than retention dam. Raised embankments can be constructed using upstream, downstream or centreline methods as shown in Figure 1.

Tailings Seepage Seepage is the movement of water (contaminated and uncontaminated) through and around the dam and impoundment. Figure 1. Embankment Types (a) Upstream (b) centerline In conducting stability analysis of these dams, flow (c) Downstream or water retention (Source: Vick 1990) nets can be used to estimate seepage direction, volume and pore pressure at points within the embankment (CANMET 1977). A flow net is a graphical solution of Darcy's law to show steady flow through porous media and is often used to show ground water flow. The variables include flow characteristics (either in terms of flow or head), boundary condition of the area to be modelled, and information on the within the area.

TAILING DAM STABILITY ANALYSIS –A CASE STUDY

Stability & seepage analysis for tailing dam at Tanzania Graphite was carried out by SGES as part of the project. Plan area of tailing dam was approximately 400mx600m.Topwidth of embankment section was 11m. Tailing dam was proposed as downstream method of construction in four phases using compacted waste rock material. U/s of the embankment had a slope of 1V:3H and D/s of the embankment had a slope of 1.5H: 1V. Plan and cross section of proposed tailing dam is provided in Figure 2 & Figure 3 respectively. Initial embankment (starter dyke) was proposed with clayey (compacted to 98%, +2% OMC). Geo fabric material was proposed to be placed between waste rock embankment, tailings and the basin of the Tailing storage facility (TSF). Toe drains were also proposed along the upstream toe of the embankment.

Figure 2. Plan of Tailing Dam

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Figure 3. Cross Section of Tailing Dam The underlain strata of the proposed tailing dam consist of overburden soil for depth of 3m followed by moderately weathered rock up to 15m. Fresh rock was present beyond 15m. Permeability of top soil was in the order of 10‐8 m/s. Slope Stability and Seepage Analysis Slope stability and steady seepage analysis were performed for each stage of embankment (pre‐deposition stage to Lift 3). Flooded condition was selected for analysis for both functional and non‐functional toe drain cases. Analysis was carried out for both static and pseudo‐static cases. The project site was located at ‘very strong’ to ‘destructive zone’ as per intensity zones in Africa (OCHA regional office for central and east Africa). A horizontal seismic coefficient of 0.05g was considered for the seismic analysis. All the analysis was carried out using ‘Slide 2’ software. Slide 2 is a 2D software program for evaluating the safety factor or probability of failure of circular and non‐circular failure surfaces in soil and rock slopes. Slide 2 analyses the stability of slip surfaces using vertical slice or non‐vertical slice limit equilibrium methods like Bishop, Janbu, Spencer and Sarma, among others. In this project study, Bishop’s simplified method was adopted for the analysis considering circular failure surface. Slide2 includes built in finite element ground water seepage analysis for steady state and transient conditions. Parameters used for the analysis are presented in Table 1, Table 2 and Table 3. Permissible factor of safety of 1.5 and 1.1 were considered for static and pseudo‐static analysis respectively, as per the South African Chamber of Mines Guideline (1996) and ANCOLD (1998) Table 1 :Embankment Properties Angle of Internal Saturated Unit Hydraulic Description (kPa) (º) Weight (kN/m3) Conductivity (m/s) Waste Rock 10 35 20 5x10-7 Clay 10 28 19 2.08x10-10 Tailings 0 32 15 3.57x10-6 Compacted Material @U/S face * 10 32 18 5x10-7

Table 2 : Properties Depth Cohesion Angle of Internal Friction Saturated Unit Description Hydraulic Conductivity (m/s) (m) (kPa) (º) Weight (kN/m3) 0-3 CLAY 10 28 19 1x10-8 3m below Rock 100 35 21 1x10-7

Table 3 : Support Properties Description Tensile Strength (kN/m) Hydraulic Conductivity (m/s)

Geofabric – (Bidim A6) 20 4.3x10-3

HDPE -1.5mm thick ** 15 1x10-12

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RESULTS Summary of factor of safety for U/S and D/S slopes under steady seepage condition are presented in Table 4. Typical analysis output for final stage condition of embankment is presented in Figure 4.

Table 4 : Summary of factor of safety for U/S and D/S slopes under steady seepage condition

Factor of Safety Stage of Tailing Dam Construction Analysis condition Slope Direction Static Seismic D/S 1.81 1.65 Functional Toe drains U/S 4.87 3.91

D/S 1.53 1.38 Pre-deposition Non-Functional Toe Drain U/S 4.31 3.29 D/S 1.56 1.42 Functional Toe drains U/S 3.47 3.05 Lift -1 D/S 1.14 1.03 Non-Functional Toe Drain U/S 3.40 2.90 D/S 1.52 1.37 Functional Toe drains U/S 5.67 3.98 Lift -2 D/S 0.95 0.85 Non-Functional Toe Drain U/S 4.64 3.34 D/S 2.22 1.89 Functional Toe drains U/S 4.98 3.83 Lift-3 D/S 1.27 1.07 Non-Functional Toe Drain U/S 4.24 3.27

Recommendations for flattening the D/S slope for lift 1, lift 2 and lift 3 stages were provided as the factor of safety is less than the permissible limits

(a) (b)

Figure 4. (a) D/s Slope Stability with functional Toe Drains –Static (b) D/s Slope stability with Non- Functional Toe Drains –Seismic

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MAJOR EVENTS AND ACHIEVEMENTS IN 1st  SGES succesfully completed the NABL Quarter & 2nd Quarter 2021 surveillance Audit for ISO/IEC 17025:2017 Standards for Site, Laboratories (fixed and A. EVENTS IN 1st Quarter & 2nd Quarter 2021 mobile)

 SGES launched Site and Geotech Lab App  Dr. CR Parthasarathy delivered a talk on the theme “Change In Practice Of Ubiquitous Spt In  Dr. C R Parthasarathy has been re-elected as Site Characterisation” conducted by IGS Chairman of Bangalore Chapter, Indian Bengaluru Chapter & Dept of Civil Engineering Geotechnical Society. (IGS-Bangalore Chapter) IISc, Bangalore for the Second Consecutive Term

 Dr. CR Parthasarathy delivered a lecture on the C. HAPPENINGS IN SECOND HALF OF 2021 theme “Overview of geotechnical Investigations with Case Studies” conducted by SJC Instituite of Technology  SGES will be conducting Online Quality Assurance/Quality Control & Testing of (PDA training) on 06 and 07 August  Mohammed Toufeeq delivered a talk on Short 2021. Term Training Programme (STTP) on Field Practices in jointly organised by SRM IST Indian Geotechnical  The other online training courses offered are: Society Student Chapter, Department of Civil Engineering, SRM Institute of Science and o Pile Integrity Test Training (PIT) Technology and The Institution of Engineers (I), o Offshore Geotechnical Practices Kattankulathur Local Centre o Geophysical Survey – Theory & Case  SGES conducted a full two day online training Studiesꞏ course - Quality Assurance/Quality Control & o Ground Improvement Techniques Testing of Deep Foundation for academicians, industrialists, practitioning engineers etc., o Geotechnical Investigation – Good Practice  SGES conducted two days online expert talk programme - Overview of Offshore Geophysical D. SGES Fights COVID-19 Survey & Geotechnical Investigation by Le HMDung, Geozard Expert. SGES implemented robust virus prevention and containment protocols and honoured its commitment to lend a helping hand to fight the B. MILESTONES AND RECOGNITION pandemic. SGES conducted Vaccination drive- Onsite SGES Office/LAB  SGES registered a new entity in United Arab Emirates.

 SGES launched mobile geotechnical laboratory with NABL accrediatation

 SGES was awarded the Top SME Business of the Year 2021

 SGES was awarded the India's Growth Champions 2021 by the Economic Times and Statista

 SGES was re-certified for ISO 9001:2015 QMS and upgraded ISO 45001:2018 OH&SMS for safety by TUV India Private Limited

“Strength and growth come only through continuous effort and struggle.” SGES - NEWS - 16 4 – Napoleon Hill

SARATHY GEOTECH & ENGINEERING SERVICES PVT LTD JUNE 2021 (ISO 9001:2015 & ISO 45001:2018 CERTIFIED COMPANY)

SGES High end upgraded brand new CSR 183 SPT, CPT & Wireline Rotary Core Test Pile Monitoring Project – IKIKE, Deltatek, Nigeria ‐ Completed

Drill Rig at Bullet train project in Surat

Pile Monitoring Project – ONGC 98/2, Sapura, India – Pile Monitoring Project – Subsea 7, KSA – Ongoing First Phase Completed

“Strength and growth come only through continuous effort and struggle.” SGES - NEWS - 16 5 – Napoleon Hill

SARATHY GEOTECH & ENGINEERING SERVICES PVT LTD JUNE 2021 (ISO 9001:2015 & ISO 45001:2018 CERTIFIED COMPANY)

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